Generator on the analog of the tunnel diode. Encyclopedia of radio electronics and electrical engineering

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In [1], a transistor analogue of a tunnel diode (ATD) is considered. A diagram of such an analog is shown in fig. 1.

Fig. 1

The zener diode VD1 is included in the base circuit of the transistor VT1. VT1 is initially closed because VD1 is closed and there is no offset based on VT1, while VT2 is open. With increasing voltage applied to terminals A and B, the current through VT2 increases quite quickly. Due to this, an "ascending" branch of the current-voltage characteristic (CVC) of the ATD is formed. After reaching the breakdown voltage of the zener diode VD1, a rapid increase in the base current VT1 begins, and, accordingly, this transistor gradually opens, and VT2 closes. This leads to a decrease in the current through the ATD, i.e. a "falling" branch of the current-voltage characteristic with negative resistance is formed.

On the second, "ascending" branch of the CVC, the ATD current is determined mainly by the current passing through VD1 and R1. However, the use of low-frequency transistors in the circuit [1] does not allow such ATD to operate at sufficiently high frequencies. This drawback can be eliminated by using RF transistors. Zener diode VD1, operating in the breakdown mode, is a source of strong noise, and therefore the ATD itself turns out to be quite "noisy". If you replace the zener diode with a string of diodes connected in series (Fig. 2), the noise of the circuit is significantly reduced.

Fig. 2

The operation of the generator on the ATD (Fig. 3), due to the presence of a section with negative resistance, requires power from a voltage source (with low internal resistance). During the measurements, it turned out that most testers at the current measurement limit up to 50 mA have a very high internal resistance and do not allow measuring the CVC of the ATD. Therefore, the author uses a current probe - a resistor with a resistance of 1 ohm. To measure the current, the voltage drop across this resistor is determined.

Fig. 3

On the "falling" branch of the CVC, the ATD, due to the presence of parasitic reactivities, often starts to generate. To eliminate such parasitic generation, the voltmeter is connected to the current probe through two 10 kΩ resistors soldered to the ends of the probe resistor. But even such measures do not completely exclude the appearance of hysteresis phenomena. There is some difference between the I–V characteristic taken in the “forward” direction (with an increase in the voltage at the ATD) and the I–V characteristic obtained with a decrease in the corresponding voltage.

On fig. 4 shows the CVC of an ATD, taken with an increase in voltage across it. As can be seen, this CVC has an N-shaped form. The voltage on the ATD, at which oscillations occur in the LC circuit (Fig. 3), has a rather narrow range (about 0,2 V). On fig. 4 this area is highlighted. From the point of view of generating oscillations, a narrow generation zone is a disadvantage, since an accurate setting of the supply voltage is required to obtain generation. However, this drawback, on the other hand, is also a certain advantage, since it becomes possible to control the generation with a relatively small change in the supply voltage.

Based on the graph shown in fig. 4, a number of ATD parameters can be determined, for example, the value of its negative resistance.

Fig. 4

Assuming that between points 1 and 2 the graph is a straight line, we will approximately determine the differential negative resistance in this section:

Rд=dU/dI=(4,8-4,3)/((6,7-24,8)*10^-3) = 5*10^-1/(-1,81*10^-2) = -27,6 (Ohm)

Returning to the consideration of the schemes presented in Fig. 1 and 2, it should be noted that the peak voltage for such circuits with a sufficiently high accuracy can be considered equal to the breakdown voltage of the zener diode or the trigger voltage of the diode chain.

The "trough" voltage is approximately 0,5 V (Fig. 1) and 1 V (Fig. 2) greater than the peak voltage, which is apparently due to the saturation voltage of the transistors. The RF voltage on the circuit was taken in the mode of reducing the supply voltage using a high-resistance RF voltmeter connected directly to the LC circuit. The graph of the change in the RF voltage on the generator circuit (effective value) is shown in fig. 5 (B - reliable part of the graph, A - branch to be clarified).

Fig. 5

Literature

1. Telezhinsky P. An analogue of a tunnel diode. - Radio, 1977, N4, p.30.
2. Gonorovsky I.S. Radio engineering circuits and signals. - M.: Soviet radio, 1977.

Author: V.Artemenko, UT5UDJ, Kiev.

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