Name of Unit : Transistor Power Amplifier Topic : Class B Push-Pull Amplifire Name of Faculty : Hutesh Baviskar

Crcuit of this type of amplifire is shown in fig . Q1 and Q2 are two NPN type transistor. Their emitter terminal are joined together and connected to ground through common resistance RE. Bypass capacitor is not connected across RE. Which indicates that the negative feedback is given. Due to this, the distortion is reduced. Topic: Push Pull Amplifier

However the gain is reduced to some extent. Biasing resistor R1 and R2 are selected such that both the transistor get cut-off bias. T2 is the output transformer. There is center tapping in the primary winding. Center tap is connected to +vcc supply. Two outer leads are connected to the collector of the transistor. Topic: Push Pull Amplifier

Loud speaker is connected across the secondary winding. Thus both the collector gets +vcc supply through half the primary winding. Loud speaker is connected across the secondary winding. T1 is the interstage transformer. It is also called the driver trnsformer. There is center tapping S in the secondary winding of this transformer. Outer leads P and Q are connected to the base of the transistor. Topic: Push Pull Amplifier

Topic: Push Pull Amplifier FIG. CLASS B PUSH- PULL AMPLIFIRE Topic: Push Pull Amplifier

WORKING:- Both the transistor are given the cut-off bias, so when there is no input signal,no current is taken from the power supply. So there is no loss of power during no signal condition. When input signal is given to the primary winding of transformer T1 , 180 degree out of phase voltage are induced in the winding PS and QS. Topic: Push Pull Amplifier

So collector current IC1 of transistor Q1 flows. This means when voltage at P becomes positive w.r.t. S, at the same time voltage at Q becomes negative w.r.t. S. When voltage at P goes towards the positive, trnsistor Q1 becomes forward biased and its base current increases. So collector current IC1 of transistor Q1 flows. Which passes through the upper half on the winding through Q1, through RE to ground. So flux is estabished in the core of the transformer. Topic: Push Pull Amplifier

The output voltage follows this. This induces voltage in the secondary winding so current flows in one direction through the voice coil of the loud speaker. Input voltage increases from zero in the positive half cycle and then reduces to zero. The output voltage follows this. When Q1 conducts ,transistor Q2 does not counduct due to the reverse bias. Topic: Push Pull Amplifier

The output voltage follows this. This induces voltage in the secondary winding so current flows in one direction through the voice coil of the loud speaker. Input voltage increases from zero in the positive half cycle and then reduces to zero. The output voltage follows this. When Q1 conducts ,transistor Q2 does not counduct due to the reverse bias. Topic: Push Pull Amplifier

Base of transistor Q2 gets positive voltage and it conduct. When the input voltage becomes negative, transistor Q1 is reverse biased so it stops conducting. Base of transistor Q2 gets positive voltage and it conduct. Its collector current IC2 flows through the lower half winding of transforner T2 through Q2,through RE to ground. Topic: Push Pull Amplifier

Waveforms at key points are shown in the same figure. This induces emf in the secondary winding and due to this current flows in the opposite direction through the voice coil of the loud speaker. Waveforms at key points are shown in the same figure. These are drawn without scale. Topic: Push Pull Amplifier

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