The Push-Pull Amplifier commonly used in power amplifier. It is employed in the output stages of the circuits. It is used to get high output power at high efficiency. The audio power amplifiers used in transistor receivers, tape recorders etc. Distortion is greatly reduced by using push-pull operation employing two transistor in a single stage. There systems are usually operated by batteries.
Two transistor Q1 and Q2 placed back employed. The emitter terminals of the two transistor Q1 and Q2 connected together. Both transistors are operated in class B operation. The input signal is applied to the input of two transistors through centre tapped step up transformer. The input transformer is called Driver transformer. Driver transformer supplies equal and opposite voltages to the base circuits of two transistors.
The output transformer has the centre tapped primary winding. The load speaker is connected across the secondary of the output transformer.
Working or Operation
The primary of driver transformer connected to AC supply. The input signal appears across the secondary AB of the driver transformer. During first half of the input signal A becomes positive and end B negative. Here we have used two NPN transistors. This will make the base emitter junction of Q1 forward biased and that of Q2 forward biased. Now the current will conduct through Q1 while Q2 remains in non conducting state.
The current conduction through transistor Q1 shown by solid arrow. The amplified signal appears in the upper half of the output transformer.
During Negative half cycle of the supply the terminal A becomes negative and B becomes positive. This will make the case emitter junction of Q1 reverse biased and that of Q2 forward biased. The transistor Q2 and transistor Q1 remains in non-conducting state.
The amplified signal appears in the lower half of the output transformer.
The centre-tapped primary of the output transformer combines two collector currents to form a sine wave output in the transformer.
- Less distortion for a given power output due absence of even harmonics.
- Elimination of dc component at output.
- Requirement of two transistors.
- Need of bulky and expensive transformer.