Power Amplifier Stage

The power stage of an amplifier usually has unity gain, and supplies current to the load. By looking at the power requirements, we can determine the amount of current which the power stage must source with minimal sagging of the signal. Above we said that the maximum voltage at the load would be about 10V. Thus, the maximum current would be 1.2A. We must now ask ourselves the question, by how much do we have to amplify the output current of the pre-amp, in order to be able to supply 1.2A. In other words, if we consider the input of the power stage to be the load of the preamp stage, what is the load that the preamp stage will drive before significant sagging occurs? The manufacturer's specification sheet says that a 741op-amp has a maximum output current of 20mA. Thus, as designers we probably don't want the 741 to have to source more than about 10mA. Since the 741 will amplify the signal to a maximum amplitude of 10V, then $\frac{10}{0.010}=1K$ will be the smallest resistance we would want the op-amp to drive. Thus, the input resistance to the transistors of the power stage must be at least 1K to avoid sagging the preamp.

From the preceding discussion we conclude that our power output stage has to be able to source 1.2A and have a minimum input resistance of approximately 1K and unity gain. To achieve this, we could use an ordinary capacitively coupled emitter follower. However, we would like to avoid this because the DC bias requires too much power for efficient operation of power amps. In addition, the AC coupling would cut down on the low frequency response. Probably the best choice for the power stage would thus be a Darlington connected push-pull circuit which is biased slightly on to avoid distortion.