Introduction

The standard multivibrator is well known. However there is a different multivibrator which never was so popular, but which in practise works rather better and is also more versatile. It is the emitter coupled multivibrator.

Available to TEC members is a Library of multivibrator circuits. This demonstrates that there is a whole family of standard multivibrators. So, also there are several versions of the emitter coupled multivibrator.

Basic Circuit

The basic circuit is shown below.

A better circuit

This alsi is a multivibrator, and is emitter coupled. But there the similarity ends.

Tr2 and Tr3 are a complimentary feedback pair (CFP), covered on the TEC public area. Tr3 is Not strictly necessary: it can be removed and the resistor values altered to suit, but the addition of Tr3 does make the circuit better behaved and cleans up the waveforms. Also shown here are current sources, CS1 and CS2, for the emitter drives. Again, these could be resistors - but the use of current sources gives linear charge and discharge ramps.

Consider the circuit starting with the CFP turned on, at time t₁. Point B is high. When this state is new, the capacitor will be discharged so point C also will be high (at Vcc). CS1 now starts to charge C1, pulling point C downwards towards 0v.

When the capacitor has charged sufficiently, point C falls below Vb, the base reference voltage to Tr1, and Tr1 conducts (time t₂). Tr1 now robs the base current that was holding Tr2 in conduction and the CFP switches off. There is now nothing to hold point B high, so CS2 starts to discharge the capacitor. Tr1 is conducting during this discharge, so Tr1 is saturated and its collector and emitter are both at about Vb (actually about one V_be drop below Vb). Eventually C1 discharges sufficiently for point B fo fall below Tr2's base voltage (i.e nearly 2 V_bes below Vb). Now current starts to flow in Tr2 and the CFP (Tr1 and Tr2) snap into conduction and the cycle re-starts.

Tr3 is part of the CFP, so it either turned hard on or turned off: transitions are very fast so the output is a good, clean squarewave, though the pull-up impedance is low and the pull down is high. Also the negative level of the waveform is clamped to Vb, and not to 0v.

It should also be apparent from the above discussion that Vcc should not be high enough that Tr1's emitter-base breakdown voltage is ever reached. If this occurs, the capacitor voltage swing is clamped. However it is easy enough to put diodes inn series with the emitters of Tr1 and Tr2 so that reverse breakdown does not happen, when much larger voltage swings can occur.

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