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**ELECTRONIC PRINCIPLES**

CE Feedback Theory Lecture 4

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**Feedback Amplifier Introduction of Two-Port Network**

Negative Feedback (Uni-lateral Case) Feedback Topology Analysis of feedback applications Close-Loop Gain Input/Output resistances EE3110 Feedback Amplifiers Ref:080130HKN

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**General Feedback Structure**

A : Open Loop Gain A = Vo / V : feedback factor = Vf / Vo EE3110 Feedback Amplifiers Ref:080130HKN

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**Negative Feedback Properties**

Negative feedback takes a sample of the output signal and applies it to the input to get several desirable properties. In amplifiers, negative feedback can be applied to get the following properties Desensitized gain : gain less sensitive to circuit component variations Reduce nonlinear distortion : output proportional to input (constant gain independent of signal level) Reduce effect of noise Control input and output impedances by applying appropriate feedback topologies Extend bandwidth of amplifier All of these properties can be achieved by trading off gain EE3110 Feedback Amplifiers Ref:080130HKN

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Gain De-sensitivity Feedback can be used to desensitize the closed-loop gain to variations in the basic amplifiler. Assume is constant. Take differentials of the closed loop gain equation gives, Divided by Av, the close loop gain sensitivity is equal to, This result shows the effects of variations in A on ACL is mitigated by the feedback amount. (1+A) is also called the desensitivity amount. Differential respected with A EE3110 Feedback Amplifiers Ref:080130HKN

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**Basic Feedback Topologies**

Depending on the input signal (voltage or current) to be amplified and form of the output (voltage or current), amplifiers can be classified into four categories. Depending on the amplifier category, one of four types of feedback structures should be used. (Type of Feedback) (Type of Sensing) (1) Series (Voltage) Shunt (Voltage) (2) Series (Voltage) Series (Current) (3) Shunt (Current) Shunt (Voltage) (4) Shunt (Current) Series (Current) EE3110 Feedback Amplifiers Ref:080130HKN

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**Series-Series Example**

CE amplifier with an un-bypassed emitter ac small signal equivalent circuit EE3110 Feedback Amplifiers Ref:080130HKN

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Feedback Amplifiers. Outline Introduction The general feedback structure Some properties of negative feedback The four basic feedback topologies The series-shunt.

Feedback Amplifiers. Outline Introduction The general feedback structure Some properties of negative feedback The four basic feedback topologies The series-shunt.

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