Rectangular pulse generators are widely used in radio engineering, television, automatic control systems and computer technology.

To obtain rectangular pulses with steep edges, devices are widely used whose operating principle is based on the use of electronic amplifiers with positive feedback. These devices include so-called relaxation oscillators - multivibrators, blocking oscillators. These generators can operate in one of the following modes: standby, self-oscillating, synchronizing and frequency division.

In standby mode, the generator has one stable equilibrium state. An external trigger pulse causes an abrupt transition of the waiting generator to a new state, which is not stable. In this state, called quasi-equilibrium, or temporarily stable, relatively slow processes occur in the generator circuit, which ultimately lead to a reverse jump, after which a stable initial state is established. The duration of the quasi-equilibrium state, which determines the duration of the generated rectangular pulse, depends on the parameters of the generator circuit. The main requirements for waiting generators are the stability of the duration of the generated pulse and the stability of its initial state. Waiting generators are used, first of all, to obtain a certain time interval, the beginning and end of which are fixed, respectively, by the front and fall of the generated rectangular pulse, as well as to expand pulses, to divide the pulse repetition rate and other purposes.

In the self-oscillatory mode, the generator has two states of quasi-equilibrium and does not have a single stable state. In this mode, without any external influence, the generator sequentially jumps from one state of quasi-equilibrium to another. In this case, pulses are generated, the amplitude, duration and repetition frequency of which are determined mainly only by the parameters of the generator. The main requirement for such generators is high frequency stability of self-oscillations. Meanwhile, as a result of changes in supply voltages, replacement and aging of elements, the influence of other factors (temperature, humidity, interference, etc.), the stability of the frequency of self-oscillations of the generator is usually low.

In synchronization or frequency division mode, the repetition rate of the generated pulses is determined by the frequency of the external synchronizing voltage (sinusoidal or pulsed) supplied to the generator circuit. The pulse repetition frequency is equal to or a multiple of the synchronizing voltage frequency.

A generator of periodically repeating relaxation-type rectangular pulses is called a multivibrator.

The multivibrator circuit can be implemented both on discrete elements and in integrated design.

Multivibrator based on discrete elements. This multivibrator uses two amplification stages covered by feedback. One feedback leg is formed by a capacitor and a resistor , and the other - And (Fig. 6.16).

states and ensures the generation of periodically repeating pulses, the shape of which is close to rectangular.

In a multivibrator, both transistors can be in the active mode for a very short time, since as a result of positive feedback, the circuit jumps into a state where one transistor is open and the other is closed.

Let us assume for definiteness that at the moment of time transistor VT1 open and saturated, and the transistor VT2 closed (Fig. 6.17). Capacitor Due to the current flowing in the circuit at previous times, it is charged to a certain voltage. The polarity of this voltage is such that to the base of the transistor VT2 a negative voltage is applied relative to the emitter and VT2 closed. Since one transistor is closed, and the other is open and saturated, the self-excitation condition is not satisfied in the circuit, since the gain coefficients of the stages
.

In this state, two processes take place in the circuit. One process is associated with the flow of the capacitor recharge current from the power supply through the resistor circuit – open transistor VT1 .The second process is due to the charge of the capacitor through a resistor
and the base circuit of the transistor VT1 , resulting in voltage at the collector of the transistor VT2 increases (Fig. 6.17). Since the resistor included in the base circuit of the transistor has a higher resistance than the collector resistor (
), capacitor charging time less time to recharge the capacitor .

open because its base is connected to the positive pole of the power supply through a resistor .

Basic
and collector
transistor voltage VT1 however, they do not change. This state of the circuit is called quasi-stable.

At a moment in time as the capacitor recharges, the voltage at the base of the transistor VT2 reaches the opening voltage and the transistor VT2 switches to active operating mode, for which
. When opening VT2 collector current increases and decreases accordingly
. Decrease
causes a decrease in the base current of the transistor VT1 , which, in turn, leads to a decrease in the collector current . Current reduction accompanied by an increase in the base current of the transistor VT2 , since the current flowing through the resistor
, branches into the base of the transistor VT2 And
.

After the transistor VT1 exits the saturation mode, the self-excitation condition is satisfied in the circuit:
. In this case, the process of switching the circuit proceeds like an avalanche and ends when the transistor VT2 goes into saturation mode, and the transistor VT1 – to cut-off mode.

Subsequently, the almost discharged capacitor (
) is charged from a power source through a resistor circuit
– basic circuit of an open transistor VT2 according to exponential law with time constant
. As a result, over time
the voltage across the capacitor increases before
and the front of the collector voltage is formed
transistor VT1 .

Transistor off state VT1 ensured by the fact that initially charged to voltage capacitor through an open transistor VT2 connected to the base-emitter gap of the transistor VT1 , which maintains a negative voltage at its base. Over time, the blocking voltage at the base changes as the capacitor recharged through the resistor circuit – open transistor VT2 . At a moment in time transistor base voltage VT1 reaches the value
and it opens.

In the circuit, the self-excitation condition is again satisfied and a regenerative process develops, as a result of which the transistor VT1 goes into saturation mode VT2 closes. Capacitor is charged to a voltage
, and the capacitor almost empty(
). This corresponds to the time , from which the consideration of processes in the scheme began. This completes the full cycle of operation of the multivibrator, since in the future the processes in the circuit are repeated.

