In most applications where AC power is driving a load, a complementary symmetry (push-pull) power amplifier is employed. This amplifier usually has a voltage gain of one, and a large current gain. It is the most efficient configuration for transforming DC power from the power supply to the AC power driving the load.
A basic complementary symmetry BJT power amplifier is shown in Fig. 3.1
The BJT's provide a current gain of , and a low output resistance. When the input is positive and greater than 0.7V, the npn BJT is on and the pnp is off. Under these conditions, the npn acts like an emitter follower, taking the input current from the base, multiplying it by , to provide current and a low source resistance to drive the load. Of course the extra current derived from the multiplication has its origin at the positive DC power supply.
When the input voltage is less the -0.7V, the npn is off and the pnp is on. Under these conditions, the pnp acts like an emitter follower, taking the input current from the base, multiplying it by , to sink current and provide a low source resistance to sink the load. Here, the extra current required by multiplication has its origin at the negative DC power supply.
The output resistance Rout is given by the parallel output resistance of the npn and the pnp (Ron||Rop). For the positive side of the cycle, since the pnp is off, . So under these conditions the output resistance is just that of the npn which is
To appreciate how power is converted from supply by the circuit, consider the following. Each power supply provides current during only half the cycle. During the positive cycle, the positive DC supply is sourcing current, while during the negative part of the cycle the negative DC supply is sinking current. Therefore, the average current provided by the positive supply is
The average power for the total cycle is then obtained by adding the power from each half cycle:
The power conversion efficiency , which gives a measure of how well power is transferred from the supply to the load, is given by the ratio of the power to the load PL to the power provided by the supplies Psupply:
You may have noticed that the complementary symmetry configuration
has a small problem which is that no signal can propagate
when -0.7 < Vs < 0.7 because both BJT's are off at this time.
This region is often referred to as the cross-over region between the
npn and pnp cycles. Hence, the accompanied distortion that results
is often referred to as cross-over distortion.
To avoid cross-over distortion, circuit designers often bias
the two transistors slightly into the forward active region
with diodes. For example, look at the circuit in Fig. 3.2.
The diodes, in conjunction with resistors R1 and R2, constitute the biasing network to make sure these is always a across both transistors. That way one BJT is always on, and the cross-over region is eliminated.