Introduction to
Basic Electronics
Lecture - 6
Basic Electronics
• Signal Amplifier
- Introduction
- Operational Amplifier
- Power Amplifier
: Class A,B,AB
- Distortion
Introduction to Amplifiers
 In "Electronics", signal amplifiers are widely used devices as they
have the ability to amplify a relatively small input voltage signal, for
example from a sensor or microphone, into a much larger output
signal to drive a relay, lamp or loudspeaker for example.
 There are many forms of amplifiers, from Operational Amplifier
and Small Signal Amplifiers up to Large Signal and Power-Amplifiers.
 Amplifiers can be thought of as a simple box or block containing the
amplifying device, such as a Transistor, Field Effect Transistor or Opamp, and which has two input terminals and two output terminals with
the output signal being greater than that of the input signal, being
"Amplified".
Introduction to Amplifiers
 An amplifier has three main properties,
- Input Resistance (Rin)
- Output Resistance(Rout)
- Gain(A).
 No matter how complicated an amplifier circuit is, a general amplifier
model can be used to show the relationship of these three properties.
Ideal Amplifier Model
Amplifier Gain(A)
 The gain of an amplifier can be said to be the relationship that exists
between the signal measured at the output with the signal measured at
the input.
Amplifier Gain(A)
 The power Gain of the amplifier can also be expressed in Decibels,
(dB).
 To calculate the gain of the amplifier in Decibels or dB, we can use the
following expressions.
Voltage Gain in dB:
av = 20 log Av
Current Gain in dB:
ai = 20 log Ai
Power Gain in dB:
ap = 10 log Ap
Operational Amplifier(OP-AMP)
Two Basic Operational Amplifier Circuits
Operational Amplifier(OP-AMP)
 The Operational Amplifier, or Op-amp as it is most commonly
called, is an ideal amplifier with infinite Gain and Bandwidth when
used in the Open-loop mode with typical d.c. gains of 100,000, or
100dB.
 The basic construction is of a 3-terminal device, 2-inputs and 1-output.
An Operational Amplifier operates from a dual positive (+V) and an
corresponding negative (-V) supply but they can also operate from a
single DC supply voltage.
 It has Infinite Input impedance, (Z∞) resulting in "No current
flowing into either of its two inputs" and zero input offset voltage
"V1 = V2". It also has Zero Output impedance, (Z=0).
Operational Amplifier(OP-AMP)
 Op-amps sense the difference between the voltage signals applied to
the two input terminals and then multiply it by some pre-determined
Gain, (A).
This Gain, (A) is often referred to as the amplifiers "Open-loop Gain".
Op-amps can be connected into two basic circuits, Inverting and Noninverting.
Operational Amplifier(OP-AMP)
Gain Bandwidth Product
Operational Amplifier(OP-AMP)
 For Negative feedback, where the fed-back voltage is in "Anti-phase"
to the input the overall gain of the amplifier is reduced.
 For Positive feedback, where the fed-back voltage is in "Phase" with
the input the overall gain of the amplifier is increased.
 By connecting the output directly back to the negative input terminal,
100% feedback is achieved resulting in a Voltage Follower (buffer)
circuit with a constant gain of 1 (Unity).
 Changing the fixed feedback resistor (Rf) for a Potentiometer, the
circuit will have Adjustable Gain.
 The Differential Amplifier produces an output that is proportional
to the difference between the 2 input voltages.
Operational Amplifier(OP-AMP)
Differential and Summing Operational Amplifier Circuits
Operational Amplifier(OP-AMP)
 Adding more input resistor to either the inverting or non-inverting
inputs Voltage Adders or Summers can be made.
 Voltage follower op-amps can be added to the inputs of Differential
amplifiers to produce high impedance Instrumentation amplifiers.
 The Integrator Amplifier produces an output that is the mathematical
operation of integration.
 The Differentiator Amplifier produces an output that is the
mathematical operation of differentiation.
 Both the Integrator and Differentiator Amplifiers have a Resistor and
Capacitor connected across the op-amp and are affected by its RC time
constant.
