Schemes of saw generators on tunnel diodes. Applications of tunnel diodes

A tunnel diode is a special diode whose characteristics differ from those of any ordinary diode or zener diode.

Both a conventional diode and a zener diode are very good conductors when forward biased, but neither of them conducts current well in a reverse biased state (the exception is the breakdown region). But in the material of the tunnel diode there are additives in a much larger volume than in a conventional diode, and its P-N junction is very narrow. The tunnel diode, due to the fact that it has a large number of additives and a very narrow P-N junction, conducts current exceptionally well in both directions.

The principle of operation of the tunnel diode

The potential required to make a tunnel diode act as a conductor, whether in forward or reverse bias mode, is very small, typically in the millivolt range. This is why tunnel diodes are known as low resistance devices. They very little oppose the movement of current in the circuit.

The most unique feature of tunnel diodes is their voltage-to-current ratio when they are forward biased. When the tunnel diode is forward biased (from point A to point B on the graph) as the voltage increases, the current also rises to a certain amount. Once this value is reached, a further increase in forward bias voltage causes the current to decrease to a minimum value (from point B to point C). In the area that is on the graph between the maximum and minimum current flows, the tunnel diode has a negative resistance. In this region of negative resistance, the current flowing through the tunnel diode actually decreases as the voltage increases. The exact opposite of the usual voltage-to-current ratio occurs. However, when the voltage beyond point C rises, this instrument exhibits the normal voltage/current relationship.

Under normal conditions, tunnel diodes operate in their negative resistance region. In this area, a slight decrease in voltage turns on this device, and a small increase turns it off. As such a kind of switch, the tunnel diode can be used either as a generator or as a high-speed switch: a specific feature of the device, low resistance, allows an almost instantaneous change in internal resistance. Tunnel diodes can also be used as amplifiers, where upward changes in applied voltage cause proportionately larger current changes in the circuit.

Semiconductor diodes are rarely used as the main elements of generator and amplifying units. Being for the most part purely passive components, they simply cannot act as a source of current or voltage required for any oscillator or amplifier. However, there are quite a few cases where, when using semiconductor diodes of certain types (tunnel diodes, Gunn diodes, avalanche-span diodes, parametric diodes), it is possible to build diode amplifying and generator circuits.

Such semiconductor devices as: tunnel diodes, Gunn diodes, avalanche-span diodes have one property in common - the presence on the CVC of the device under certain conditions of a section with negative differential resistance. In each of these devices, the physical effects that cause the appearance of such a section are different. In a tunnel diode, this is a sharp drop in the tunnel effect with an increase in the electric field strength in a semiconductor above a certain critical value, in a Gunn diode, it is a feature of the band structure of gallium arsenide, in an avalanche-span diode, it is the specificity of avalanche breakdown at high frequencies of the applied voltage. It should be noted that these cases are not the only ones. An example is the widely known and popular in the 30s. kristadin Loseva, also representing a semiconductor diode introduced into a special breakdown mode.

Today, diode self-oscillators of the microwave range are most widely used. They use Gunn diodes and avalanche transit diodes. Under certain conditions, such generators can be converted into amplifiers and used for resonant amplification of microwave signals. However, due to the increased noise level and practical irrationality, amplifiers based on Gunn diodes and avalanche-transit diodes are used extremely rarely.

A special type of amplifying devices in the microwave range is the so-called. parametric amplifiers. They are built on the basis of special parametric diodes. The principle of operation of such amplifiers is very close to how the diode mixers described above work. Two signals are applied to the parametric diode, as well as in mixers. With a certain coordination of these signals and the correct choice of the operating mode of the diode, it is possible to redistribute the power of the incident signals in favor of one of them (useful) on the nonlinear conductivity or capacitance of the diode. At the same time, the frequency conversion of this signal is also possible. Microwave parametric amplifiers are very difficult to tune and quite unstable. Their main advantage is a uniquely low noise level. Therefore, they are most often used in radio telescopes and deep space communication systems.

Of greatest interest and practical value may be tunnel diodes. Generator and amplifying devices based on them can be used in radio receivers, radio microphones, measuring equipment, etc.

A simplified diagram of a self-oscillator based on a tunnel diode is shown in fig. 3.6-42.

