Ref:080130HKN EE3110 Feedback Amplifier s 1 Lecture 2 Feedback Amplifier • Introduction of Two-Port Network • Negative Feedback (Uni-lateral C ase) • Feedback Topology • Analysis of feedback application s – Close-Loop Gain – Input/Output resistances
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)
EE3110 Feedback Amplifiers
Two-Port Network (y-parameters)(Short-Circuit Admittance)Short-circuit input admittanceAt port 1Short-circuit reverse transadmittanceAt port 2Short-circuit forwardtransadmittanceShort-circuit output admittance
EE3110 Feedback Amplifiers
Two-Port Network (h-parameters)(hybrid)Short-circuit input impedanceAt port 1Open-circuit reverse voltage gainAt port 2Short-circuit forwardcurrent gainOpen-circuit output admittance
EE3110 Feedback Amplifiers
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
EE3110 Feedback Amplifiers
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)
EE3110 Feedback Amplifiers
y-parameter example
EE3110 Feedback Amplifiers
y-parameter example (Cont)
EE3110 Feedback Amplifiers
General Feedback Structure A : Open Loop Gain A = Vo / V : feedback factor = Vf / Vo
EE3110 Feedback Amplifiers
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
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
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)
EE3110 Feedback Amplifiers
Feedback Structure (Series-Shunt)Voltage amplifier voltage-controlled voltage source Requires high input impedance, low output impedanceVoltage-voltage feedbackVoltage Gain Calculation:
EE3110 Feedback Amplifiers
Input/Output Resistance (Series-Shunt)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)
EE3110 Feedback Amplifiers
h-parameter Modeling
EE3110 Feedback Amplifiers
Uni-lateral
EE3110 Feedback Amplifiers
Series-Shunt ExampleEquivalent circuitIt is observed that:Series connection in input portsShunt connection in output ports Series-Shunt connection h-parameter should be used.
EE3110 Feedback Amplifiers
h-parameter analysis1
EE3110 Feedback Amplifiers
EE3110 Feedback Amplifiers
Feedback Structure (Series-Series)
EE3110 Feedback Amplifiers
Input/Output Resistance (Series-Series)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)
EE3110 Feedback Amplifiers
Series-Series ExampleCE amplifier with an un-bypassed emitter ac small signal equivalent circuit
EE3110 Feedback Amplifiers
Feedback Network with z-parameterReduce equivalent circuit
EE3110 Feedback Amplifiers
Close loop analysis
EE3110 Feedback Amplifiers
Final Rin and Rout
EE3110 Feedback Amplifiers
Feedback Structure (Shunt-Shunt)
EE3110 Feedback Amplifiers
Input/Output Resistance (Shunt-Shunt)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)
EE3110 Feedback Amplifiers
Shunt-Shunt ExampleCE amplifierac small signal equivalent circuitShunt-Shunt connection found! y-parameter
EE3110 Feedback Amplifiers
Feedback Networky-parameter modeling
EE3110 Feedback Amplifiers
EE3110 Feedback Amplifiers
Feedback Structure (Shunt-Series)
EE3110 Feedback Amplifiers
Input/Output Resistance (Shunt-Series)Input Resistance:Output Resistance (Closed loop output resistance with zero input voltage)
EE3110 Feedback Amplifiers
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
EE3110 Feedback Amplifiers
SupplementaryFind the input and output resistance from - Two port network, and - Circuit theory
EE3110 Feedback Amplifiers
Circuit Theory
EE3110 Feedback Amplifiers
Two Port Network
EE3110 Feedback Amplifiers
EE3110 Feedback Amplifiers