Chapter 4 of the document covers multistage amplifiers, which enhance gain and modify characteristics using configurations like cascode and cascade connections, as well as different coupling methods such as RC, direct, and transformer coupling. Each method has distinct advantages and disadvantages, influencing frequency response and biasing. The overall gain of a multistage amplifier is determined by the product or sum of the gains of individual stages, depending on whether decibels or linear values are used.

1. EE201-SEMICONDUCTOR
DEVICES
CHAPTER 4 : MULTISTAGES
AMPLIFIER

2. Introduction
∗ 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.

3. Working principles
a) Cascode connection
- Places one transistor on top of another
b) Cascade connection
- Provides stages in series
c) Coupling
- Circuitry used to connect the output of one stage of a
multistage amplifier to the input of next stage.

4. Block diagram

5. The Gain
∗ 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

6. Type of multistage
∗ Depending on the manner which the different
amplifier stages are connected, one of the
following amplifiers may result :
a) RC coupling @ capacitor coupling
b) Direct coupling (CE-CC, Darlington pair)
c) Transformer coupling

7. RC COUPLING

8. Cont’
∗ 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

9. Advantages
∗ 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

10. Disadvantage
∗ Affects the lower frequency response of amplifier

11. DIRECT COUPLING

12. Cont’
∗ 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)

13. Advantages
∗ Simple circuit
∗ Can be used to amplify zero and low frequency signal
∗ Used in differential and op-amp

14. Disadvantages
∗ 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

15. DARLINGTON PAIR

16. Cont’
∗ 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

17. TRANSFORMER COUPLING

18. Cont’
∗ 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

19. Advantages
∗ 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)

20. Disadvantages
∗ Poor frequency response due to inductance and inter-
winding capacitance tend to reduce the usable
bandwidth of these amplifier

21. Frequency response curve of
multistage

22. Cont’
∗ 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