Short slides on Multistage Transistor and Feedback.

2. Multistage
Transistor Amplifier
-Define Multistage Transistor
Amplifier?
-Write down the names of multistage
transistor amplifier?
Mohammad Shakirul Islam
151-15-311

3. MULTISTAGE TRANSISTOR AMPLIFIER
A transistor circuit containing more than one stage of
amplification is known as multistage transistor amplifier.
Fig: Block diagram of a 3-stage amplifier
In a multistage amplifier, a number of single
amplifiers are connected.

4. THE NAMES OF MULTISTAGE
TRANSISTOR AMPLIFIER?
a) R-C coupled amplifier
b) Transformer coupled amplifier
c) Direct coupled amplifier

5. (i) In RC coupling, a capacitor is used as the coupling device. The capacitor connects
the output of one stage to the input of the next stage in order to pass the a.c. signal on
while blocking the d.c. bias voltages.
(ii) In transformer coupling, transformer is used as the coupling device. The
transformer coupling provides the same two functions but permits in addition
impedance matching.
(iii) In direct coupling or d.c. coupling, the individual amplifier stage bias conditions
are so designed that the two stages may be directly connected without the necessity
for d.c. isolation.
THE NAMES OF MULTISTAGE
TRANSISTOR AMPLIFIER?

6. Jannatul
Ferdous
Id: 151-15-
4888

7. WHAT IS
FEEDBACK?
The process of injecting a fraction of output
energy of device back to the input is known as
feedback.
Principles of Electronic Feedback:
1. Positive Feedback.
2. Negative Feedback.

9. SHAMMI AKTER
Define Rc coupled
transistor amplifier &
mention why transformer
coupling is used?
ID NO : 151-15-4970
RC Coupled Trasistor Amplifier &
Transformer Coupled Amplifier edback

10. RC COUPLED TRANSISTOR AMPLIFIER

11. TRASFORMER-COUPLED AMPLIFIER

12. WE USE IT BECAUSE.....
 No signal power is lost in the collector or base resistor.
 An excellent impedance matching can be achieved in a
transformer coupled amplifier.
 Due to excellent impedance matching, it provides higher gain.

13. H.M TASFIK KIBRIA
151-15-4985

14. Positive feedback: When the feedback energy (voltage or current) is in phase with the input
signal and thus aids it, it is called positive feedback.
In this Figure, both amplifier and feedback network introduce a phase shift of 180°. The
result is a 360° phase shift around the loop, causing the feedback voltage Vf to be in phase
with the input signal Vin.

15. Negative feedback: When the feedback energy (voltage or current) is out of phase with the
input signal and thus opposes it, it is called negative feedback.
In This figure, As we can see, the amplifier introduces a phase shift of 180° into the circuit
while the feedback network is so designed that it introduces no phase shift (i.e., 0° phase shift).
The result is that the feedback voltage Vf is 180° out of phase with the input signal Vin.

16. Hasanul Ferdous
151-15-5197

17. Short Notes
Gain: The ratio of the output electrical quantity to the input one of the amplifier is
called its gain.
it can be current gain or voltage gain or power gain.
The gain of a multistage amplifier is equal to the product of gains of individual
stages. For instance, if G1, G2 and G3 are the individual voltage gains of a three-
stage amplifier, then total voltage gain G is given by :
G = G1 × G2 × G3

18. Frequency response: The voltage gain of an amplifier varies with signal frequency. It is
because reactance of the capacitors in the circuit changes with signal frequency and hence affects
the output voltage.
The curve between voltage gain and signal frequency of an amplifier is known as frequency
response. The gain of the amplifier increases as the frequency increases from zero till it becomes
maximum at fr, called resonant frequency. If the frequency of signal increases beyond fr, the gain
decreases.

19. Decibel Gain: Although the gain of an amplifier can be expressed as a number, yet it is of
great practical importance to assign it a unit. The unit assigned is bel or decibel (db). The common
logarithm (log to the base 10) of power gain is known as bel power gain i.e.
Power Gain= log10 (Pout/Pin ) bel
1 bel = 10 db
If the two powers are developed in the same resistance or equal resistances, then,
P1 = (Vin
2 / R) = Iin
2 R
Pout = (Vout
2 / R) = Iout
2 R

20. Bandwidth: The range of frequency over which the voltage gain is equal to or greater than
70.7% of the maximum gain is known as bandwidth.
The voltage gain of an amplifier changes with frequency. Referring to the frequency response
in Fig, it is clear that for any frequency lying between f1 and f2 the gain is equal to or greater
than 70.7% of the maximum gain. Therefore, (f1 - f2 ) is the bandwidth.

21. SADEKA HAQUE
ID-151-15-5210
NEGATIVE FEEDBACK
Define Negative
Feedback & also mention
the advantage of
Negative feedback.

22. What is Negative Feedback ???
Negative feedback. When the feedback energy (voltage or current) is out of
phase with the input signal and thus opposes it, it is called negative
feedback.
Fig : Negative Feedback

23. 1. GAIN STABILITY
2. NOISE REDUCTION
3. REDUCTION IN NONLINEAR DISTORTION
4. BANDWIDTH CAN BE INCREASED
5. INCREASE IN INPUT IMPEDENCE
6. DECREASE IN OUTPUT IMPEDENCE
Advantage Of Negative Feedback Advantages
of Negative Feedback

24. Md. Ferdouse Ahmed Foysal
ID-151-15-5274
PRINCIPLES OF NEGATIVE FEEDBACK

25. 1. When negative voltage feedback is applied, the gain of the amplifier is
reduced. Thus, the gain of above amplifier without feedback is 10,000
whereas with negative feedback, it is only 100.
Fig: Principles of negative voltage feedback in an amplifier.

26. Fig: Principles of negative voltage feedback in an amplifier.
2. When negative voltage feedback is employed, the voltage actually
applied to the amplifier is extremely small. In this case, the signal
voltage is 101 mV and the negative feedback is 100 mV so that voltage
applied at the input of the amplifier is only 1 mV.

27. Fig: Principles of negative voltage feedback in an amplifier.
3. n a negative voltage feedback circuit, the feedback fraction m is
always between 0 and 1.

28. Fig: Principles of negative voltage feedback in an amplifier.
The gain with feedback is sometimes called closed-loop gain while the gain without
feedback is called open-loop gain. These terms come from the fact that amplifier and
feedback circuits form a “loop”. When the loop is “opened” by disconnecting the feedback
circuit from the input, the amplifier's gain is Av , the “open-loop” gain. When the loop is
“closed” by connecting the feedback circuit, the gain decreases to Avf , the “closed-loop”
gain.

29. Shahariar Rabby
151-15-5424
VOLTAGE GAIN WITH NEGATIVE
FEEDBACK

30. Here,
Av = Gain;
Avf = Gain with Feedback;
eg = Signal Voltage;
e0 = Output Voltage;
mv = Fraction Of Output Voltage
Feedback;

31. The output e0 must be equal to the
input voltage eg − mv e0 multiplied
by gain Av of the amplifier i.e.,

32. But e0 / eg is the voltage gain of
the amplifier with feedback.
∴ Voltage gain with negative feedback is

33. It may be seen that the gain
of the amplifier without
feedback is Av. However,
when negative voltage
feedback is applied, the
gain is reduced by a factor
1 + Av mv. It may be noted
that negative voltage
feedback does not affect
the current gain of the
circuit.