The push-pull amplifier uses two complementary power transistors arranged in a symmetrical configuration to amplify an input signal. There are different classes of linear amplifiers - Class A always conducts but is inefficient, Class B has zero quiescent current but high distortion, and Class AB balances these tradeoffs. The class is determined by the quiescent current. Feedback amplifiers have gains that are stable over temperature and reduce distortion.

1. Power Amplifiers
The push-pull amplifier is a typical arrangement for an
audio power stage.
+V
Vi
-V
In its simplest form it comprises two complementary
power transistors (npn and pnp).

2. Amplifier Class
Class A, B, AB and C are known as linear amplifiers because
they operate over a continuous sinewave, ie analogue signals.
Class A: Transistors Q1
and Q2 are always
conducting causing the
quiescent current IQ to be
above the maximum and
continuous.
These amplifiers have
very low distortion (zero
crossover) but run very
hot (low efficiency), used
in ‘high end’ audio
systems.
Class B: Quiescent
current IQ is zero giving a
much lower power
consumption but suffer
from large crossover
distortion.
+V
IQ
Q1
Vi
Load
Q2
-V
Class AB: By allowing a
small amount of
quiescent current the
crossover distortion can
be reduced significantly.
Class C: With efficiencies approaching 100% these are used in very
specialised applications such as radio transmitters
Class ‘D’ : Reserved for switching amplifiers where the transistors are
either fully ‘on’ or ‘off’ used in stepper motor control applications.

3. Power Amplifiers
Three common push-pull amplifier configurations
The class of an amplifier is determined by the amount of
quiescent current flowing in the output transistors.

4. Distortion
No electronic circuits are ideal, all impose limits on the amplitude and frequency of
the signals that pass through them.
Sinewave: Pure signal without
distortion, as expected.
Clipping: caused by overdriving
the transistors
Harmonic distortion: caused by clipping,
poor earthing or coupling of ‘stray’
signals into the original.
Crossover distortion: due to both output
transistors being cut-off at small voltages
either side of zero,(class B).

5. Amplifier Bandwidth
The cut-off frequencies, f1 and f2, define the bandwidth
Bandwidth = f2 – f1
Gain
dB
Am
bandwidth
f1
f2
frequency
Hz
Am is called the midband gain, which in an ideal amplifier is flat through the
frequency range, as above.
The operational gain is called the bandwidth and is the gain between the cut-off frequencies f1 and f2, also known
as the half-power points or -3dB points.

6. Feedback Amplifiers
By using feedback the gain of an amplifier is independent of transistor parameters
RI
RF
By using high stability close tolerance components for R1 and
R2 the amplifier gain is made very stable over a wide
temperature range

7. Gain Bandwidth Product
The GB product is a term that quantatively states the
performance of an amplifier.
Gain dB 100
80
60
-20dB/decade
40
20
1
0
1
10
100 1000 10000
100000 f Hz.
This quantity is commonly specified for operational amplifiers, and allows
circuit designers to determine the maximum gain that can be achieved
from the device for a given frequency (or bandwidth) and vice versa.

8. Effects of Feedback
Negative feedback is widely used in amplifier circuits for the
following reasons,
•gain is independent of transistor parameters
•Can increase or decrease input impedance (depending on type
of feedback)
•Can increase or decrease output impedance (depending on type
of feedback)
•Reduces distortion (increases linearity)
•Bandwidth is increased
Ways to apply feedback; series voltage (voltage amplifier), series current
(transconductance amplifier), ; shunt current (current amplifier), shunt
voltage (transresistance amplifier).