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AP3302 Pt3 Contents

AP3302 Pt3 Section 2Contents

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AP 3302 Pt. 3

Section 2

CHAPTER 7

Monostable and Bistable Multivibrators

The action of the circuit may be explained as follows:

Stable state. Before the application of a trigger pulse, the circuit is in its stable state with V1 cut off by the bias voltage applied to its grid and V2 conducting. Vg1 is at a negative voltage and Va1 at h.t. +; Vg2 is limited to zero volts by the flow of grid current through R2 and Va2 is at a low working voltage.

Instant A. The positive trigger pulse applied to V1 lifts Vg1 above cut-off and V1 conducts. The usual multivibrator avalanche then takes place causing the circuit to 'flip' to its unstable state where V1 is conducting and V2 is cutoff. Vg1 rises to zero volts, where it is clamped by grid current limiting, and Va1 falls to a low working value. Vg2 is driven below cut-off and Va2 rises.

Interval A to B. This is the normal multivibrator relaxation period during which Va2 is at a voltage just below h.t. + as determined by the potential divider RL2, R3, R1. With only the valve capacitance of the stage to charge, i.e. no cross-coupling capacitor connected to V2 anode, Va2 rises very quickly. C2 discharges through R2 towards the aiming voltage causing Vg2 to risee exponentially.

Instant B. Vg2 reaches cut-off and V2 conducts. The usual avalanche then occurs and results in the circuit 'flopping' back to its original stable state, in which V2 is conducting and V1 cut off. Vg2 rises to zero volts, where it is clamped by grid current limiting, and Va2 falls to its working value. Vg1 is driven well below cut-off and Va1 rises by the same amount as Vg2 has risen from cut-off.

Interval B to C. In this relaxation period Va1 rises exponentially to h.t. + as C2 charges through RL1. Vg1 returns rapidly to its original level and remains there until the next trigger pulse arrives. The circuit can stay in this stable state indefinitely. Vg1 is held below cut-off by the bias voltage and Va1 is at h.t. +; Vg2 is limited to zero volts and Va2 is at its working voltage.

Instant C. On receipt of the next trigger pulse the action is repeated as from instant A.

The output may be taken either from the anode of V2 (Va2) or from the anode of V1 (Va1) depending upon whether a positive or a negative-going output is required. The output from V2 anode has the better shape since there is no capacitor connected to this electrode. The p.r.f. of the circuit is determined by the repetition rate of the trigger pulses, one complete cycle of output being obtained for each trigger pulse. The pulse duration AB is determined by the time constant C2R2 and by the aiming voltage and may be controlled by varying R2 The instant of time at which the trailing edge of the pulse occurs relative to the leading edge can therefore be varied.

This circuit may also be triggered by applying negative-going trigger pulses to the anode of V1. Each negative pulse is applied through C2 to V2 grid and appears as an amplfied and inverted pulse, ie positive-going at V2 anode, whence it is applied to V1 grid via R3 to cut on V1.

The transistor equivalent of the circuit just discussed is known as the collector-coupled flip-flop. The circuit and waveforms are shown in Fig 3. The action of the circuit may be deduced from the description given for the action of the valve anode-coupled flip-flop.

Emitter-coupled Flip-flop

The basic circuit of an emitter-coupled flip-flop is illustrated in Fig 4. Here, the base of TR2 is connected through R3 to the negative supply -Vc. The base current supplied through R3 is then such that TR2, in the absence of triggering, conducts heavily and the large voltage drop across the common emitter resistor RE makes the emitters of both transistors negative with respect to earth. Since the base of TR2 is connected through R3 to a negative voltage greater than that at its emitter TR2 conducts. The base of TR1 however is connected to a point of fixed voltage which is only one volt negative with respect to earth; VB1 is therefore positive with respect to its emitter and TR1 is cut off. The circuit remains in this stable state, with TR2 on and TR1 off, until TR1 is made to conduct by a negative-going input pulse to its base. 


 

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