As follows from the timing diagram (Fig. 6.17), in a multivibrator, periodically repeating rectangular pulses can be removed from the collectors of both transistors. In the case when the load is connected to the collector of the transistor VT2 , pulse duration determined by the process of recharging the capacitor , and the duration of the pause – the process of recharging the capacitor .

Capacitor recharge circuit contains one reactive element, therefore, where
;
;.

Thus, .

Recharge process ends at the moment of time , When
. Consequently, the duration of the positive pulse of the collector voltage of the transistor VT2 is determined by the formula:

.

In the case when the multivibrator is made on germanium transistors, the formula is simplified, since
.

Capacitor recharging process , which determines the duration of the pause between transistor collector voltage pulses VT2 , proceeds in the same equivalent circuit and under the same conditions as the process of recharging the capacitor , only with a different time constant:
. Therefore, the formula for calculating similar to the formula for calculating :

.

Typically, in a multivibrator, the pulse duration and pause duration are adjusted by changing the resistance of the resistors And .

The duration of the fronts depends on the opening time of the transistors and is determined by the charging time of the capacitor through the collector resistor of the same arm
. When calculating a multivibrator, it is necessary to satisfy the condition of saturation of an open transistor
. For transistor VT2 excluding current
capacitor recharge current
. Therefore, for the transistor VT1 saturation condition
, and for a transistor VT2 -
.

Frequency of generated pulses
. The main obstacle to increasing the pulse generation frequency is the long pulse rise time. Reducing the duration of the pulse front by reducing the resistance of the collector resistors can lead to failure of the saturation condition.

With a high degree of saturation in the considered multivibrator circuit, cases are possible when, after turning on, both transistors are saturated and there are no oscillations. This corresponds to a strict self-excitation mode. To prevent this, you should select an open transistor operating mode near the saturation limit in order to maintain sufficient gain in the feedback circuit, and also use special multivibrator circuits.

If the pulse duration equal to duration , which is usually achieved at , then such a multivibrator is called symmetrical.

The rise time of the pulses generated by the multivibrator can be significantly reduced if diodes are additionally introduced into the circuit (Fig. 6.18).

When, for example, a transistor turns off VT2 and the collector voltage begins to increase, then to the diode VD2 reverse voltage is applied, it closes and thereby turns off the charging capacitor from the collector of the transistor VT2 . As a result, the capacitor charge current no longer flows through the resistor , and through a resistor . Consequently, the duration of the front pulse of the collector voltage
is now determined only by the process of closing the transistor VT2 . A diode works the same way. VD1 when charging a capacitor .

Although in such a circuit the rise time is significantly reduced, the charging time of the capacitors, which limits the duty cycle of the pulses, remains virtually unchanged. Time constants
And
cannot be reduced by reducing . Resistor in the open state of the transistor, it is connected through an open diode in parallel with the resistor .As a result, when
The power consumption of the circuit increases.

Multivibrator on integrated circuits(Fig. 6.19). The simplest circuit contains two inverting logic elements LE1 And LE2, two timing chains
And
and diodes VD1 , VD2 .

At a moment in time
and at the exit LE2
. As a result, at the entrance LE1 through a capacitor , which is charged to voltage
, voltage is applied and LE1 goes to zero state
. Since the output voltage LE1 decreased, then the capacitor starts to discharge. As a result, the resistor a voltage of negative polarity will arise, the diode will open VD2 and capacitor will quickly discharge to voltage
. After this process is completed, the input voltage LE2
.

At the same time, the capacitor is charging in the circuit. and over time the input voltage LE1 decreases. When at a point in time voltage
,
,
. The processes begin to repeat themselves. The capacitor charges again , and the capacitor discharges through an open diode VD1 . Since the resistance of the open diode is much less than the resistance of the resistors , And , capacitor discharge And occurs faster than their charge.

Input voltage LE1 in the time interval
determined by the capacitor charging process :, Where
;
– output resistance of the logic element in the one state;
;
, where
. When
, the formation of the pulse at the output of the element ends LE2, therefore, the pulse duration

.

The duration of the pause between pulses (time interval from before ) is determined by the process of charging the capacitor , That's why

.

The duration of the front of the generated pulses is determined by the switching time of the logic elements.

In the time diagram (Fig. 6.20), the amplitude of the output pulses does not change:
, since during its construction the output resistance of the logic element was not taken into account. Taking into account the finiteness of this output resistance, the amplitude of the pulses will change.

The disadvantage of the considered simplest multivibrator circuit based on logic elements is the hard self-excitation mode and the associated possible absence of an oscillatory mode of operation. This drawback of the circuit can be eliminated if you additionally introduce an AND logical element (Fig. 6.21).

When the multivibrator generates pulses, the output LE3
, because the
. However, due to the strict self-excitation mode, it is possible that when the power supply voltage is turned on, due to the low rate of voltage rise, the charging current of the capacitors And turns out to be small. In this case, the voltage drop across the resistors And may be less than threshold
and both elements( LE1 And LE2) will find themselves in a state where the voltages at their outputs
. With this combination of input signals at the output of the element LE3 tension will arise
, which through a resistor supplied to the element input LE2. Because
, That LE2 is transferred to the zero state and the circuit begins to generate pulses.

