AP 3302 Pt. 3
Section 2
CHAPTER 10
Electronic Switching Circuits
Quench Circuit as a Low-power Switch
Fig
4 shows how an oscillator may be controlled by means of a ‘quench’ circuit.
TR1 is the quench or switching transistor which is normally conducting, with
its base held at zero volts by the flow of base current through R. TR2 and its
associated circuit form an inverted Hartley oscillator. Since TR1 is normally
conducting, the current drawn by it through the oscillator coil L effectively
damps the oscillator tuned circuit and prevents oscillations. Only when TR1
is switched off by the positive-going trigger pulses on its base is the oscillator
free to oscillate for the duration of the trigger pulse. This low-power circuit
may be used as the first stage in a ‘cal pip generator’ (see p145).
Gating Principles and Circuits
Gating is a switching process which may be likened to opening a gate to allow the passage of something only at and for a required time. In radar, gating circuits are used to allow input signals to pass to the output terminals at definite instants of time and for required periods of time. Gating is normally carried out at low-voltage, low-power stages in an equipment.
A simple diode gate is illustrated in
Fig 5. The input consists of a train of positive-going pulses. A gating square
wave is applied to the diode as shown. On the positive-going half-cycles of
the gating waveform the diode conducts and the input signal pulses then appear
at the output terminals. 
On
the negative-going half-cycles of the gating waveform the diode is cut off and
there is no output - even although the signal pulses are still present at the
input.
Fig 6 illustrates a pentode gate together with its associated waveforms. The valve is normally biased beyond cut-off both on the control grid g1 and on the suppressor grid g3. The gating square wave is applied to one grid (g3 in this example) and the signal input is applied to the other grid g1. The gating square wave lifts the suppressor grid above cut-off during the periods AB and CD and the signal input to g1 then causes the valve to conduct. Thus the only signals which produce an output are those which appear at the control grid g1 during the positive-going portions of the gating waveform, when anode current is allowed to flow. The circuit thus selects input signals according to the time they arrive. A gating circuit is therefore a time selection circuit. The diode D is normally inserted to limit the suppressor grid voltage to zero volts.
A simple transistor gate is illustrated in Fig 7. The base-emitter junction of the transistor is normally held cut off by the negative bias applied to the emitter. The positive-going signal input pulses are applied to the emitter but these are of insufficient amplitude to cut on the transistor.
