Push pull amplifier (class b power amplifier)

This PPT explains the circuit diagram and the working of push pull amplifier

1. Push pull amplifier (class B power amplifier) Dr.R.Hepzi Pramila Devamani , Assistant Professor of Physics, V.V.Vanniaperumal College for Women, Virudhunagar

2. Push pull amplifier The audio power amplifier used in transistor receivers , tape recorders , record players , PA systems etc make use of the push-pull circuit. Push pull amplifiers have increased power output , efficiency and less distortion. A combination of two class B amplifiers working together is called push-pull amplifier.

3. CIRCUIT DIAGRAM

4. Push pull amplifier The push-pull amplifier circuit uses two identical transistors whose emitter terminals are connected together. The circuit uses two transformer one at the input and one at the output. The input transformers has a center-tapped secondary winding. It provides opposite polarity inputs to the two transistors.  

5. Push pull amplifier The primary of the output transformer is also center tapped. The collector terminals of the two transistors are connected to the supply voltage through the primary of this transformer. The load ( eg , loud speaker) is connected across the secondary of the output transformer. The turns ratio is properly chosen to match the load and output impedence of the transistor.  

6. Push pull amplifier Under well-matched conditions , there is maximum power delivered to the load by the amplifier. The resistors are used to bias the amplifier under class B condition ( i,e , the operating point is at cut off ) so that practically no current flows in the absence of an applied signal.  

7. Working of Push pull amplifier The class B amplifier is biased at cut-off (stop). If there is no signal input , both the transistor are cut-off . When a sine wave voltage is applied as the input across the primary of the input transistor (IT) , a sine wave voltage of the same phase appears in the upper half of the secondary IT.  

8. Working of Push pull amplifier The combined action of the two transistors is shown by the composite transfer characteristics curve. The two curves are inverted with respect to each other since the current directions in them are opposite. The transfer characteristics relates of each transistor.  

9. Working of Push pull amplifier Thus the input to base of transistor is a sine wave voltage , which has the same phase as in the input voltage , i.e. , the transistor gets the positive voltage during the first half of the sine wave. The voltage appearing at the lower half of the secondary of IT had a phase opposite to that of the input voltage. So, the transistor gets a positive voltage only during the second half of the sine wave.  

10. Working of Push pull amplifier Since the transistor are biased at cut-off , the transistor amplifiers the first half of the sine wave and amplifiers the second half of the input signal. The transistor conducting current for the first half of the signals through the upper half of the primary of the output transformer (O.T) , the current flows from the center-tap upward. This produce induced voltage in the secondary of the O.T and this part of output is in phase with the first half of the input signal.  

11. Working of Push pull amplifier The transistor conducting only during the second half of the signal through the lower half of the primary of the O.T , the current from the center-tap flows downward. This produces induced voltage in the secondary of the O.T and this is in phase with the second half of the input signal. Thus one complete cycle of the input signal gets power-amplified to produce an output signal which is a replica of the input signal.  

12. Working of Push pull amplifier We see that during one half cycle of the input signal , the circuit pushes the signal high and during the next half cycle , the circuit pulls the signal low at the output. For this reason the circuit is called the push-pull amplifier.

13. ADVANTAGES The output is true replica of the input signal. The d.c components of the collector current in the output transformer are in opposite directions. This prevents the transformer core from getting into saturation in the transformer. High efficiency with less distortion can be secured with the class B push-pull amplifier. The maximum efficiency = 79%. There will be no hum in the amplifier output since the ripple currents due to power supply flow in opposite directions in the output transformer.

14. THANK YOU