Double-ended or push-pull amplifier makes use of two identical transistors in a single stage. It consists of two loops in which the transistor collector currents flow in opposite directions but add in the load.
Push-pull amplifiers eliminate all of three drawbacks. These drawbacks are; (i) The total harmonic distortion is unacceptability high for most audio applications except in cheaper amplifiers. The second harmonic distortion is usually the greatest offender, (ii) Maximum output power is delivered at a load resistance that is different from the value for minimum harmonic distortion. Consequently, the amplifier is designed to operate into a load that provides slightly less than maximum output power, but a significantly lower total distortion, (iii) The output transformer is subject to saturation problems due to flow of direct current in transformer primary.
The push-pull is a power amplifier and is frequency used in the output stage of electronic circuits. It is employed whenever high output at high efficiency is required. The audio power amplifiers used in transistor receivers, tape recorders, record-players, public address systems etc. make use of such circuits. These systems are usually operated by batteries or cells and, therefore, amplifier efficiency is of prime importance.
Basic Principle of Working of Push-Pull Amplifier
The basic principle on which a push-pull amplifier operates is that the input signal is converted before amplification, into two separate signals, which are identical except for a 180° phase difference. Each of these signals is applied as the input to one of the transistors of the push-pull amplifier. Since the inputs to the two transistors are 180° out of phase, so the output of the transistors. The output transformer is connected in the collector circuits of the transistors in such a way that the collector currents of the two transistors combine to provide an overall signal having the same waveform as the original input signal. The magnitude of this combined output signal is larger than the input signal. Push-pull amplifiers can be operated in class A, class AB or class B mode.
Fig. Complementary Symmetry Push-Pull Amplifier
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