Introduction to feedback (block diagram and types of feedback) , Analysis at middle, low and high frequency of multi-stage amplifier with RC coupling and direct coupling, cascade amplifiers-Darlington Pair.

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