To build rectangular pulse generators, along with discrete elements and LEs in an integrated design, operational amplifiers are used.

therefore the voltage at the inverting input
depends not only on the voltage at the output of the amplifier, but is also a function of time, since
.

A positive voltage is applied to the non-inverting input
. Voltage
remains constant, and the voltage at the inverting input
increases over time, tending to the level
, since the process of recharging the capacitor takes place in the circuit .

However, for now
, the state of the amplifier determines the voltage at the non-inverting input and the output level is maintained
.

At a moment in time The voltages at the inputs of the operational amplifier become equal to:
. Further slight increase
leads to the fact that the differential (difference) voltage at the inverting input of the amplifier
turns out to be positive, so the output voltage decreases sharply and becomes negative
. Since the voltage at the output of the operational amplifier has changed polarity, the capacitor subsequently recharges and the voltage on it, as well as the voltage at the inverting input, tend to
.

At a moment in time again
and then the differential (difference) voltage at the amplifier input
becomes negative. Since it acts on the inverting input, the voltage at the output of the amplifier jumps again to the value
. The voltage at the non-inverting input also changes abruptly
. Capacitor , which by the time charged to a negative voltage, recharges again and the voltage at the inverting input increases, tending to
. Since in this case
, then the voltage at the amplifier output remains constant. As follows from the time diagram (Fig. 6.23), at the moment of time the full cycle of operation of the circuit ends and in the future the processes in it are repeated. Thus, periodically repeating rectangular pulses are generated at the output of the circuit, the amplitude of which at
equal to
. Pulse duration (time interval
) is determined by the time it takes to recharge the capacitor according to the exponential law from
before
with time constant
, Where
– output impedance of the operational amplifier. Because during the pause (interval
) the capacitor is recharged under exactly the same conditions as during the formation of pulses, then
. Hence, the circuit works as a symmetrical multivibrator.

occurs with time constant
. With a negative output voltage (
) diode open VD2 and the capacitor recharge time constant , which determines the duration of the pause,
.

A standby multivibrator or monovibrator has one stable state and provides the generation of rectangular pulses when short trigger pulses are applied to the input of the circuit.

Single vibrator based on discrete elements consists of two amplification stages covered by positive feedback (Fig. 6.25).

transistor VT1 depends on the collector current of the transistor VT2 . This circuit is called an emitter-coupled single-vibrator. The circuit parameters are calculated in such a way that in the initial state, in the absence of input pulses, the transistor VT2 was open and rich, and VT1 was in cutoff mode. This state of the circuit, which is stable, is ensured when the following conditions are met:
.

Let us assume that the monovibrator is in a stable state. Then the currents and voltages in the circuit will be constant. Transistor base VT2 through a resistor connected to the positive pole of the power supply, which, in principle, ensures the open state of the transistor. To calculate the collector
and basic currents we have a system of equations

.

Having determined from here the currents
And , we write the saturation condition in the form:

.

Considering that
And
, the resulting expression is significantly simplified:
.

On a resistor due to the flow of currents ,
voltage drop is created
. As a result, the potential difference between the base and emitter of the transistor VT1 is determined by the expression:

If the condition is met in the circuit
, then the transistor VT1 closed. Capacitor at the same time charged to voltage. The polarity of the voltage across the capacitor is shown in Fig. 6.25.

When the transistor VT1 opens, capacitor turns out to be connected to the base-emitter region of the transistor VT2 such that the base potential becomes negative and the transistor VT2 goes into cut-off mode. The circuit switching process is avalanche-like in nature, since at this time the self-excitation condition is satisfied in the circuit. The switching time of the circuit is determined by the duration of the transistor switching processes VT1 and turn off the transistor VT2 and is a fraction of a microsecond.

When the transistor turns off VT2 through a resistor collector and base currents stop flowing VT2 . As a result, the transistor VT1 remains open even after the end of the input pulse. At this time on the resistor voltage drops
.

State of the circuit when the transistor VT1 open and VT2 closed and quasi-stable. Capacitor through a resistor , open transistor VT1 and resistor turns out to be connected to the power source in such a way that the voltage on it has opposite polarity. A capacitor recharging current flows in the circuit , and the voltage across it, and therefore at the base of the transistor VT2 strives for a positive level.

Voltage change
is exponential in nature: where
. Initial voltage at the base of the transistor VT2 determined by the voltage to which the capacitor is initially charged and residual voltage on the open transistor:

The limiting voltage value to which the voltage at the base of the transistor tends VT2 , .

It is taken into account here that through a resistor not only the capacitor recharging current flows , but also current open transistor VT1 . Hence, .

At a moment in time voltage
reaches release voltage
and transistor VT2 opens. Appearing collector current creates an additional voltage drop across the resistor , which leads to a decrease in voltage
. This causes a decrease in the base and collector currents and a corresponding increase in voltage
. Positive increment of transistor collector voltage VT1 through a capacitor transmitted to the base circuit of the transistor VT2 and contributes to an even greater increase in its collector current . A regenerative process again develops in the circuit, ending with the transistor VT1 closes and the transistor VT2 goes into saturation mode. This completes the process of generating an impulse. The pulse duration is determined by putting
: .