Rice. 3.6-42. Simplified diagram of a self-oscillator based on a tunnel diode

Since there is a voltage-stable negative resistance section on the CVC of the tunnel diode, when a parallel oscillatory circuit is connected to it, it can generate. In this case, the negative resistance of the diode will compensate for the losses, and undamped oscillations can arise and be maintained in the circuit. Modern tunnel diodes can generate at frequencies up to 1 GHz or more. However, due to the small size of the CVC section of a diode with negative resistance, the power given by it at any frequency is a fraction of a milliwatt. So that the shape of the generated oscillations is not distorted, as a rule, a partial inclusion of the diode in the generator circuit is used. The main condition for generation is the excess of the loop loss resistance value over the negative resistance value of the tunnel diode. Taking into account that the parallel loss resistance in real oscillatory circuits significantly exceeds the negative resistance of the tunnel diode, a partial inclusion of the diode in the circuit (through the coil tap) is used.

Part of the power of the generated oscillations will be released on the internal resistance of the bias source, so it should be as small as possible. Since the required bias voltage is very small (for example, for germanium tunnel diodes of the order of 0.1 ... 0.15 V), the tunnel diodes are usually powered from a voltage divider (Fig. 3.6-43). However, this can lead to wasteful power consumption of the power supply (which is important for subminiature devices). Therefore, to power tunnel diodes, sources with the lowest possible output voltage should be used. The output impedance of the voltage divider is chosen within 5 ... 10 ohms, and only in devices where the greatest efficiency is required, it can be increased to 20 ... 30 ohms. The negative resistance of the tunnel diode should exceed the resistance of the divider by 5...10 times. It is not advisable to shunt such small resistances with capacitors to reduce high-frequency energy losses, since in some cases this can lead to unstable operation of the generator, especially if its mode was selected according to the maximum output power. It should be borne in mind that for stable operation of the generator, it is necessary to maintain a stable position of the operating point of the diode. If the supply voltage changes by at least 10% (for example, due to the discharge of a chemical battery), the normal operation of the generator may be disturbed. Sometimes it is advisable to use a pre-stabilized voltage or use non-linear resistances in the divider (stabilizing current in the upper arm, and voltage in the lower arm). So, if in the oscillator circuit (Fig. 3.6-43) instead of the resistance R2, a low-power germanium diode is used in direct connection, as shown in fig. 3.6-44, the stability of the generator will improve, and when the supply voltage changes within 1 ... 1.5 V, no additional adjustments are required.

Rice. 3.6-43. Scheme of a self-oscillator on a tunnel diode powered by a voltage divider

Rice. 3.6-44. Scheme of a self-oscillator based on a tunnel diode with a non-linear resistance in the power circuit

All the above methods of voltage stabilization somewhat complicate the circuits, and in some cases increase the power consumption, so they are not widely used. In real equipment, tunnel diodes are most often used in conjunction with transistors. It is known that in a transistor, the emitter current depends relatively little on the collector supply voltage, especially if the transistor bias is stabilized in some way. Therefore, when the diode is powered by the emitter current of the transistor, it is possible to obtain a gain not only in stability, but also in efficiency. The latter increases here due to the fact that the losses on the upper arm of the divider are eliminated, and the additional power consumed by the tunnel diode is small.

On fig. 3.6-45, 3.6-46, 3.6-47 three examples of the application of a tunnel diode oscillator are presented. When designing such generators, one should strive to obtain the maximum quality factor of the oscillatory circuit in order to increase the power delivered to the load.

Rice. 3.6-45. The simplest tunnel diode transmitter

Rice. 3.6-46. Improved Tunnel Diode Transmitter Circuit

Rice. 3.6-47. Local oscillator on a tunnel diode

To increase the power, you can also include two or more diodes in the generator circuit (Fig. 3.6-48). In this case, the diodes are best connected in series with direct current. Then the voltage at the lower resistance of the divider should be twice as much as for one tunnel diode, i.e. losses on the upper arm are reduced. It must be borne in mind that the resistance of the lower arm must necessarily consist of two identical resistances, and their midpoint must be connected by direct current to the midpoint of the two diodes. Otherwise, stable operation of two diodes connected in series is impossible. For alternating current, diodes can be connected in parallel or in series. In the diagram shown in fig. 3.6-48 each diode is connected to a separate winding. To get the most power, the connection of each diode to the circuit should be adjusted individually.

