Download Direct-Coupled Multistage Amplifiers

DR S & S S GANDHY ENGG. COLLEGE
Multistage Amplifiers
E.C. Sem 3
Enrollment no.
Submitted to :- Parul Mam
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Concept
 Two or more amplifiers can be connected in a cascaded
arrangement with the output of one amplifier driving the
input of the next.
 Each amplifier in a cascaded arrangement is known as a
stage.
 The basic purpose of a multistage arrangement is to
increase the overall voltage gain.
Multistage Amplifier
Multistage Voltage Gain
 The overall voltage gain, Av of cascaded amplifiers, as
shown in fig. is the product of the individual voltage
gains.
 Av = Av1Av2Av3 …Avn
 where n is no. of stages.
Multistage Amplifier
Multistage Voltage Gain
 Amplifier voltage gain is often expressed in decibels
(dB) as follows:
 Av(dB) = 20 log Av
 This is particularly useful in multistage systems
because the overall voltage gain in dB is the sum of the
individual voltage gains in dB.
 Av(dB) = Av1(dB) + Av2(dB) + . . . + Avn(dB)
Multistage Amplifier
Capacitively-Coupled Multistage
Amplifier
 For purposes of illustration, we will use the two-stage
capacitively coupled amplifier in fig.
 Notice that both stages are identical common-emitter
amplifiers with the output of the first stage
capacitively coupled to the input of the second stage.
 Capacitive coupling prevents the dc bias of one stage
from affecting that of the other but allows the ac signal
to pass without attenuation because XC  0 at the
frequency of operation.
 Notice, also, that the transistors are labeled Q1 and Q2.
Multistage Amplifier
Capacitively-Coupled Multistage
Amplifier
Multistage Amplifier
Loading Effects
 In determining the voltage gain of the first stage, you must consider the
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loading effect of the second stage.
Because the coupling capacitor C3 effectively appears as a short at the
signal frequency, the total input resistance of the second stage presents
an ac load to the first stage.
Looking from the collector of Q1, the two biasing resistors in the second
stage, R5 and R6, appear in parallel with the input resistance at the base
of Q2.
In other words, the signal at the collector of Q1 “sees” R3, R5, R6, and
Rin(base2) of the second stage all in parallel to ac ground.
Thus, the effective ac collector resistance of Q1 is the total of all these
resistances in parallel.
The voltage gain of the first stage is reduced by the loading of the
second stage because the effective ac collector resistance of the first
stage is less than the actual value of its collector resistor, R3.
Remember that Av = Rc/re.
Multistage Amplifier
Multistage Amplifier
Multistage Amplifier
Multistage Amplifier
Direct-Coupled Multistage
Amplifiers
 A basic two-stage, direct-coupled amplifier is shown in fig. Notice that there
are no coupling or bypass capacitors in this circuit.
 The dc collector voltage of the first stage provides the base-bias voltage for the
second stage.
 Because of the direct coupling, this type of amplifier has a better low-frequency
response than the capacitively coupled type in which the reactance of coupling
and bypass capacitors at very low frequencies may become excessive.
 The increased reactance of capacitors at lower frequencies produces gain
reduction in capacitively coupled amplifiers.
 Direct-coupled amplifiers can be used to amplify low frequencies all the way
down to dc (0 Hz) without loss of voltage gain because there are no capacitive
reactances in the circuit.
 The disadvantage of direct-coupled amplifiers, on the other hand, is that small
changes in the dc bias voltages from temperature effects or power-supply
variation are amplified by the succeeding stages, which can result in a
significant drift in the dc levels throughout the circuit.
Multistage Amplifier
Direct-Coupled Multistage
Amplifiers
Multistage Amplifier