Govt. WPC, Thrissur
Electronic Circuits
Module -1
Syllabus: Transistor biasing – need - load line – operating point – stabilization of operating point Biasing circuits – requirements - list - fixed and voltage divider bias circuits. Single Stage CE Amplifier
with voltage divider biasing - the principle of operation - expression for voltage gain, current gain,
power gain, input and output impedances– simple problems - frequency response – bandwidth. Emitter
follower –circuit diagram - features – applications. Multistage amplifier - need – overall gain – gain in
dB – simple problems - methods of inter-stage coupling - RC coupled, transformer coupled, and direct
coupled multistage amplifiers - working principle - frequency response - applications – comparison
Transistor biasing

It is the process of setting a transistor DC operating voltage or current conditions to the correct
level so that any AC input signal can be amplified correctly by the transistor OR

It is defined as the
o
proper flow of zero signal Ic and
o
the maintenance of proper Vce during the passage of a signal.
Biasing circuit

The circuit that provides biasing to the transistor is known as a ‘biasing circuit’.
Need for biasing

To stabilize the Q point against
o
Changes in temperature
o
Variation in transistor parameters
o
Aging of components

To set the Q point at the center of the DC load line

T operate the transistor in active region as an amplifier
DC load line

A line drawn on the output characteristics of the transistor circuit which gives the value of Ic
and Vce when no signal is applied.
AC load line

A line drawn on the output characteristics of the transistor circuit which gives the value of Ic
and Vce when signal is applied.
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Q point (Operating point)

It is the zero signal values of Ic and Vce.

It is the point where the transistor can be operated efficiently to get maximum output.

Q point must be located in the active region of the transistor to get distortion-free output.
Fig 1: Load lines and Q point
Stabilisation

The process of making Q point independent of temperature changes or inherent variation in
transistor parameters is called stabilisation.
Unstabilisation
A transistor or amplifier is unstabilised when its Q point is changed.
Causes of Unstabilisation

It occurs because of a change in Ic

Ic changes due to
o
Change in temperature
o
Inherent variation in transistor parameters
Thermal Runaway
The self-destruction of an unstabilised transistor due to rise in temperature is called thermal run away.
Figure 2: Thermal runaway process
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Methods for achieving stabilization

Bias Stabilization:
Use Fixed Bias, Voltage Divider Biasing methods for necessary biasing

Thermal Stabilization:
Use Heat Sinks to prevent temperature rise thereby preventing thermal runaway.
Stability factor

The rate of change of collector current IC with respect to the collector leakage current ICO at
constant β and IB is called Stability factor.

Lower the S value, higher the thermal stability of transistor

Ideal value of Stability factor =1
Methods of transistor biasing

Base Resistor or Fixed biasing

Voltage-divider biasing

Emitter resistor biasing

Feedback resistor biasing
Fixed biasing method/ Base resistor biasing

Basic Configuration:
 A single resistor 𝑅𝐵 is connected between the base of the transistor and the supply
voltage Vcc.
 The emitter is usually grounded (for NPN transistors).

Operation:
 The base resistor 𝑅𝐵 sets the base current 𝐼𝐵
 The base-emitter voltage determines 𝐼𝐸 and hence the collector current 𝐼𝑐

Stability factor S = β+1.
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
Advantages:
 Simple circuit
 Uses few components
 Disadvantages
 Poor thermal stability (High stability factor)
 Poor stabilisation
Figure 3: Fixed biasing
Voltage divider biasing
 Configuration
 Two resistors R1 and R2 are connected to VCC and provide necessary biasing.
 The resistor RE in the emitter provides stabilization.
 The name voltage divider comes from the voltage divider formed by R1 and R2.
 Operation
 The voltage drop across R2 forward biases the base-emitter junction
 This causes the base current and hence collector current flow in the zero signal
conditions
,
 Stability factor =1
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 Advantages:
 Provides high stabilization
 High thermal stability
 Disadvantages
 Requires more components
 Design and analysis is complex
Figure 5: Voltage divider biasing
Amplification
The process of increasing the strength/amplitude of a weak signal is called amplification
Types of Amplifiers

Based on the number of stages of Amplification
 Single-stage Amplifiers −This has only one transistor circuit, which is a single-stage
amplification.
 Multi-stage Amplifiers −This has multiple transistor circuit, which provides multistage
amplification.