Operational Amplifier(OP-AMP)
Differentiator and Integrator Operational Amplifier Circuits
Power Amplifiers
 Small signal amplifiers are generally referred to as "Voltage"
amplifiers as they convert a small input voltage into a much larger
output voltage.
 Sometimes an amplifier is required to drive a motor or feed a
loudspeaker and for these types of applications where high switching
currents are needed Power Amplifiers are required.
 The main function of Power amplifiers (also known as large signal
amplifiers) is to deliver power.
 The power amplifier works on the basic principle of converting the
DC power drawn from the power supply into an AC voltage signal
delivered to the load.
Amplifier Classes
 By changing the amplifiers Base bias voltage different ranges or modes
of operation can be obtained and these are categorized according to their
Class.
 Audio Power Amplifiers are classified in order according to their
circuit configurations and mode of operation being designated different
classes of operation in alphabetical order such as A, B, C, AB, etc.
 These different classes of operation range from a near linear output but
with low efficiency to a non-linear output but with a high efficiency.
 There are typical maximum efficiencies for the various types or class of
amplifier, with the most commonly used being:
 Class A
- a maximum theoretical efficiency of less than 40%
 Class B
- with a maximum theoretical efficiency of about 70%
 Class AB - which an efficiency rating between that of Class A and
Class B
Class A
 Class A Amplifier operation is were the entire input signal
waveform is faithfully reproduced at the amplifiers output as the
transistor is perfectly biased within its active region, thereby never
reaching either of its Cut-off or Saturation regions.
Class A
Single Ended Amplifier Circuit
Class B
 Unlike the Class A amplifier above that uses a single transistor for its
output stage, the Class B Amplifier uses two complimentary
transistors (an NPN and a PNP) for each half of the output waveform.
 One transistors for the positive half of the waveform and another for
the negative half of the waveform. This means that each transistor
spends half of its time in the Active region and half its time in the Cutoff region.
Class B
Class B
Class AB
 The Class AB Amplifier is a compromise between the Class A and
the Class B configurations.
 While Class AB operation still uses two complementary transistors in
its output stage a very small biasing voltage is applied to the Base of the
transistor to bias it close to the Cut-off region when no input signal is
present.
 An input signal will cause the transistor to operate as normal in its
Active region thereby eliminating any crossover distortion. A small
Collector current will flow when there is no input signal but it is much
less than that for the Class A amplifier configuration.
 This means then that the transistor will be "ON" for more than half a
cycle of the waveform. This type of amplifier configuration improves
both the efficiency and linearity of the amplifier circuit compared to
Class A.
Class AB
Class AB
Amplifier Distortion
 Distortion of the signal waveform may take place because:
1. Amplification may not be taking place over the whole signal cycle
due to incorrect biasing.
2. The input may be too large, causing the amplifier to limit.
3. The amplification may not be linear over the entire frequency
range of inputs.
 This means then that
during the amplification
process of the signal
waveform, some form
of Amplifier Distortion
has occurred.
Amplitude Distortion
 Amplitude distortion occurs when the peak values of the frequency
waveform are attenuated causing distortion due to a shift in the Q-point
and amplification may not take place over the whole signal cycle.
 This non-linearity of the output waveform is shown below.
Amplitude Distortion due to Incorrect Biasing
Frequency Distortion
 Frequency Distortion occurs in a transistor amplifier when the level
of amplification varies with frequency.
 Many of the input signals that a practical amplifier will amplify consist
of the required signal waveform called the "Fundamental Frequency" plus
a number of different frequencies called "Harmonics" superimposed onto
it.
Frequency Distortion
 Normally, the amplitude of these harmonics are a fraction of the
fundamental amplitude and therefore have very little or no effect on the
output waveform.
 However, the output waveform can become distorted if these harmonic
frequencies increase in amplitude with regards to the fundamental
frequency.
Phase Distortion
 Phase Distortion or Delay Distortion occurs in a non-linear
transistor amplifier when there is a time delay between the input signal
and its appearance at the output.
Phase Distortion
 If we call the phase change between the input and the output zero at
the fundamental frequency, the resultant phase angle delay will be the
difference between the harmonic and the fundamental.
 This time delay will depend on the construction of the amplifier and
will increase progressively with frequency within the bandwidth of the
amplifier.