feedback amplifiers
TRANSCRIPT
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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