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.
4. Push pull amplifier
The push-pull amplifier circuit uses two identical transistors
𝑇1and 𝑇2 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 𝑉𝐶𝐶 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
oUnder well-matched conditions , there is maximum
power delivered to the load by the amplifier.
oThe resistors 𝑅1, 𝑅2 𝑎𝑛𝑑 𝑅𝐸 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 𝑇1𝑎𝑛𝑑 𝑇2 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
oThe combined action of the two
transistors is shown by the
composite transfer characteristics
curve.
oThe two curves are inverted with
respect to each other since the
current directions in them are
opposite.
oThe transfer characteristics relates
𝐼𝐵 𝑎𝑛𝑑 𝐼𝐶 of each transistor.
9. Working of Push pull amplifier
Thus the input to base of transistor 𝑇1 is a sine wave voltage ,
which has the same phase as in the input voltage , i.e. , the
transistor 𝑇1 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 𝑇2 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 𝑇1amplifiers
the first half of the sine wave and 𝑇2 amplifiers the second half of the
input signal.
•The transistor 𝑇1 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 𝑇2 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.
