Power Calculations for Class B Push-Pull Amplifier
The power delivered by the ac source is split between the transistor and the resistors in the bias circuitry. The ac signal source adds an insignificant additional amunt of power since base currents are small relative to collector currents. Part of the power to the transistor goes to load, and the other part is dissipated by the transistor itself. The following equations specify the various power relationships in the circuit.
The average power supplied by the dc source is
iCC(t) is the total current and is composed of two components: the dc current through the base bias resistor and diode combination, and the ac collector current through transistor, Q1. Under quiescent conditions (i.e. zero input) Q1 is in cutoff mode. Collector current flows during the positive half of the output signal waveform. Therefore we only need to integrate this component of the power supply signal over the first half cycle.
The maximum values of collector current and power delivered to the transistor are
The ac output power, assuming a sinusoidal input, is
The maximum ac output power is found by substituting ICmax for IC1max to get
The total power supplied to the stage is the sum of the power to the transistor and the power to the bias and compensation circuitry.
If we subtract the power to the load from the power supplied to the transistors, we find the power being dissipated in the transistors the power dissipated by a single transistor is one half of this value. Thus,
we are assuming that the base current is negligible.
The efficiency of the Class B push-pull amplifier is the ratio of the output power to the power delivered to the transistor. Thus we neglect the power dissipated by the bias circuitry.
This amplifier is more efficient than a Class A amplifier. It is often used in output circuits where efficiency important design requirement.
Therefore,
In choosing a transistor, it is important that the power rating is equal to or exceeds the maximum power Pmax.
