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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.E.
• Bypass capacitor is not connected across RBypass 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
• 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
FIG. CLASS B PUSH- PULL AMPLIFIRETopic: 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
• 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
• 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
• 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
• 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
• 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
THANK YOU
Topic: Push Pull Amplifier