MULTISTAGE AMPLIFIERS

UNIT-II

INTRODUCTION

If the voltage or power gain obtained from a

single stage small signal amplifier is not

sufficient for a practical application , we go

for more than one stage of amplification to

achieve required voltage and power gain.

Such an amplifier is called a multistage

amplifier

INTRODUCTION

In multistage amplifiers, the output of one stage is fed as the input to the next as shown in figure below

INTRODUCTION

A multistage amplifier using two or more single stage common emitter amplifier is called as cascaded amplifiers

A multistage amplifier with CE as the first stage and CB as the second stage is called as cascode amplifier.

Such cascade and cascode connections are also possible in FET amplifiers

Comparison of various multistage configurations

Use of coupling schemes When amplifiers are cascaded, it is necessary

to use a coupling network between the output of one amplifier and the input of the following amplifier. This type of coupling is called as inter stage coupling

These coupling networks serves the following– It transfers the ac output of one stage to the input

of the next stage – It isolates the dc conditions of one stage to the

next stage – It also provides impedance matching

Different coupling schemes

• Resistance –Capacitance couplingor

Capacitive Coupling

• Direct Coupling

• Transformer Coupling

Capacitive Coupling

Note that there is a single dc source (VCC), as well as the signal ground, common to both stages.

Direct Coupling

Direct coupling is just what it sounds like – two amplifier stages are direct coupled if the output of the first stage is connected to the input of the second without the use of capacitors.

Transformer Coupling

N1 gives the number of turns in the primary coil and N2 is the number of turns in the secondary coil. The turns ratio may be expressed as N1:N2 as shown in the figure, a:1 (where a=N1/N2), or 1:a (where a=N2/N1).

Frequency response

• The voltage gain and phase shift of the gain depend on the frequency range over which the amplifier operate

• In general the entire frequency range can be divided into three ranges– Mid frequency range– Low frequency range – High frequency range

Mid -Frequency range

• In this frequency range the voltage gain is practically constant and is not affected by the changes of the capacitances in the circuit

• The reactance Xc=1/WCc of the coupling capacitor in series between the amplifying stages is very small so that it can be neglected

• Therefore the voltage gain may be considered as Aom=1

Low frequency range

• In this frequency range , the circuit behaves like the simple high pass circuit as shown in figure

• From the circuit

Low frequency range

• Where • Therefore the magnitude of the voltage gain

at low frequency response is

• And the phase angle is

• At f=fL,

High frequency range• Above the mid band

frequency, the transistorbehaves like the simple lowpass circuit as shown in figure

• From the circuit,

High frequency range

• Therefore

• Let we get• Therefore the magnitude of the voltage gain is

• and phase is

• .

Frequency response

Analysis of cascaded RC coupled BJT amplifiers