After the end of the pulse, the capacitor is charged in the circuit. through a circuit consisting of resistors
,and emitter circuit of an open transistor VT2 . At the initial moment, the base current transistor VT2 equal to the sum of the capacitor charge currents : current , limited by the resistance of the resistor
, and the current flowing through the resistor . As the capacitor charges current the base current of the transistor decreases and accordingly decreases VT2 , tending to a stationary value determined by the resistor . As a result, at the moment the transistor opens VT2 voltage drop across resistor turns out to be greater than the stationary value, which leads to an increase in the negative voltage at the base of the transistor VT1 . When the voltage across the capacitor reaches
the circuit returns to its original state. Duration of the capacitor recharging process , which is called the recovery stage, is determined by the relation.

Minimum repetition period of one-shot pulses
, and the maximum frequency
. If the interval between input pulses is less , then the capacitor will not have time to recharge and this will lead to a change in the duration of the generated pulses.

The amplitude of the generated pulses is determined by the voltage difference across the transistor collector VT2 in closed and open states.

A one-shot can be implemented on the basis of a multivibrator, if one feedback branch is made not capacitive, but resistor and a voltage source is introduced
(Fig. 6.27). Such a circuit is called a single-vibrator with collector-base connections.

To the base of the transistor VT2 negative voltage is applied and it is closed. Capacitor charged to voltage
. In the case of germanium transistors
.

Capacitor , acting as a boost capacitor, is charged to voltage
. This state of the circuit is stable.

When applied to the base of the transistor VT2 unlocking pulse (Fig. 6.28), the processes of opening the transistor begin to take place in the circuit VT2 and closing the transistor VT1 .

When switching the circuit, the front of the output pulse is formed, which is usually removed from the collector of the transistor VT1 . Subsequently, the circuit undergoes a process of recharging the capacitor .Voltage on it
, and therefore the voltage at the base transistor VT1 changes according to exponential law
,Where
.

When at a point in time the base voltage reaches
, transistor VT1 opens, voltage on its collector
the transistor decreases and turns off VT2 . In this case, a cutoff of the output pulse is formed. We obtain the pulse duration if we put
:

.

Because
, That . Slice duration
.

Subsequently, a capacitor charging current flows in the circuit through a resistor
and the base circuit of the open transistor VT1 . The duration of this process, which determines the recovery time of the circuit,
.

The amplitude of the output pulses in such a one-shot circuit is almost equal to the voltage of the power source.

One-shot logic gate. To implement a one-shot on logical elements, AND-NOT elements are usually used. The block diagram of such a one-shot device includes two elements ( LE1 And LE2) and timing chain
(Fig. 6.29). Inputs LE2 combined and it works as an inverter. Exit LE2 connected to one of the inputs LE1, and a control signal is supplied to its other input.

its input circuit. The circuit generates a rectangular pulse with a short-term decrease (time ) input voltage
. After a time interval equal to
(not shown in Fig. 6.29), at the output LE1 the voltage will increase. This voltage surge across the capacitor passed to the input LE2. Element LE2 switches to state “0”. Thus, at the input 1 LE1 after an interval of time
tension begins to take effect
and this element will remain in the state of one, even if after time
voltage
will again become equal to logical “1”. For normal operation of the circuit, it is necessary that the input pulse duration
.

As the capacitor charges output current LE1 decreases. Accordingly, the voltage drop by :
. At the same time, the voltage increases slightly
, striving for tension
, which when switching LE1 in state “1” there was less
due to the voltage drop across the output resistance LE1. This circuit state is temporarily stable.

At a moment in time voltage
reaches the threshold
and element LE2 switches to state “1”. To input 1 LE1 signal is given
and it switches to the log state. "0". In this case, the capacitor , which is in the time interval from before charged, begins to discharge through the output resistance LE1 and diode VD1 . After time has passed , determined by the capacitor discharge process , the circuit returns to its original state.

Thus, the output LE2 a rectangular pulse is generated. Its duration, depending on the time of decrease
before
, is determined by the relation
, Where
– output impedance LE1 in state "1". Circuit recovery time , where
– output impedance LE1 in state "0"; – internal resistance of the diode in the open state.

and the voltage at the inverting input is small:
, Where
voltage drop across the diode in the open state. The voltage at the non-inverting input is also constant:
, and since
, then the output voltage is maintained constant
.

When submitted at the time input pulse of positive polarity amplitude
the voltage at the non-inverting input becomes greater than the voltage at the inverting input and the output voltage suddenly becomes equal to
. At the same time, the voltage at the non-inverting input also increases abruptly to
. At the same time the diode VD closes, capacitor begins to charge and the positive voltage increases at the inverting input (Fig. 6.32). Bye
voltage is maintained at the output
. At a moment in time at
the polarity of the output voltage changes and the voltage at the non-inverting input takes on its original value, and the voltage begins to decrease as the capacitor discharges .

Because
, That
.

The recovery time of the circuit is determined by the duration of the capacitor discharge process from
before
and taking into account the accepted assumptions
.

Generators based on operational amplifiers provide the formation of pulses with an amplitude of up to tens of volts; The duration of the rises depends on the frequency band of the operational amplifier and can be a fraction of a microsecond.

A blocking oscillator is a relaxation-type pulse generator in the form of a single-stage amplifier with positive feedback created using a transformer. The blocking oscillator can operate in standby and self-oscillating modes.