Based on the parameter that is amplified at the output
 Voltage Amplifiers −The amplifier circuit that increases the voltage level of the input
signal, is called a Voltage amplifier.
 Power Amplifiers −The amplifier circuit that increases the power level of the input signal,
is called a Power amplifier.

Based on the method of coupling one stage to the other
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 RC Coupled amplifier − A Multi-stage amplifier circuit that is coupled to the next stage
using a resistor and capacitor (RC) combination can be called an RC coupled amplifier.

Transformer Coupled amplifier − A Multi-stage amplifier circuit that is coupled to the
next stage, with the help of a transformer, can be called a Transformer coupled amplifier.
 Direct Coupled amplifier − A Multi-stage amplifier circuit that is coupled to the next stage
directly, can be called a direct coupled amplifier.

Based on the Transistor Configuration
 CE amplifier − The amplifier circuit that is formed using a CE-configured transistor
combination is called a CE amplifier.
 CB amplifier − The amplifier circuit that is formed using a CB-configured transistor
combination is called a CB amplifier.

CC amplifier − The amplifier circuit that is formed using a CC-configured transistor
combination is called a CC amplifier.
Single-stage CE amplifier
 When 1 transistor with its associated circuitry is used to increase the strength of the weak
signal, the circuit is known as a single-stage transistor amplifier.
 Here, the transistor works in the Common emitter configuration.
 Working:
 When a weak signal (Vin) is applied to the base of the transistor, a small current 𝐼𝐵
flows through it.
 This increases the Collector Current𝐼𝐶 as (𝐼𝐶 =β×𝐼𝐵 )
 The value of Rc is high, therefore large voltage appears across it(𝑉𝑜𝑢𝑡 =𝐼𝑐 ×𝑅𝑐 )

Components and functions
 Resistors R1 and R2 (Biasing resistors)
Form a voltage divider network to provide the necessary biasing
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 Resistor 𝑅𝐸 (Emitter Resistor)
Provides stability to the amplifier
 Coupling Capacitor (𝐶1 )
Couples the input signal to the base of the transistor while blocking DC components.
 Bypass Capacitor (𝐶𝐸 )
Bypasses the emitter resistor (𝑅𝐸 ) for AC signals
 Collector Resistor (𝑅𝐶 )
Determines the voltage gain of the amplifier and helps in converting the amplified
current signal to a voltage signal.
 Coupling Capacitor ((𝐶2 )
Couples the amplified signal from the collector to the next stage or output while
blocking DC components
 Gain:
𝑅𝐴𝐶 =(𝑅𝐶 ×𝑅𝐿 )/( 𝑅𝐶 +𝑅𝐿 )
Problems
1. An amplifier circuit is shown in the figure below. If the input voltage is 1 mV, find the output
voltage. Assume β=80 and 𝑅𝑖𝑛 =2KΩ
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Ans:
AC load resistance, 𝑹𝑨𝑪 = (𝑹𝑪 ×𝐴𝑉 𝑹𝑪 +𝑹𝑳 )
= (6×6)/(6+6)
= 3kΩ
𝑨𝑽 = β× (𝑹𝑨𝑪 /𝑹𝒊𝒏 )
Voltage gain,
= 80×(3/2)
= 120
𝑨𝑽
=
(Vout/Vin)
120
=
(Vout/ 1mV)
Therefore Vout = 120×1mV=120mV
2. In a transistor amplifier when a signal changes by 0.012 V, the base and collector current
change by 10µA and 0.8 mA respectively. If the collector load Rc=4KΩ, and 𝑅𝐿 =6KΩ,
determine the a) current gain b) input impedance c) ac load d) voltage gain e) power gain
Ans:
∆𝐼𝐵 = 10µA, ∆𝐼𝐶 = 0.8mA, ∆𝑉𝐵𝐸 = 0.012V
𝑅𝐿 = 6kΩ, 𝑅𝑐 =4kΩ
Current gain, β= (∆𝑰𝑪 /∆𝑰𝑩 )
= 0.8mA/10µA
= 80
Input impedance, 𝑹𝒊𝒏 = (∆𝑽𝑩𝑬 /∆𝑰𝑩 )
= 0.012V/ 10 µA
= 1.2KΩ
AC load resistance, 𝑹𝑨𝑪 = (𝑹𝑪 ×𝑹𝑳 )/( 𝑹𝑪 +𝑹𝑳 )
= (4×6)/(4+6)
= 2.4kΩ
Voltage gain 𝑨𝑽 = β× (𝑹𝑨𝑪 /𝑹𝒊𝒏 )
= 80×(2.4/1.2)
= 160
Power gain 𝑨𝑷 = 𝜷𝟐 × (𝑹𝑨𝑪 /𝑹𝒊𝒏 )
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= 80×80×(2.4/1.2)
= 12800
Frequency Response of RC Coupled Amplifier

