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AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 10

Electronic Switching Circuits

As the forward bias is increased the tunnel current increases to a peak value at A (Fig 2). It then falls with further increase in forward bias to a level known as the ‘valley current’ at B. Thereafter the characteristic is that of a normal forward-biased junction diode. For a germanium tunnel diode the peak and valley current points occur at about 50mV and 350mV respectively.

Notice that between points A and B the current is falling as the voltage is increased. The tunnel diode therefore behaves as a negative resistance over this region. By suitable arrangements, such a device can be made to provide amplification. A tuned circuit load has a certain dynamic impedance at its resonant frequency. If a negative resistance is placed in parallel with this load, the normal (positive) resistive damping losses of the tuned circuit may be completely cancelled by the negative resistance and the circuit will then oscillate. By suitable adjustment of the standing bias on the tunnel diode the negative resistance value is made such that the circuit does not quite oscillate. The arrangement now works as a tuned amplifier. This is something quite new for a diode.

The two main applications of the tunnel diode are in switching circuits, where the device is switched from the peak to the valley points by an input current, and in low-noise amplifiers and oscillators for extremely high frequencies, where the tunnel diode is biased in its negative resistance region.

In the switching application, the characteristic of Fig 2 shows that a given value of current x may correspond to three different applied voltage levels, P, Q and R. in most switching circuits, voltages between A and B are unstable and the tunnel diode is usually considered as a two-state (bistable) device, one state to the left of the peak current point and the other to the right of the valley point. To switch from one state to the other a small change in input current in one sense or the other is all that is required.

A useful tunnel diode-transistor trigger circuit is shown in Fig 3. When the circuit is first switched on it is arranged that the tunnel diode is switched to the peak current-low voltage region. The base emitter junction of the transistor has therefore a low voltage across it and the collector current is negligible. The output voltage is thus at the negative supply voltage level. As the input current is increased the tunnel diode switches rapidly to the valley current-high voltage state and the transistor conducts heavily. The output voltage thus rises rapidly towards earth. By changing the input current by small amounts in this way very rapid changes in output voltage may be obtained. High-speed low-voltage switching circuits of this type find many uses in computers.


 

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