Standby mode blocking-generator When operating in standby mode, the circuit has a single stable state and generates rectangular pulses when trigger pulses are received at the input. The stable state of the blocking oscillator on a germanium transistor is achieved by including a bias source in the base circuit. When using a silicon transistor, no bias source is required because the transistor is closed at zero base voltage (Figure 6.33).

the base voltage decreases. Such a connection is realized by appropriately connecting the beginning of the transformer windings (shown by dots in Fig. 6.33).

In most cases, the transformer has a third (load) winding to which the load is connected .

The voltages on the windings of the transformer and the currents flowing in them are related to each other as follows:
,
,
,
Where
,
– transformation coefficients;
– number of turns of the primary, secondary and load windings, respectively.

The duration of the transistor switching process is so short that during this time the magnetizing current practically does not increase (
). Therefore, the current equation when analyzing the transient process of turning on a transistor is simplified:
.

base current
and the actual current flowing in the base circuit of the transistor,
.

Thus, the initial change in base current
as a result of processes occurring in the circuit, leads to a further change in this current
, and if
, then the process of changing currents and voltages has an avalanche-like character. Consequently, the condition for self-excitation of the blocking oscillator:
.

In the absence of load (
) this condition is simplified:
. Because
, then the self-excitation condition in the blocking generator is satisfied quite easily.

The process of opening the transistor, accompanied by the formation of a pulse front, ends when it goes into saturation mode. In this case, the self-excitation condition ceases to be satisfied and the top of the pulse is subsequently formed. Since the transistor is saturated:
, then voltage is applied to the primary winding of the transformer
and reduced base current
, as well as load current
, turn out to be constant. The magnetizing current during the formation of the pulse apex can be determined from the equation
, from where, under zero initial conditions, we obtain
.

Thus, the magnetizing current in the blocking generator, when the transistor is saturated, increases in time according to a linear law. In accordance with the current equation, the collector current of the transistor also increases according to a linear law
.

Over time, the transistor's saturation level decreases as the base current remains constant.
, and the collector current increases. At some point in time, the collector current increases so much that the transistor switches from saturation mode to active mode and the self-excitation condition of the blocking oscillator begins to be fulfilled again. It is obvious that the duration of the pulse apex is determined by the time during which the transistor is in saturation mode. The boundary of the saturation mode corresponds to the condition
. Hence,
.

From here we get the formula for calculating the duration of the pulse apex:

.

Magnetizing current
during the formation of the top of the pulse, it also increases at the moment of the end of this process, i.e., when
, reaches the value
.

Since the voltage of the power source is applied to the primary winding of the pulse transformer when the top of the pulse is formed , then the amplitude of the pulse on the load
.

When the transistor switches to active mode, the collector current decreases
. A voltage is induced in the secondary winding, leading to a decrease in base voltage and current, which in turn causes a further decrease in the collector current. A regenerative process develops in the circuit, as a result of which the transistor goes into cutoff mode and a pulse cutoff is formed.

The avalanche-like process of closing the transistor has such a short duration that the magnetizing current during this time practically does not change and remains equal
. Consequently, by the time the transistor closes in inductance energy stored
. This energy is dissipated only in the load , since the collector and base circuits of the closed transistor are open. In this case, the magnetizing current decreases exponentially:
, Where
– time constant. Flowing through a resistor the current creates a reverse voltage surge across it, the amplitude of which is
, which is also accompanied by a voltage surge at the base and collector of the closed transistor
. Using the previously found relation for
, we get:

,

.

The process of dissipation of energy stored in a pulse transformer, which determines the recovery time of the circuit , ends after a time interval
, after which the circuit returns to its original state. Additional collector voltage surge
may be significant. Therefore, in the blocking generator circuit, measures are taken to reduce the value
, for which a damping circuit consisting of a diode is connected parallel to the load or in the primary winding VD1 and resistor , whose resistance
(Fig. 6.33). When a pulse is formed, the diode is closed, since a voltage of reverse polarity is applied to it, and the damping circuit does not affect the processes in the circuit. When a voltage surge occurs in the primary winding when the transistor is turned off, a forward voltage is applied to the diode, it opens and current flows through the resistor . Because
, then the collector voltage surge
and reverse voltage surge on are significantly reduced. However, this increases the recovery time:
.

A resistor is not always connected in series with the diode , and then the amplitude of the burst turns out to be minimal, but its duration increases.

impulses. We will consider the processes occurring in the circuit, starting from the moment of time , when the voltage on the capacitor reaches the value
and the transistor will open (Fig. 6.36).

Since the voltage on the secondary (base) winding remains constant during the formation of the top of the pulse
, then as the capacitor charges, the base current decreases exponentially
, Where
– resistance of the base-emitter region of the saturated transistor;
– time constant.

In accordance with the current equation, the collector current of the transistor is determined by the expression
.

From the above relations it follows that in a self-oscillating blocking oscillator, during the formation of the top of the pulse, both the base and collector currents change. As can be seen, the base current decreases over time. The collector current, in principle, can both increase and decrease. It all depends on the relationship between the first two terms of the last expression. But even if the collector current decreases, it is slower than the base current. Therefore, when the base current of the transistor decreases, a moment in time occurs , when the transistor comes out of saturation mode and the process of forming the top of the pulse ends. Thus, the duration of the top of the pulse is determined by the relation
. Then we can write the current equation for the moment of completion of the formation of the top of the pulse:

.