Frequency response curve is a graph that indicates the relationship between voltage gain and
function of frequency.
Figure 6: Frequency response

At low frequencies (<50Hz)
 The reactance of coupling capacitor CC is quite high; hence, a small part of the signal
will pass from one stage to the next.
 The voltage gain falls at low frequencies.

At high frequencies (>20 KHz)
 The reactance of CC is very small and it behaves as a short circuit.
 The voltage gain falls at high frequencies.

At mid frequencies (50 Hz to 20 KHz)
 The voltage gain of the amplifier is constant
Bandwidth

The range of frequencies for which the gain is greater than or equal to 70.7 % of the
maximum value of gain.

In Figure 6, The range of frequencies f1 to f2 is called the bandwidth

The lower value of frequency f1 is called lower cut-off frequency.

The higher value of frequency f2 is called higher cut-off frequency.
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Emitter follower

The Emitter-follower circuit is also known as a common collector configuration

It is called the emitter follower configuration because the emitter voltage follows the base
voltage.

It is mostly used as a voltage buffer.

Features
 High input impedance and low output impedance
 No voltage gain. The voltage gain is nearly 1.
 Input and output ac voltages are in phase

The biasing is provided either by the base resistor method or by the potential divider method
Figure 7: Emitter follower

Applications:
 Buffer Amplifier
 Impedance matching
Multistage amplifier

A multistage amplifier is an electronic amplifier consisting of two or more single-stage
amplifiers connected together.

The output of the first stage is coupled to the input of the next stage using a coupling device.

The process of joining two amplifier stages using a coupling device can be called as Cascading.
Figure 8: 2-stage amplifier
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Purpose of the coupling device

To transfer the AC from the output of one stage to the input of the next stage.

To block the DC from passing from the output of one stage to the input of the next stage.
Need for multistage amplifier/ Advantages

Increased Gain

Stability and Noise Reduction

Frequency Response Improvement
Gain of multistage amplifier

The overall gain of a multistage amplifier is the product of the gains of the individual stages :
Gain (A) = A1×A2×A3 ×A4 ×... ×An.
Where A1 is the gain of the first stage
A2 is the gain of the second stage
An is the gain of the nth stage

If the gain of each amplifier stage is expressed in decibels (dB), the total gain is the sum of
the gains of the individual stages:
Gain in dB (A) = A1 + A2 + A3 + A4 + ... An
Gain in decibel

Voltage gain 𝐴𝑣 = (Vout/ Vin)

In decibel , voltage gain = 20 𝑙𝑜𝑔10 (Vout/ Vin) dB

Power gain = (Pout/Pin)

In decibel , voltage gain = 10 𝑙𝑜𝑔10 (Pout/ Pin) dB
Reasons for expressing gain in Decibel

Using dB, gains of cascaded stages can be added instead of multiplied.

dB scale handles very large or very small values more easily.

dB unit Matches human perception and simplifies handling wide value ranges.
Problems
1. Consider a three-stage amplifier with the following stage gains:
Stage 1 gain Av1: 10,
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Stage 2 gain Av2: 20,
Stage 3 gain Av3: 5
Find overall gain?
Ans: Overall gain Av= Av1×Av2×Av3
= 10×20×5
= 1000
Overall gain in dB = 20 𝑙𝑜𝑔10 Av dB
= 20 𝑙𝑜𝑔10 1000 dB
= 60
2. Given

The output voltage Vout is 10 volts.

