transistor amplifier a part of multistage amplifier electronics class -12

2. MULTISTAGE AMPLIFIER
• INTRODUCTION:
• Generally ,voltage gain or power gain obtained from a single stage
amplifier is less. Therefore in order to achieve greater voltage and
power gain ,we have to use more than one stage of amplification.
• A circuit containing more than one stage of amplification is called
multistage amplifier.

3. AV =
Where, AV1 = V1
Vi V1
AV2 = V2 AV3 = V3
V2
X XX X
 Over all the voltage gain of a multistage amplifier is equal to the product of the gain
of the individual stages.

5. Gain in decibel(dB)
• The decibel is a logarithmic measurement of the ratio of one power to
another or one voltage to another.
• Usually ,the voltage gain or power gain of an amplifier is represented
in decibels.
Power gain in dB=10 log 10AP
10
10

6. • Note : 1dB=
1
10th
bel
Power gain in dB Voltage gain in dB Current gain in dB
1dB = log Po We know that,
Pi
Hence ,
Power gain in dB=10 log 10AP
Gp=
Current gain in dB=20 log 10Ai
G v =
Gì=

7. Why do we measure gain in dB?
• It allows us to denote, both very small as well as very large quantities
of linear scale.
• The human ear responds to the intensity of sound which is used by
logarithmic scale.
Note : The overall voltage gain of a multistage amplifier in decibels is given by

9. 2.
3.

10. Types of coupling amplifiers:
1. RC coupling amplifier.
2. Direct coupling amplifier.
3. Transformer coupling amplifier.

13. Advantages of RC coupling:
 It is inexpensive.
 It has good frequency response.
 It has low distortion.
 Overall gain is high.
Disadvantages of RC coupling:
• The gain is reduced because of loading effect.
• It cannot amplify very low frequency signal.
• Power loss due to large number of resistors.
Applications:
Used as a voltage amplifiers such as radio and TV receivers.

14. This amplifier is also called as DC amplifier and is used to amplify very low frequencies including Direct
current or zero frequency.
Two stage direct coupled amplifier:

15. • The output of first stage is directly connected to the base of the next transistor.
• Moreover ,there is no input and output coupling capacitors.
• Required DC conditions of the first stage are obtained from R1 and R2 [along with Vcc].
• The DC voltage of first stage provides the required DC bias for the second stage.
• The signal to be amplified is applied directly to the input of the first stage. Due to transistor
action, it appears in the amplified form across the collector resistor of transistor T1. This voltage
then drives the base of the second transistor T
• Advantages:
• 1. The circuit arrangement is very simple.
• 2. The circuit cost is less.
• 3. It can amplify very low frequencies [even zero frequency].
• Disadvantages:
• 1. It cannot amplify high frequency signals.
• 2. It has poor temperature stability.
Working:
 Applications:
 Used in electrocardiogram, computers and
instrumentation amplifiers.
 Regulator circuits.

16. • Capacitive reactance is defined as a opposition offered by the capacitor to the
flow of alternating current.
• It is denoted by
 Coupling capacitance CC : couples the Ac signal to next stage.

17. Frequency response of RC coupled amplifier
The curve drawn between voltage gain and the signal frequency of an amplifier is known
as frequency response.

18. At low frequency:
CC offers large reactance.
Hence, gain decreases with decrease in
frequencies.

19. • At high frequency:
•
CC offers very low reactance.
loading effect increases
voltage gain decreases

20. • At mid frequency:
As frequency increases,
XC decreases,
AV increases and
also at the same time,
decrease in XC
increases the loading effect
which decreases the gain AV .

21. LOW FREQUENCY RANGE MID FREQUENCY RANGE HIGH FREQUENCY
RANGE
At low frequency,
CC offers large reactance.
Hence, gain decreases with
decrease in frequencies.
At mid frequency,
As frequency increases,
XC decreases,
AV increases and
also at the same time,
decrease in XC increases the
loading effect which reduces
the gain AV .
Therefore, gain remains
almost constant.
At high frequency,
CC offers very low
reactance.
therefore loading effect
increases which
reduces the voltage
gain.

22. Frequency response of direct coupled amplifier