Rice. 3.6-48. Self-oscillator on two tunnel diodes

A tunnel diode generator can also be built using a quartz resonator that sets the oscillation frequency. An example of such a scheme is shown in Fig. 3.6-49.

Historically, tunnel diodes appeared much later than transistors and tubes. Small dimensions and weight, high reliability and cost-effectiveness led to a rapid expansion of their scope. Current-voltage characteristic of a tunnel diode - type N(Fig. 7). Therefore, the oscillator circuit is obtained simply: a parallel alternating current circuit is connected to the diode (Fig. 8.44 b), and the direct current mode is chosen so that the operating point O is in the falling section of the characteristic (Fig. 7).

Fig.7. Volt-ampere characteristic and tunnel diode oscillator circuit

The DC mode must be provided taking into account the internal resistance of the source R i. To do this, it is necessary to solve a system of two equations:

The graphic solution of the system is shown in Figure 8.44 a.

Let's consider two cases.

In the first case, with the slope of the characteristic | S(U 0)| > 1/R i, there are three possible states that satisfy the equations of the system - points A, O, B. Analysis, taking into account the capacitance of the diode itself, shows that only points A and B, located on the growing sections of the characteristic, are stable. If the rest point (point O) is located on the section of the characteristic with a negative slope, then the state of the circuit will be unstable and the operating point will spontaneously shift to one of the extreme positions (to point A or point B).

In the second case, when the slope of the characteristic | S(U 0)| < 1/R i, there is only one state that satisfies the equations - point O. It turns out to be stable and therefore the operating point can be set in any section of the current-voltage characteristic with a negative slope, therefore, the self-excitation phase condition is satisfied. The amplitude condition of self-excitation will be satisfied if | S(U 0)| > G Eh, where G E - the conductivity of the circuit at the points of connection of the diode.

The oscillation frequency is

and can be changed with WITH K. The oscillation amplitude is changed by changing the point of connection of the diode to the oscillatory circuit. If coils L 1 and L 2 are not connected by a single magnetic field, then the turn-on coefficient of the circuit is equal to

If the coils L 1 and L 2 form a single coil with a common magnetic field, then the diode is connected to the inductive branch with a switching coefficient equal to

Where n 1 and n 2 - the number of turns in the parts of the coil indicated in the diagram L 1 and L 2 .

Block capacity WITH B is selected from the condition

Advantages of the scheme:

the ability to operate in a very wide frequency range (from units of kilohertz to tens of gigahertz);

high stability of parameters when the temperature changes over a wide range;

low level of self-noise;

low energy consumption from power sources;

long service life;

low sensitivity to radiation.

The disadvantage of the circuit is the low output power, which is due to the small intervals of currents and voltages within the falling section of the characteristic (with a negative slope). For example, a generator based on a single tunnel diode with a peak current of up to 10 mA provides power not exceeding a few milliwatts. To obtain more power, it is necessary to use diodes with high peak currents.

On the Internet today you can find a huge number of circuits for radio transmitting devices. These compact transmitters in the common people were called a bug, a device for wiretapping.

Basically, well-known bug designs are repeated by a novice amateur, but without some experience, it is very difficult to assemble and test a professional microphone, since bugs are quite difficult to tune. In other words, at home it is very difficult to assemble a stable bug with a range of more than 200 meters, unless of course the device does not work on the satellite band.

Today we will consider the design of a radio bug, where a tunnel diode is used as a generator.

There are not very many designs of such bugs on the Internet, there are only a couple of bug circuits on tunnel diodes, let's break the tradition of standard circuits and consider a new version of the transmitter structure without transistors!

The above is not entirely true, because after testing the circuit, it became clear that one still cannot do without a transistor, however, here the transistor is used only to amplify the signal from the microphone.

The circuit is wound on a plastic frame with a diameter of 5 mm, contains 7 turns (for the FM band), a wire with a diameter of 0.6-1 mm.