The input voltage Vin is 0.1 volts.
Calculate the voltage gain Av in dB?
Ans: Gain in dB= 20 𝑙𝑜𝑔10 (Vout/ Vin) dB
= 20 𝑙𝑜𝑔10 (10/ 0.1) dB
=20 𝑙𝑜𝑔10 100 dB
=40
3. Given:

The output power Pout is 100 milliwatts (mW).

The input power Pin is 1 milliwatt (mW).
Calculate the power gain Ap in dB?

Ans: Power gain Ap = 10 𝑙𝑜𝑔10 (Pout/ Pin) dB
= 10 𝑙𝑜𝑔10 (100 mW/ 1 mW) dB
= 10 𝑙𝑜𝑔10 100 dB
=20
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RC-coupled transistor multistage amplifier
Figure 9: RC coupled multistage amplifier

A coupling capacitor Cc is used to connect the output of first stage to the input of next
stage.

The resistances R1, R2 and RE form the biasing and stabilization network

The coupling capacitor CC transmits a.c. signal but blocks d.c.

Working:
 When an AC signal is applied to the base of the first transistor, it gets amplified
and appears across its collector load Rc
 The amplified signal from the collector load RCR_CRC is passed to the base of
the next stage through a coupling capacitor Cc.
 The second transistor stage further amplifies the signal received from the first
stage.
 By cascading multiple stages in this manner, the signal undergoes successive
amplification, resulting in an overall increase in gain.

Total gain is less than the product of individual gain due to loading effect.

Frequency response:
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
Advantages:
 Excellent frequency response
 Low cost
 Compact circuit

Disadvantages
 Smaller voltage and current gain
 Poor impedance matching

Application
 Widely used in audio equipment such as stereo amplifiers, home theatre
systems, and musical instrument amplifiers
 Used for amplifying signals in telephone lines, modems, and networking
devices.
Transformer coupled Multistage amplifier
Figure 10: Transformer-coupled amplifier

A transformer-coupled amplifier is a type of amplifier circuit that uses a coupling
transformer to connect the previous stage to the next stage.

The collector load is replaced by the primary winding of the transformer.

The secondary winding is connected between the potential divider and the base of 2nd
stage, which provides the input to the 2 nd stage.
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
Frequency response;
 The gain of the amplifier is constant only for a small range of frequencies
 Therefore, the amplification of audio signals will not be uniform, and it may
introduce frequency distortion.

Advantages:
 It provides excellent impedance matching.
 Gain achieved is higher.
 Efficient in operation

Disadvantages
 Frequency distortion is higher.
 Transformers produce hum noise.
 Transformers are bulky and costly.
 Poor frequency response

Applications:
 Used widely for the amplification of radio frequency signals i.e. above 20
KHz
 Mostly used for impedance matching purposes.
 Used for Power amplification.
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Direct coupled Amplifier
Figure 10: Directly coupled amplifier

The output of one stage is directly coupled to the next stage through a simple wire without
any coupling device.

In this method, complementary transistors are used.

If npn is used in the first stage, pnp is used in the next stage.

No coupling devices are used

Frequency response:

Advantages:
 The circuit arrangement is simple
 The circuit is of low cost because of the absence of expensive coupling devices

Disadvantages
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 It cannot be used for amplifying high frequencies.
 The operating point is shifted due to temperature variations.

Applications:
 Low frequency amplifications.
 Low current amplifications
Comparison of multistage amplifiers
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