After some transformations we have
. The resulting transcendental equation can be simplified under the condition
. Using the exponential series expansion and limiting ourselves to the first two terms
, we obtain a formula for calculating the duration of the pulse apex
, Where
.

During the formation of the top of the pulse due to the flow of the base current of the transistor, the voltage on the capacitor changes and by the time the transistor closes it becomes equal
. Substituting the value into this expression
and integrating, we get:

.

When the transistor switches to the active operating mode, the self-excitation condition begins to be fulfilled again and an avalanche-like process of its closing occurs in the circuit. As in the standby blocking generator, after the transistor is closed, the process of dissipation of the energy stored in the transformer occurs, accompanied by the appearance of surges in the collector and base voltages. After this process is completed, the transistor continues to be in the off state due to the fact that the negative voltage of the charged capacitor is applied to the base . This voltage does not remain constant, since in the closed state of the transistor through the capacitor and resistor recharge current flows from the power source . Therefore, as the capacitor recharges the voltage at the base of the transistor increases exponentially
, Where
.

When the base voltage reaches
, the transistor opens and the pulse formation process begins again. Thus, the duration of the pause , determined by the time the transistor is in the off state, can be calculated if we put
. Then we get
.For a blocking oscillator on a germanium transistor, the resulting formula is simplified, since
.

Blocking generators have a high efficiency, since practically no current is consumed from the power source during the pause between pulses. Compared to multivibrators and monovibrators, they allow you to obtain a higher duty cycle and shorter pulse duration. An important advantage of blocking generators is the ability to obtain pulses whose amplitude is greater than the power source voltage. To do this, it is enough that the transformation ratio of the third (load) winding
. In a blocking generator, if there are several load windings, it is possible to carry out galvanic isolation between the loads and receive pulses of different polarities.

The blocking oscillator circuit is not implemented in an integrated design due to the presence of a pulse transformer.

Stable Square Pulse Generator

Clock generators (GTIs) are a kind of master mechanisms in most complex digital circuits. At the output of the GTI, electrical pulses repeating at a certain frequency are formed. Most often they have a rectangular shape. Based on these oscillations, the operation of all digital chips included in the device is synchronized. In one clock cycle, one atomic operation is performed (that is, indivisible, one that cannot be performed or not partially performed).

Voltage pulses can be generated with varying degrees of accuracy and stability. But the more demanding the circuit is regarding the reference frequency, the more accurate and stable the generator should be.

The most common:

1.Classical (analog) generators. They are easy to assemble, but have low stability or generate pulses that are not quite square. As a simple example, LC circuits or circuits based on them.

2. Quartz (based on quartz crystals). Here quartz acts as a highly selective filter. The circuit is characterized by a high degree of stability and ease of assembly.

3.Based on programmable chips (such as Arduino). The solutions also generate stable pulses, but unlike quartz ones, they can be controlled in specified ranges and generate several reference frequencies at once.

4. Autogenerators. These are controlled GTIs, working primarily with modern processors, and are most often integrated directly into the chip.

Thus, the following are suitable for the role of stable rectangular pulse generators in circuit design:

• Quartz
• And programmable (based on programmable chips).

Separately, it is worth mentioning the circuits of classical single- and multivibrators operating using logic elements. This class of GTI can definitely be used in digital circuits, since it is capable of generating a stable frequency.

High stability crystal oscillator

One of the implementation examples.

Rice. 1. Crystal oscillator circuit

The circuit is based on a quartz resonator and a CMOS inverter based on the principle of a Pierce oscillator.

Capacitors of increased capacity Ca and Cb are responsible for increasing stability.

Multivibrators based on logic elements

The simplest multivibrator circuit looks like this.

Rice. 2. Multivibrator circuit

In fact, this is an oscillatory circuit based on capacitors and resistances. Logic elements make it possible to cut off smooth edges of increasing and decreasing voltage when charging/discharging a capacitor in an oscillatory circuit.

The stress generation graph will look like this.

Rice. 3. Stress generation graph

Capacitor C1 is responsible for the pulse duration, and C2 is responsible for the pause between pulses. The slope of the edge depends on the reaction time of the logic element.

The indicated circuit has one drawback - a self-excitation mode is possible.

To eliminate this effect, another additional logical element is used (see diagram below - LE3).

Rice. 4. C multivibrator circuit

Operational amplifier generators

The same oscillatory circuit, but with op-amp integration, will look like this.

Rice. 5. Oscillatory circuit diagram

Rice. 6. Graph of pulse formation at its output

The circuit mentioned above generates pulses whose duration is equal to the pause time, which does not always have to be the case.

You can introduce asymmetry into the generation frequency as follows.

Rice. 7. Pulse generator circuit

Here, the time of the pulses and the pauses between them are determined by the different resistor values.

Generator based on NE555

The NE555 chip is a universal timer that can operate in multi- or one-shot mode.

There are many analogues of this microcircuit: 1006VI1, UPC617C, ICM7555, etc.

One of the simple options for constructing generators of stable rectangular pulses with the ability to adjust the frequency can be seen below.

