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’