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Some Important Amplifier parameters

Voltage gain Current gain Input Impedance Output Impedance Bandwidth

These parameters are nearly constant for a given amplifier.

But some times we need to change these parameters.

This can be achieved by a technique known as

FEEDBACKFEEDBACK

FEEDBACK

When a part of output is combined to the input, feedback is said to be exist.

Thus the process of combining output energy or signal (current or voltage) back to the input is known as feedback.

It is of 2 types. Negative Feedback Positive Feedback

Comparison of Feedbacks Comparison of Feedbacks

-ve Feedback +ve Feedback

Output signal is out of phase with the input signal.

Due to this, magnitude of input signal reduces.

Otherwise called Degenerative or Inverse FB.

Output signal is in phase with the input signal.

Due to this, magnitude of input signal increases.

Otherwise called Regenerative or Direct FB.

Advantages of Feedback(s)

Advantages: Gain Stabilization Reduction in Distortion Reduction in Noise Increase in Input

Impedance Decrease in Output

Impedance Increase in range of

uniform amplification.

Advantages: Increase in Gain

-ve Feedback +ve Feedback

-ve Feedback +ve Feedback

Reduction in Gain

Increase in Distortion

Decrease in Stability.

Disadvantage of +ve Feedback

A

β

RLVo

+

-Vin

βVo

SIMPLE BLOCK DIAGRAM OF A FEEDBACK AMPLIFIER

Vs

SAMPLING

NETWORK

AMPLIFIER WITH GAIN

A

FEEDBACK NETWORK

WITH GAIN β

Vi

Vo

RL

Vf

Mixer or Comparator

Vs + vf

TOPICS

Derivation of equation for closed loop voltage gain or transfer gain

Proof for Advantages of Feedback Classification of feedback amplifiers Topologies based on samplers and mixers

AMPLIFIER WITH GAIN

‘A’Vo

+

-Vin

SIMPLE BLOCK DIAGRAM OF AMPLIFIER WITHOUT FEEDBACK

A= Open Loop voltage gain of an amplifier

β = voltage gain of feedback network

Af = closed loop voltage gain of an amplifier

Advantages of –ve Feedback

• Transfer Gain• Increase in BW• Decrease in Distortion• Decrease in Noise• Increase in Stability

• BACK

bandwidth Range of frequencies for which an

amplifier provides best performance is called BW.

Two frequencies take part in this bandwidth

Lower cut-off frequency (f1)and

Upper cut-off frequency (f2)

BW = f2 - f1

EFFECT OF fb ON BANDWIDTH

Af = closed loop voltage gain of an amplifier β = voltage gain of feedback network Am = gain of amplifier at mid frequency Al = gain of amplifier at low frequencies Ah = gain of amplifier at high frequencies f1 = lower cut off frequency of amplifier without FB f2 = higher cut off frequency of amplifier without FB

Large signal is applied such that transistor is slightly out of linear operation.

Due to this, output signal is slightly distorted. Now if –ve feedback is introduced in the

circuit, voltage gain slightly reduces. Output signal amplitude remains the same. Now if we measure distortion in both the

cases, it can be found that it is reduced by a factor of (1+Aβ).

D ?= D/ (1+Aβ) BACK

Noise always exist in an amplifier With the application of feedback,

magnitude of noise reduces by (1+Aβ) similar to distortion.

N ?= N/ (1+Aβ)

BACK

OHP SHEET NO-3 Sensitivity is now defined as the ratio of

percentage change in voltage gain with feedback to the ratio of percentage change in voltage gain without feedback.

De-sensitivity is the inverse of sensitivity.

BACK

Depending upon the magnitudes of input impedance and output impedance w.r.t source resistance (Rs) and load resistance (RL), amplifiers are classified into four types.› Voltage Amplifiers› Current Amplifiers› Trans-conductance Amplifiers› Trans-resistance Amplifiers

› Voltage Amplifiers: Output Voltage Vo is proportional to signal

voltage Vs. Refer OHP sheet – 4

› Current Amplifiers:

Output Current (Io) or (IL) is proportional to signal

current Is. Refer OHP Sheet – 4

› Trans-conductance Amplifiers: Output Current (Io) or (IL) is proportional to input

voltage Vs. Refer OHP Sheet – 5

› Trans-resistance Amplifiers: Output Voltage Vo is proportional to input signal

current Is. Refer OHP Sheet – 5

BACK

Feedback topologies

VOLTAGE SERIESVOLTAGE SHUNTCURRENT SERIESCURRENT SHUNT

VOLTAGE SERIES FEEDBACK

Vs

SAMPLING

NETWORK

AMPLIFIER WITH GAIN

A

FEEDBACK NETWORK

WITH GAIN β

Vi

Vo

RL

Vf

Vs + vf

MIXER OR

COMPARATOR