PUSH-PULL AMPLIFIER:
The distortion introduced by the non-linearity of the dynamic transfer characteristic may be
eliminated by a circuit known as a known as push-pull configuration. It employs two active
devices and requires input signals 180 degrees out of phase with each other.
The above figure shows a transformer coupled push-pull amplifier. The circuit consists of two
centre tapped transformers T1 & T2 and two identical transistors Q1 and Q2.The input transformer
T1 does the phase splitting. It provides signals of opposite polarity to the transistor inputs. The
output transformer T2 is required to couple the ac output signal from the collector to the load.
On application of a sinusoidal signal, one transistor amplifies the positive half-cycle of the
input, whereas the other transistor amplifies the negative half cycle of the same signal. When a
transistor is operated as class-B amplifier, the bias point should be fixed at cut-off so that
practically no base current flows without an applied signal.
Consider an input signal (base current of the form ib1 I bmCost applied to Q1 .
The output current of this transistor is given as,
i1 I C B0 B1Cost B2 Cos2t B3Cos3t --------- (1)
The corresponding input signal to Q2 is
ib 2 ib1 I bm Cost
The output current of this transistor is obtained by replacing t by t in expression for i1 .
i.e.
i2 (t ) i1 (t ) ------------------ (2)
i2 I C B0 B1Cost
= I C B0 B1Cost B2Cos2t B3Cos3t -- (3)
As illustrated in the above fig., the current i1 & i2 are in opposite directions through the output
transformer windings. The total output current is the proportional to the difference between the
collector currents in the two transistors. i.e.
i k i1 i2 2k B1Cost B3Cos3t -------- (4)
This expression shows that a push-pull circuit cancels out all even harmonics in the output and will leave
the third harmonic as the principal source of distortion. This is true only when the two transistors are
identical. If their characteristics differ appreciably, the even harmonics may appear.
ADVANTAGES OF PUSH-PULL SYSTEM:
Because no even harmonics are present in the output of a push-pull amplifier ,such a circuit will
give more output per active device for given amount of distortion. Also, a push-pull arrangement
may be used to obtain less distortion for given power output per transistor.
It can be noticed that the dc component of the collector current oppose each other magnetically
in the transformer core. This eliminates any tendency towards core saturation and consequent
non-linear distortion that might arise from the curvature of the magnetization curve.
Another advantage of this system is that the effects of ripple voltages that may be contained in
the power supply because of inadequate filtering will be balanced out. This cancellation results
because the currents produced by this ripple voltage are in opposite directions in the transformer
winding and so will not appear in the load.
CLASS-B AMPLIFIER
The circuit for the class-B push pull system is the same as that for the class A system except that
the devices are biased approximately at cut-off. The above circuit (class A) operates in class B if
R2 =0 because a silicon transistor is essentially at cut –off if the base is shorted to the emitter.
ADVANTAGES OF CLASS B OPERATION
(1) It is possible to obtain greater power output
(2) Efficiency is higher