The range of the bug is small, only 20-30 meters, and even then with fine tuning. The generator starts to work even when the voltage is 0.5-0.6 volts, the standard voltage is -1.5 volts, you should not apply higher. The current consumption is only 1.5-2 mA! One AA battery can last for half a year.
The bug is only suitable for "close fights", to monitor the neighboring house, etc., it will be your third ear.

Despite the rather simple design, stability at a fairly high level, the frequency alloy was almost not observed during the experiments.
The sensitivity of the microphone is up to 4 meters, the microphone itself is used from a mobile phone headset.

The main parameters of the bug:
Supply voltage 0.5...2 Volt
Range - 30 meters
Operating frequency - 88-108 MHz

A transistor with a 520 ohm resistor in the collector circuit forms a voltage divider, its operating point is set by a trimming resistor of 68 k, the resistor is adjusted so that the voltage on the transistor collector is 0.2-0.3 volts, thus providing a normal voltage to power the generator, this is the second purpose of the transistor.

Antenna - a piece of stranded wire with a length of 20 cm, with the exception of the latter, the range of the bug drops to 5-6 meters.

Tunnel diode can be used type AI201/301 or from imported interior - 1N3713

The main advantage is the compact dimensions of the device and low-voltage power supply; when assembled on smd components, the entire structure can be placed in a raincoat button.

List of radio elements

The simplest way is to build self-oscillator circuits using tunnel diodes. Since the tunnel diode is a two-terminal device with a negative resistance, stable in voltage, when a parallel oscillatory circuit is connected to it, it can generate. In this case, the negative resistance of the diode will compensate for the losses, and undamped oscillations can arise and be maintained in the circuit. Ordinary low-frequency tunnel diodes work well at frequencies equal to units of megahertz.

Higher frequency diodes, in which the junction capacitance and lead inductance are reduced, generate at frequencies of thousands of megahertz. However, due to the small values ​​of the section of the current-voltage characteristic of a diode with negative resistance, the power given by it at any frequency is fractions of a mW. So that the shape of the generated oscillations is not distorted, as a rule, a partial connection of the diode to the generator circuit is used. In this case, the loss resistance given to the diode terminals must be equal to its negative resistance. In real circuits, the reduced loss resistance is chosen to be greater than the negative one. resistance of the tunnel diode in order to guarantee reliable excitation of the generator with changes in temperature, supply voltage and frequency.

Considering that the parallel loss resistance in real oscillatory circuits significantly exceeds the resistance of the tunnel diode, the tap must be made from a small part of the circuit turns (Fig. 1). Part of the vibrational power will be released on the internal resistance of the bias source, so it should be as small as possible.

Rice. 1

Typically, tunnel diodes are powered by a voltage divider, which wastes power. Indeed, for germanium diodes, the bias voltage in the generation mode is 0.1-0.15 V, and the minimum voltage of the vast majority of chemical current sources is 1.2-2 V, which is why it is necessary to use voltage dividers in the power circuit. In this case, approximately 80-90% of the total power consumption is dissipated in the divider. For reasons of economy, it is advisable to use sources with the lowest possible voltage to power tunnel diodes. The output resistance of the voltage divider is chosen in the range of 5-10 ohms, and only in devices where the greatest efficiency is required, it is increased to 20-30 ohms. The negative resistance of the tunnel diode should exceed the resistance of the divider by 5-10 times. It is not advisable to shunt such small resistances with capacitors to reduce high-frequency energy losses, since in some cases this can lead to unstable operation of the generator, especially if its mode was selected according to the maximum output power.

The negative resistance of the tunnel diode is highly dependent on the position of the operating point, so that if the supply voltage changes by 10%, the normal operation of the generator can be completely disrupted. Therefore, when powering diodes from chemical current sources - batteries, accumulators, it is very difficult to ensure their stable operation. It is most advisable to power them from mercury oxide elements, the voltage of which changes slightly during operation, and in some cases it is necessary to use a pre-stabilized voltage or use non-linear resistances in the divider - in the upper arm, stabilizing the current, and in the lower arm - voltage. So, if in the oscillator circuit (Fig. 2, a) instead of the resistance R2, a germanium diode D11 is used in direct connection, as shown in fig. 2, b, the stability of the generator will improve and when the supply voltage changes from 1.5 to 1 V, no adjustments are required.

