A push-pull amplifier is a type of amplifier that can drive a current in either direction through the load. In terms of distortion and performance, push-pull amplifiers are more efficient than single ended amplifier because of its push-pull arrangement. Into this arrangement, one transistor amplifies the positive half cycle whereas the other transistor amplifies the negative half cycle of the signal i.e. whole signal is amplified at the output. The output stage of a typical push-pull amplifier comprises of two identical BJT’s or Mosfet’s i.e one source current through the load while another sink current from the load. In our previous articles, we explain power amplifier, Amplifier classes, Class A power amplifier theory etc in detail. Here we are going to explain Push-pull amplifier circuit, operation, harmonic distortion, advantages and disadvantages in detail.
All audio power amplifiers used in the record player, transistor radio receivers, tape recorders etc makes use of push-pull arrangement because these systems are usually operated by batteries where efficiency is the primary factor. Let’s have a look at its circuit diagram and operation.
Push Pull Circuit:
As shown in push-pull amplifier circuit diagram below, two transistors T1 & T2 are placed back to back. Both the transistors are operated in class B operation i.e. the collector current is almost zero in the absence of the signal. Through a driver transformer, the input signal is given to the circuit from the driver stage.The secondary of driver transformer is centre tapped which supplies equal and opposite voltages to the base circuits of the two transistors. Cente tap primary of the output transformer is connected in the collector circuit and at the secondary side, the load is connected. To get maximum output across the load, a suitable impedance matching is obtained by designing proper turn ratio of the output transformer. As shown in the figure above, the supply voltage is connected between the centre tap of the output transformer and the base.
Push-pull Amplifier operation:
In a push-pull amp, the input signal appears across the secondary AB of the driver transformer. During the first positive half cycle of the signal, end A of the transformer winding become positive and end B become negative. By doing so, the base-emitter junction of the transistor T1 is forward biased and T2 is reverse biased which allow the current conduction across the transistor T1 and make the transistor T2 in the cut-off state. Therefore, the first half cycle of the signal is amplified by the transistor T1 and appears in the upper half of the primary of the output transformer.
During the negative half cycle of the signal, end B of the transformer winding become positive and end A become negative. By doing so, the base-emitter junction of the transistor T2 is forward biased and T1 is reverse biased which allow the current conduction across the transistor T2 and make the transistor T1 in the cut-off state. Therefore, the first half cycle of the signal is amplified by the transistor T2 and appears in the lower half of the primary of the output transformer.
Complete sine wave output in the secondary is obtained when the centre tapped primary of the output transformer combines the two halves of the cycle. By proper impedance matching, maximum power can be transferred to the load. As a result, the load resistance RL when referred o the primary side, its value will be given as:
RL’ = ( 2 N1 / N2 )2 * RL
N1 = No. of Primary turns between centre tap and either end of the output transformer
N2 = No. of secondary turns
Harmonic Distortion in Push-pull Amplifier:
When we apply a sinusoidal signal of frequency ω at the input of the amplifier, we get oppositely phased base currents represented by the following equations:
Ib1 = Ib sin ωt
Ib2 = Ib sin (ωt + π)
As we know, due to the nonlinearity of the transistor,even if the input is sinusoidal, the output current contains harmonics. The output i.e. collector current of the first transistor is then represented as;
Ic1 = I0 + I1 sin ωt + I2 sin 2ωt + I3 sin 3ωt + ….
As the collector current of second transistor is 180o out of phase, it can be represented as;
Ic2 = I0 + I1 sin (ωt + π) + I2 sin 2(ωt + π) + I3 sin 3(ωt + π) + ….
Ic2 = I0 + I1 sin (ωt + π) + I2 sin (2ωt + 2π) + I3 sin (3ωt + 3π) + ….
Ic2 = I0 – I1 sin ωt + I2 sin 2ωt – I3 sin 3ωt + ….
Since, matched pair of complementary transistors is available in the market, therefore we assume that the characteristics of the two transistors are identical. The power developed in the load R
In his amplifier, the voltage induced in the secondary of the output transformer is proportional to the difference of the two collector currents i.e. Ic1 – Ic2.The power developed in the load RL depends on upon the output voltage.
Note: In push pull amplifiers all even harmonics are cancelled.
Advantages and Disadvantages of push-pull Amplifier:
Push Pull Amplifier Advantages:
Following of its advantages are:
- Due to Class B operation, their collector efficiency is quite high.
- They give more AC output power per device.
- Distortion free output is obtained.
- Output Transformer used for push-pull amplifier circuit are lighter, smaller and less expensive than transformers of comparable quality that are used in single ended circuits.
- In the core of the output transformer, the DC components for output currents of the two devices oppose each other magnetically which eliminates the tendency of the core to saturate.
Push Pull Amplifier Disadvantages:
Following of its disadvantages are:
- It requires two equal and opposite voltages at the input, therefore, driver stage has to be employed.
- Unequal amplification of the two halves of the signal introduces more distortion.
- Two identical transistors are required.
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