1. Electronic Devices & Circuits
DR S & S S GANDHY ENGG. COLLEGE
Electronic Devices & Circuits
Multistage Amplifiers
E.C. Sem 3
Enrollment no.
Submitted to :- Parul Mam

2. 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

3. 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

4. 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

5. 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

6. Capacitively-Coupled Multistage Amplifier

7. Loading Effects
In determining the voltage gain of the first stage, you must consider the 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

8. Multistage Amplifier

9. Multistage Amplifier

10. Multistage Amplifier

11. 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

12. Direct-Coupled Multistage Amplifiers