Rice. 8. Variant of the stable rectangular pulse generator circuit

Here, the circuit includes various capacitors (C1, C2, C3, there may be more of them), and trimming resistors (R2, R3, and R4 is responsible for the output current level).

The frequency calculation formula is as follows.

We will look at the Arduino-based generator in a separate article.

Publication date: 07.01.2018

Readers' opinions

• Vitaly / 11/23/2018 - 17:11
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Pulse generators are devices that are capable of creating waves of a certain shape. The clock frequency in this case depends on many factors. The main purpose of generators is considered to be the synchronization of processes in electrical appliances. Thus, the user has the opportunity to configure various digital equipment.

Examples include clocks and timers. The main element of devices of this type is considered to be an adapter. Additionally, capacitors and resistors along with diodes are installed in the generators. The main parameters of devices include the indicator of excitation of oscillations and negative resistance.

Generators with inverters

You can make a pulse generator with your own hands using inverters at home. To do this, you will need a capacitorless adapter. It is best to use field resistors. Their impulse transmission parameter is at a fairly high level. Capacitors for the device must be selected based on the power of the adapter. If its output voltage is 2 V, then the minimum should be at 4 pF. Additionally, it is important to monitor the negative resistance parameter. On average, it must fluctuate around 8 ohms.

Rectangular pulse model with regulator

Today, a rectangular pulse generator with regulators is quite common. In order for the user to be able to adjust the maximum frequency of the device, it is necessary to use a modulator. Manufacturers present them on the market in rotary and push-button types. In this case, it is best to go with the first option. All this will allow you to fine-tune the settings and not be afraid of a failure in the system.

The modulator is installed in the square pulse generator directly on the adapter. In this case, soldering must be done very carefully. First of all, you should thoroughly clean all contacts. If we consider capacitorless adapters, their outputs are on the top side. Additionally, there are analog adapters, which are often available with a protective cover. In this situation, it must be removed.

In order for the device to have high throughput, resistors must be installed in pairs. The oscillation excitation parameter in this case must be at the level. As the main problem, the rectangular pulse generator (the diagram is shown below) has a sharp increase in operating temperature. In this case, you should check the negative resistance of the capacitorless adapter.

Overlapping pulse generator

To make a pulse generator with your own hands, it is best to use an analog adapter. In this case, it is not necessary to use regulators. This is due to the fact that the level of negative resistance can exceed 5 ohms. As a result, the resistors are subject to quite a large load. Capacitors for the device are selected with a capacity of at least 4 ohms. In turn, the adapter is connected to them only by output contacts. The main problem with the pulse generator is the asymmetry of oscillations, which occurs due to overloading of the resistors.

Symmetrical pulse device

It is possible to make a simple pulse generator of this type only using inverters. In such a situation, it is best to select an adapter of the analog type. It costs much less on the market than the capacitorless modification. Additionally, it is important to pay attention to the type of resistors. Many experts advise choosing quartz models for the generator. However, their throughput is quite low. As a result, the oscillation excitation parameter will never exceed 4 ms. Plus, there is the risk of the adapter overheating.

Considering all of the above, it is more advisable to use field-effect resistors. in this case it will depend on their location on the board. If you choose the option when they are installed in front of the adapter, in this case the excitation rate of oscillations can reach up to 5 ms. In the opposite situation, you can’t count on good results. You can check the operation of the pulse generator by simply connecting a 20 V power supply. As a result, the level of negative resistance should be around 3 ohms.

To keep the risk of overheating to a minimum, it is additionally important to use only capacitive capacitors. The regulator can be installed in such a device. If we consider rotary modifications, then the modulator of the PPR2 series is suitable as an option. According to its characteristics, it is quite reliable today.

Generator with trigger

A trigger is a device that is responsible for transmitting a signal. Today they are sold unidirectional or bidirectional. For the generator, only the first option is suitable. The above element is installed near the adapter. In this case, soldering should be done only after thoroughly cleaning all contacts.

You can even choose an analog adapter directly. The load in this case will be small, and the level of negative resistance with successful assembly will not exceed 5 Ohms. The parameter for excitation of oscillations with a trigger is on average 5 ms. The main problem the pulse generator has is this: increased sensitivity. As a result, these devices are not capable of operating with a power supply higher than 20 V.

increased load?

Let's pay attention to the microcircuits. Pulse generators of this type imply the use of a powerful inductor. Additionally, only an analog adapter should be selected. In this case, it is necessary to achieve high system throughput. For this purpose, capacitors are used only of the capacitive type. They must be able to withstand at least 5 ohms of negative resistance.

A wide variety of resistors are suitable for the device. If you choose them of a closed type, then it is necessary to provide for them a separate contact. If you still stop at field resistors, then the phase change in this case will take quite a long time. Thyristors are practically useless for such devices.

Models with quartz stabilization

The pulse generator circuit of this type provides for the use of only a capacitorless adapter. All this is necessary so that the oscillation excitation index is at least at the level of 4 ms. All this will also reduce thermal losses. Capacitors for the device are selected based on the level of negative resistance. Additionally, the type of power supply must be taken into account. If we consider pulse models, then their output current level is on average at around 30 V. All this can ultimately lead to overheating of the capacitors.

