8.3.1 Introduction: Push ?Pull Electronics

Push ?pull operation is well known in electronics where, for example (Figure 8.9), a pair of transistors T ₁ and T ₂ are coupled at their emitters (marked E). Because of a sufficiently large resistance R _t ('the long tail') there is a constant current I flowing equally between the two transistors in the steady state. On driving the input (the base of T ₁) positive, the current in T1, is enhanced by ? I but, because of the constant-current source at the coupled emitters, the current into T ₂ is reduced by ? I. The base at transistor T ₂ is shown to be fixed at 'ground' potential but could equally well have been driven by a second input provided that the polarity was opposite to that at the base input of T ₁. The ' push-pull' outputs are then capable of driving another similar circuit. This circuit is a remarkably high-speed circuit because both transistors are operated with either unity voltage gain or unity current gain which then gives the potential for working at the limit of their gain-bandwidth product signified by the transition frequency f _t. The emitter E is the output for the first transistor (T ₁) for driving transistor T ₂. Consequently, the transistor T ₁ is effectively operated as an emitter follower with unity voltage gain. The transistor T