l02 feedback amplifiers.ppt

Lecture 2 Feedback Amplifier

Introduction of Two-Port NetworkNegative Feedback (Uni-lateral Case)Feedback TopologyAnalysis of feedback applicationsClose-Loop GainInput/Output resistances

EE3110 Feedback Amplifiers

Two-Port Network (z-parameters)(Open-Circuit Impedance)


Two-Port Network (y-parameters)(Short-Circuit Admittance)Short-circuit input admittanceAt port 1Short-circuit reverse transadmittanceAt port 2Short-circuit forwardtransadmittanceShort-circuit output admittance


Two-Port Network (h-parameters)(hybrid)Short-circuit input impedanceAt port 1Open-circuit reverse voltage gainAt port 2Short-circuit forwardcurrent gainOpen-circuit output admittance


Two-Port Network (g-parameters)(inverse-hybrid)Open-circuit input admittanceAt port 1Short-circuit reverse current gainAt port 2Open-circuit forwardcurrent gainShort-circuit output impedance


z-parameter exampleNote: (1) z-matrix in the last circuit = sum of two former z-matrices (2) z-parameters is normally used in analysis of series-series circuits (3) Z12 = Z21 (reciprocal circuit) (4) Z12 = Z21 and Z11 = Z22 (symmetrical and reciprocal circuit)


y-parameter example


y-parameter example (Cont)


General Feedback Structure A : Open Loop Gain A = Vo / V : feedback factor = Vf / Vo


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


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


Basic Feedback TopologiesDepending 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)


Feedback Structure (Series-Shunt)Voltage amplifier voltage-controlled voltage source Requires high input impedance, low output impedanceVoltage-voltage feedbackVoltage Gain Calculation:


Input/Output Resistance (Series-Shunt)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)


h-parameter Modeling


Uni-lateral


Series-Shunt ExampleEquivalent circuitIt is observed that:Series connection in input portsShunt connection in output ports Series-Shunt connection h-parameter should be used.


h-parameter analysis1


Feedback Structure (Series-Series)


Input/Output Resistance (Series-Series)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)


Series-Series ExampleCE amplifier with an un-bypassed emitter ac small signal equivalent circuit


Feedback Network with z-parameterReduce equivalent circuit


Close loop analysis


Final Rin and Rout


Feedback Structure (Shunt-Shunt)


Input/Output Resistance (Shunt-Shunt)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)


Shunt-Shunt ExampleCE amplifierac small signal equivalent circuitShunt-Shunt connection found! y-parameter


Feedback Networky-parameter modeling


Feedback Structure (Shunt-Series)


Input/Output Resistance (Shunt-Series)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)


Summary

Feedback Structure Close loop gain Input impedance Output impedance Parameter used Series-Shunt h-parameter Series-Series z-parameterShunt-Shun y-parameterShunt-Series g-parameter


SupplementaryFind the input and output resistance from - Two port network, and - Circuit theory


Circuit Theory


Two Port Network


l02 feedback amplifiers.ppt

Documents

feedback factor

feedback amplifierintroduction

types of feedback structures

output voltage

circuit input impedanceat

input signal voltage

output signal

z21 reciprocal circuit