Rice. 2

In the above diagrams of self-oscillators at a frequency of 465 kHz, the L1 coil is wound on a 4-section polystyrene frame with a diameter of 4 mm with an F-1000 ferrite core with a diameter of 2.8 and a length of 12 mm. The coil winding contains 220 turns of PEV 0.13 wire with a tap from 18 turns. The high frequency voltage on the circuit is 1 Veff.

All the stabilization methods mentioned above somewhat complicate the circuits, and in some cases increase the power consumption, so they have not found wide application. In equipment, tunnel diodes are most often used in conjunction with transistors. It is known that in a transistor, the emitter current depends relatively little on the collector supply voltage, especially if the transistor bias is stabilized in some way. Therefore, when feeding the diodes with the emitter current of the transistor, you can get a gain not only in stability, but also in efficiency. The latter increases here due to the fact that the losses on the upper arm of the divider are eliminated, and the additional power consumed by the tunnel diode is small.

In addition to generators tuned to a fixed frequency, tunnel diodes can also be used in range generators. True, in this case, it is necessary to more carefully select the connection between the diode and the circuit in order to maintain the amplitude of the oscillation and the power in the load at a given level in the entire overlapped range. An example of such a use of a tunnel diode is the local oscillator circuit for a superheterodyne receiver, described in the Radio magazine No. 5, 1962. The local oscillator circuit is obtained in this case even simpler than on a transistor (Fig. 3).

Rice. 3

The total number of turns in the coil L1 is preserved, and for connection with the tunnel "diode, a winding L2 is wound on top of L1 from the side of its grounded end, containing 10 turns of PELSHO 0.15 wire. The connection winding with the L3 converter remains approximately the same, but for the greatest sensitivity, the number of turns The capacitances of capacitors C1 and C2 remain unchanged, The tunnel diode is powered from a common source.In this case, the resistance R2 should be equal to 1.2 kΩ.The tunnel diode should be selected with a maximum current of no more than 1.5 mA. It is more rational for to power the diode, apply the stabilization circuit mentioned above with the help of a transistor.For this, the LF amplifier is redone according to the circuit shown in Fig. 4. A DC connection is introduced between the transistors of the LF amplifier.The bias to the base of the transistor T1 is removed from the emitter of the transistor T2 through the R4D1 circuit, and resistances R2, R3. The resulting negative current feedback maintains the emitter current, and hence the voltage across the resistances R2 and R3, almost constant when the supply voltage is reduced by 25-30% of the nominal value (it is better to increase the supply voltage to 9 B).

rice. 4

To power the tunnel diode, a voltage of 2 V is used, supplied to the divider through the resistance R2 (Fig. 3), which in this case is taken to be 430 ohms. The adjustment begins with checking how the voltage at the emitter of the transistor T2 changes when the supply voltage decreases from 6 to 4.5 V or from 9 to 6 V. If the voltage changes by no more than 5-10%, then setting the supply voltage equal to 5.2 V (or 7.5 V at 9 V), go to the generator setting. To do this, the rotor of the variable capacitor C2 is placed in the middle position and, by adjusting the resistance values ​​R1 or R2 (Fig. 3), the maximum oscillation amplitude is achieved. Then check the uniformity of generation over the entire range. If oscillations fail in any of its sections, the winding of the L2 coil should be increased by several turns and the uniformity of generation should be checked again during the restructuring. Having finished tuning the local oscillator, the number of turns of the local oscillator connection winding with the L3 converter is selected until the optimal sensitivity is obtained.

When designing generators based on tunnel diodes, one should strive to obtain the maximum quality factor of the oscillatory circuit in order to increase the power delivered to the load. To increase power, you can also include two or more diodes in the oscillator circuit. In this case, as follows from the consideration of the energy ratios, it is advantageous to connect the diodes in series with direct current. Then the voltage at the lower resistance of the divider will be twice as high as for one tunnel diode, and the losses on the upper arm will decrease. It must be borne in mind that the resistance of the lower arm must necessarily consist of two identical resistances, and their midpoint must be connected by direct current to the midpoint of two diodes (Fig. 5). Otherwise, stable operation of two diodes connected in series is impossible. For alternating current, diodes can be connected in parallel or in series. In the diagram shown in fig. 5 each diode is connected to a separate winding. To get the most power, the coupling of each tunnel diode to the loop must be adjusted individually.

rice. 5

Tunnel diodes can also be used in aperiodic amplifier circuits. However, as indicated in the literature, such aperiodic amplifiers in the ranges of long and medium waves are not very practical due to the difficulty in separating the load and the signal source. It should also be taken into account that transistors, with comparable power consumption, have a large gain in real circuits compared to tunnel diodes.

