Negative Feedback Amplifiers Guided by : Prof. S. S. Patil Presentation by: Vinay Jasud (52) Vinod Arote (54)
TOPICS TO BE COVERED Introduction Concept of Feedback Feedback Topologies General Characteristics of Negative Feedback Amplifiers Analysis Of Feedback Amplifiers Voltage Series Feedback Voltage Shunt Feedback Current Series Feedback Current Shunt Feedback
Introduction Feedback (Definition) It is the process in which part of the output signal is returned back to the input. Types of Feedback 1.Positive Feedback 2. Negative Feedback
Amplifiers Without Feedback Amplifier “A” Vi Vo
Amplifiers With Feedback Sampling Network Amplifier “A” Mixer Vo Vs Vi Vo Feedback Signal Vf Feedback Network Feedback factor
Transfer characteristics with feedback (Af ) Stabilization of Gain Effect on input resistance Effect on output resistance Effect on Bandwidth Effect on nonlinear distortion Effect on noise General Characteristics Of Negative Feedback Amplifiers
Transfer characteristics with feedback (Af ) A = Transfer gain without feedback A = Transfer gain with feedback f f o Where X = Feedback Signal X = output signal f o Transfer gain with feedback is given by
Stabilization of Gain Fractional Change in amplification with feedback is given by Factors affecting stability are aging, temperature changes, replacement of component. f f
Sensitivity and Desensitivity
Effect on input resistance and output resistance
Analysis Of Feedback Amplifiers Step 1: Identify the topology. Step 2: Draw the basic amplifier circuit without feedback. Step 3: Use Thevenin’s theorem for voltage source and Norton’s theorem for current sou.rce. Step 4: Replace each device by it’s h-parameter. Step 5: Indicate Xo and Xf and evaluate . Step 6: Calculate open loop gain A. Step 7: Calculate D, Af, Rif and Rof.
Voltage Series Feedback Make V0=0 If Vf=0 then voltage sampling. Feedback voltage Vf is in series with the input voltage Vs .Therefore series mixing. voltage sampling + series mixing =Voltage series feedback Transistor emitter follower circuit
Approximate h-parameter equivalent circuit
Current Series Feedback Set output current Io=0 if Vf=0 then current sampling. Feedback voltage Vf is in series with the input voltage Vs .Therefore series mixing. Therefore current series feedback. CE transistor amplifier with unbypassed Re
h-parameter equivalent circuit
Current Shunt Feedback
Set output current Io=0 if Vf=0 then current sampling.
Feedback signal is in parallel . Hence it is current shunt feedback.