2. Outlines
1 Classification of Amplifier
2 Distortion in Amplifier
3 Frequency Response of an Amplifier
4 Step Response of an Amplifier
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3. Classification of Amplifier
Amplifiers are described in various ways like,
1 Frequency range
2 Methods of operation
3 Application specific
4 Type of load
5 Method of interstage coupling etc
Classification of amplifier
⇒ Class A
⇒ Class B
⇒ Class AB
⇒ Class C
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4. Class A, B, AB, C
⇒ A Class A amplifier is one in which the operating point and the input
signal are such that the current in the output circuit (in the collector
or drain electrode) flows all at all the times. A class A amplifier
operates essentially over a linear portion of its characteristics.
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5. Continued-
⇒ A Class B amplifier is one in which the operating point is at an
extreme end of its characteristics, so that the quiescent power is very
small. If the signal voltage is sinusoidal then amplification takes place
only for half of the cycle.
⇒ A Class AB amplifier is one in which the operating point between the
two extremes defined for class A and class B.
⇒ A Class C amplifier is one in which the operating point is chosen so
that the output current (or voltage) is zero for more than one-half of
the input sinusoid signal cycle.
Note: Class AB and Class B operation are used with untuned power
amplifiers, whereas Class C amplifier is used with tuned radio frequency
amplifiers.
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6. Distortion in Amplifiers
⇒ Any output waveform is not an exact replica of the input-signal
waveform because of various distortion. It may arise due to
(a) Inherent nonlinearity in the characteristics of the transistor or FET.
(b) Influence of associated circuit.
Types of distortion
1. Nonlinear distortion
2. Frequency distortion
3. Phase distortion
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7. Nonlinear distortion
This type of distortion results from production of new frequency in the
output, which was not present in the input. Ex- Let x(t) and y(t) are
input and output of an electrical circuit.
y(t) = a0x(t) + a1x2
(t) + a2x3
(t) (1)
⇒ Eq (1) is nonlinear mathematical expression.
⇒ Let x(t) = cos ωt, Input signal frequency → ω
⇒ Output signal frequency → ω, 2ω, 3ω
This distortion is sometimes referred as amplitude distortion.
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8. Frequency distortion
This type of distortion exists when the signal components of different
frequencies are amplified differently. Let input signal x(t) is defined as
follow
x(t) = a0 cos ωt + a1 cos 2ωt + a2 cos 3ωt (2)
on the other side output signal as
y(t) = b0 cos ωt + b1 cos 2ωt + b2 cos 3ωt (3)
If
b0
a0
=
b1
a1
=
b2
a2
−→ No Frequency distotion
and
b0
a0
6=
b1
a1
6=
b2
a2
−→ Frequency distotion
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9. Phase-shift distortion
It results from unequal phase shifts of signals of different frequencies. Let
input signal x(t) is defined as
x(t) = a0 cos(ωt + θ0) + a1 cos(2ωt + θ1) + a2 cos(3ωt + θ2) (4)
on the other side output signal as
y(t) = b0 cos(ωt + φ0) + b1 cos(2ωt + φ1) + b2 cos(3ωt + φ2) (5)
If
θ0 − φ0 = θ1 − φ1 = θ2 − φ2 = K −→ No phase-shift distortion
and
θ0 − φ0 6= θ1 − φ1 6= θ2 − φ2 6= K −→ phase-shift distortion
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10. Frequency Response of an Amplifier
From above circuit, we find using the complex variable s.
V0(s) =
Vi (s)R1
R1 + 1
sC1
= Vi (s)
s
s + 1
R1C1
(6)
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