ee201 -chapter 4
DESCRIPTION
Introduction to feedback (block diagram and types of feedback) , Analysis at middle, low and high frequency of multi-stage amplifier with RC coupling and direct coupling, cascade amplifiers-Darlington Pair.TRANSCRIPT
EE201-SEMICONDUCTOR
DEVICES
CHAPTER 4 : MULTISTAGES
AMPLIFIER
Amplifiers containing one active device have limited gain, input impedance, output impedance and power handling capabilities.
Multistage amplifier is used to increase the gain or modify the characteristics of the amplifier to satisfy a particular specification.
Introduction
a) Cascode connection- Places one transistor on top of anotherb) Cascade connection- Provides stages in seriesc) Coupling - Circuitry used to connect the output of one
stage of a multistage amplifier to the input of next stage.
Working principles
Block diagram
The overall gain of a multistage amplifier is the product of the gain of the individual stage
Gain (A) = A1A2A3A4….AN
If the gain of each amplifier stage is expressed in decibels (dB) the total gain is the sum of the gain of individual stage
Gain in dB (A) = A1+A2+A3+A4+…AN
The Gain
Depending on the manner which the different amplifier stages are connected, one of the following amplifiers may result :
a) RC coupling @ capacitor couplingb) Direct coupling (CE-CC, Darlington pair)c) Transformer coupling
Type of multistage
RC COUPLING
One way to connect various stage is via capacitor Coupling capacitor is used to block the flow of dc
current between the output of one amplifier stage and the input of the next stage
Cont’
Make dc bias at the output of one stage is different from dc bias voltage at the input to next stage
Prevent the flow of dc signal current between stage
Provides enough high frequency to keep capacitive reactance small
Advantages
Affects the lower frequency response of amplifier
Disadvantage
DIRECT COUPLING
In this type of amplifier, the output of one stage of amplifier is connected to the input of the next stage directly without using any reactive component like capacitor, inductor.
As a result, the frequency response of this amplifier is quite flat all the way down to dc (0Hz)
Cont’
Simple circuit Can be used to amplify zero and low
frequency signal Used in differential and op-amp
Advantages
At high frequency, gain decrease Transistor parameter change with
temperature, hence output is affected by temperature variation
Any noise appearing at the input also present at the output in amplified form due to high gain
Disadvantages
DARLINGTON PAIR
Collector of two BJT are tied together and emitter of one is direct coupled to the base of the other
The combination is used in amplifier circuit as if were a single transistor having three terminal – B,C,E
Is important and useful configuration because has high B
BD = B1B2
Cont’
TRANSFORMER COUPLING
Transformer coupling is where the primary winding is in the output of one stage and secondary winding is in the input circuit of the following stage
The ac signal is passed from one stage to the next stage without the possibility of dc current flowing between the two windings
Cont’
Low power dissipation Capability of designing a turn ratio that
result in maximum power transfer between stage
Often used in narrow band application (RF amplifier)
Advantages
Poor frequency response due to inductance and inter-winding capacitance tend to reduce the usable bandwidth of these amplifier
Disadvantages
Frequency response curve of multistage
The greater the number of identical stages, the lower cut-off frequency is increasing and the upper cut-off frequency is decreasing
Therefore, this will reduce the bandwidth of the multistage amplifier
Cont’