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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 valve equivalent of this circuit is the cathode-coupled flip-flop, the circuit and waveforms of which are illustrated in Fig 5. Examination of Figs 4 and 5 will show that the two circuits are similar, so that the action of the valve version may be deduced from the previous paragraphs. An output is sometimes taken from the cathode (output 2); this is a low impedance source and is useful for matching to other stages. Like the anode-coupled flip-flop, this circuit may be triggered by applying negative-going trigger pulses to Vi anode.

Bistable Multivibrator

The bistable trigger or toggle is a two-state circuit in which both states are stable. In each stable state one of the two transistors (or valves) is cut off and the other is conducting, and the circuit is capable of remaining indefinitely in either stable state. To change over from one state to the other the circuit must be suitably triggered and, to go through a full cycle, two triggering pulses - one to each stage - are needed.

The bistable multivibrator has many applications: it can be used as a frequency divider, as part of a counting circuit or as a storage device in a computer (see p 584 of AP 3302, Part 1B). We shall see something of these applications later in these notes. The bistable multivibrator has been given many names, but the name most commonly used is the "Eccles-Jordon" circuit. Let us now examine the basic Eccles-Jordan circuit using valves.

Eccles-Jordan Bistable Circuit

The basic circuit of an Eccles-Jordan bistable multivibrator is shown in Fig 6a. This circuit differs from the anode-coupled astable multi-vibrator in two respects:

a. The cross-coupling impedances are resistors so that they provide d.c. coupling between anode and grid of opposite valves.

b. The grid resistors of both valves are returned to a negative bias point.

RL1, R1, R2 and RL2, R3, R4 each act as a potential divider chain connected between h.t.+ and the negative bias. Thus each grid voltage lies somewhere between the negative bias and h.t. + voltage levels. It is usually arranged, by correct selection of bias and resistor values, that the maximum grid voltage of the conducting valve is zero when the other valve is cut off. Thus in Fig 6b, if V1 is cut off, the three resistors each have 100V dropped across them and the division of voltage is as shown. The voltage division in the potential divider connected to the anode of the conducting valve is, of course, different (Fig 6c). If V2 is conducting, its anode current will produce an additional voltage drop across RL2. If this is sufficient to bring Va2 down to + 50V then across RL2 we have a voltage drop of 150V. We thus have the remainder, ie 150V, divided equally between R3 and R4 (75V across each) and Vg1 is held at - 25V with respect to earth, sufficient to hold V1 cut off.


 

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