To avoid such problems, many experts advise installing zener diodes. They are soldered directly onto the adapter. To do this, you need to clean all contacts and check the cathode voltage. Auxiliary adapters for such generators are also used. In this situation they play the role of a dial-up transceiver. As a result, the oscillation excitation parameter increases to 6 ms.

Generators with capacitors PP2

The generator of high-voltage pulses with capacitors of this type is formed quite simply. Finding elements for such devices on the market is not a problem. However, it is important to choose a high-quality microcircuit. Many people purchase multi-channel modifications for this purpose. However, they are quite expensive in the store compared to regular types.

Transistors for generators are most suitable unijunction ones. In this case, the negative resistance parameter should not exceed 7 ohms. In such a situation, one can hope for the stability of the system. To increase the sensitivity of the device, many advise using zener diodes. However, triggers are used extremely rarely. This is due to the fact that the throughput of the model is significantly reduced. The main problem of capacitors is considered to be the amplification of the limiting frequency.

As a result, the phase change occurs with a large margin. To set up the process properly, you must first configure the adapter. If the negative resistance level is at 5 ohms, then the maximum frequency of the device should be approximately 40 Hz. As a result, the load on the resistors is removed.

Models with PP5 capacitors

A high-voltage pulse generator with the specified capacitors can be found quite often. Moreover, it can be used even with 15 V power supplies. Its throughput depends on the type of adapter. In this case, it is important to decide on resistors. If you select field models, then it is more advisable to install the adapter of the capacitorless type. In this case, the negative resistance parameter will be around 3 ohms.

Zener diodes are used quite often in this case. This is due to a sharp decrease in the level of the limiting frequency. In order to level it, zener diodes are ideal. They are usually installed near the output port. In turn, it is best to solder resistors near the adapter. The indicator of oscillatory excitation depends on the capacitance of the capacitors. Considering 3 pF models, note that the above parameter will never exceed 6 ms.

Main generator problems

The main problem with devices with PP5 capacitors is considered to be increased sensitivity. At the same time, thermal indicators are also at a low level. Due to this, there is often a need to use a trigger. However, in this case it is still necessary to measure the output voltage. If it exceeds 15 V with a block of 20 V, then the trigger can significantly improve the operation of the system.

Devices on MKM25 regulators

The pulse generator circuit with this regulator includes only closed-type resistors. In this case, microcircuits can even be used in the PPR1 series. In this case, only two capacitors are required. The level of negative resistance directly depends on the conductivity of the elements. If the capacitor capacitance is less than 4 pF, then the negative resistance can even increase to 5 ohms.

To solve this problem, it is necessary to use zener diodes. In this case, the regulator is installed on the pulse generator near the analog adapter. The output contacts must be thoroughly cleaned. You should also check the threshold voltage of the cathode itself. If it exceeds 5 V, then an adjustable pulse generator can be connected to two contacts.

The 555 integrated timer chip was developed 44 years ago, in 1971, and is still popular today. Perhaps no microcircuit has served people for so long. They collected everything on it, they even say that number 555 is the number of options for its application :) One of the classic applications of the 555 timer is an adjustable rectangular pulse generator.
This review will describe the generator, specific application will be next time.

The board was sent sealed in an antistatic bag, but the microcircuit is very wooden and static cannot easily kill it.


Mounting quality is normal, the flux is not washed




The generator circuit is standard for obtaining a duty cycle of pulses ≤2

The red LED is connected to the output of the generator and blinks at a low output frequency.
According to Chinese tradition, the manufacturer forgot to put a limiting resistor in series with the upper trimmer. According to the specification, it must be at least 1 kOhm so as not to overload the internal switch of the microcircuit, however, in reality the circuit works with lower resistance - up to 200 Ohms, at which generation fails. Adding a limiting resistor to the board is difficult due to the layout of the printed circuit board.
The operating frequency range is selected by installing a jumper in one of four positions
The seller indicated the frequencies incorrectly.


Really measured generator frequencies at a supply voltage of 12V
1 - from 0.5Hz to 50Hz
2 - from 35Hz to 3.5kHz
3 - from 650Hz to 65kHz
4 - from 50kHz to 600kHz

The lower resistor (according to the diagram) sets the pulse pause duration, the upper resistor sets the pulse repetition period.
Supply voltage 4.5-16V, maximum output load - 200mA

The stability of output pulses in ranges 2 and 3 is low due to the use of capacitors made of ferroelectric ceramics of the Y5V type - the frequency creeps away not only when the temperature changes, but even when the supply voltage changes (by several times). I didn’t draw any graphs, just take my word for it.
On other ranges the pulse stability is acceptable.

This is what it produces on range 1
At maximum resistance of trimmers


In meander mode (upper 300 Ohm, lower at maximum)


In maximum frequency mode (upper 300 ohms, lower to minimum)


In the minimum pulse duty cycle mode (upper trimmer at maximum, lower at minimum)

For Chinese manufacturers: add a 300-390 Ohm limiting resistor, replace the 6.8uF ceramic capacitor with a 2.2uF/50V electrolytic capacitor, and replace the 0.1uF Y5V capacitor with a higher quality 47nF X5R (X7R)
Here is the finished modified diagram


I didn’t modify the generator myself, because... These disadvantages are not critical for my application.

Conclusion: the usefulness of the device becomes clear when any of your homemade products require pulses to be sent to it :)
To be continued…