Resonant tunnel diode amplifiers are relatively easy to build. They can be performed, for example, according to the oscillator circuit, in which the feedback coefficient is insufficient to excite oscillations. Such circuits have all the disadvantages of regenerative amplifiers: instability of the regeneration threshold, the possibility of excitation when the load changes, narrowing the bandwidth with increasing gain. However, such amplifiers can work quite stably if you do not strive to get maximum gain from them. A circuit with this use of a tunnel diode is shown in fig. 6. The figure shows a diagram of the input part of a direct gain receiver with a ferrite antenna. It is known that in order to match the resistance of the antenna circuit with the input resistance of the transistor, the transformation ratio of the transformer formed by the windings of the coils L1 and L2 is much less than unity.

Rice. 6. The top plate of capacitor C1 must be grounded.

This leads to the fact that the signal voltage at the base of the transistor turns out to be 15-20 times less than the voltage on the L1C1 circuit. In the diagram shown in Fig. 6, the coupling coefficient is chosen much more than usual and the tap to the base of the transistor T1 is made from 1/5 of the total number of turns of the coil L1. In this case, the L1C1 circuit turns out to be heavily shunted, its band expands and the receiver sensitivity drops. However, when the tunnel diode is connected to the additional winding L3, the circuit is partially “unloaded”, its attenuation and bandwidth return to normal. In this way, it is possible to obtain a gain in the sensitivity of the receiver by a factor of 4-5. The number of turns of the winding L3 is chosen so that the attenuation of the circuit is not fully compensated, and the amplifier is not excited. However, to get maximum sensitivity, you need to get as close to the drive threshold as possible, so the bias of the tunnel diode is made adjustable. The winding of the L1 coil contains 200 turns of PELSHO 0.15 wire, wound in one layer turn to turn on a ferrite rod 110 mm long, 8.4 mm in diameter with a tap from 44 turns. The winding of the L3 coil contains 8-10 turns of PELSHO 0.15 wire, it is wound near the grounded end of the L1 coil. The disadvantage of the proposed scheme is that the input circuit overlap coefficient decreases, since due to the increased coupling coefficient, the input capacitance of the transistor T1 will have a stronger effect. In addition, the recalculated capacitance of the tunnel diode will be added to the capacitance of the circuit. Therefore, if a sufficiently large overlap is required, it is advisable to use a tunnel diode with a minimum capacitance.

It is more advantageous to use regenerative amplifiers for a fixed frequency, for example, in a superheterodyne IF amplifier (Fig. 7). To do this, an additional winding for a tunnel diode is wound on one of the IF circuits. It is better to make the bias of the diode stabilized. This will allow you to get close enough to the regeneration threshold and get a 8-10 times gain in gain. It must be taken into account that the bandwidth of the IF amplifier narrows sharply if the inclusion of the tunnel diode was not foreseen in advance. In some cases, when a diode is connected, the amplifier may be excited, although the coupling coefficient is insufficient for generation. This is because the gain of the cascade with the tunnel diode connected becomes greater than the maximum stable value.

rice. 7

When experimenting with tunnel diodes, current and voltage surges must be avoided, otherwise the diode may fail. Connect and disconnect the diode only when the power is off.

Literature

1. S. G. Madoyan, Yu. S. Tikhovtsev. A. F. Trutko - Tunnel diode. Collection "Semiconductor devices and their applications" edited by Ya. A. Fedotov. 7.
2. K. S. Rzhevkin "Tunnel Diode" Mass Radio Library "issue 452, State Energy Publishing House, 1962
3. Akchurin E. A., Styblik V. A. Generators on tunnel diodes with increased power, Radio engineering, 1963, vol. 18, No. 11.
4. Williams, Hamilton How to make tunnel diodes even more useful, Electronics, June 7. 1963, V 36. No. 23.

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