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Lecture 2 Feedback Amplifier. Introduction of Two-Port Network Negative Feedback (Uni-lateral Case) Feedback Topology Analysis of feedback applications Close-Loop Gain Input/Output resistances. At port 1. At port 2. Open-circuit forward transimpedance. Open-circuit input impedance.

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lecture 2 feedback amplifier
Lecture 2 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

two port network z parameters

At port 1

At port 2

Open-circuit forward

transimpedance

Open-circuit

input impedance

Open-circuit

output impedance

Open-circuit reverse

transimpedance

Two-Port Network (z-parameters)

(Open-Circuit Impedance)

EE3110 Feedback Amplifiers

two port network y parameters
Two-Port Network (y-parameters)

(Short-Circuit Admittance)

At port 1

At port 2

Short-circuit forward

transadmittance

Short-circuit

input admittance

Short-circuit

output admittance

Short-circuit reverse

transadmittance

EE3110 Feedback Amplifiers

two port network h parameters
Two-Port Network (h-parameters)

(hybrid)

At port 1

At port 2

Short-circuit forward

current gain

Short-circuit

input impedance

Open-circuit

output admittance

Open-circuit reverse

voltage gain

EE3110 Feedback Amplifiers

two port network g parameters
Two-Port Network (g-parameters)

(inverse-hybrid)

At port 1

At port 2

Open-circuit forward

current gain

Open-circuit

input admittance

Short-circuit

output impedance

Short-circuit reverse

current gain

EE3110 Feedback Amplifiers

z parameter example
z-parameter example

Note: (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
y-parameter example

EE3110 Feedback Amplifiers

y parameter example cont

Note: the y-matrix is equal to the sum of two former ones.

Therefore, y-parameters is normally used in analysis of shunt-shunt circuits

What connection should be for h- or g- parameters?

y-parameter example (Cont’)

EE3110 Feedback Amplifiers

general feedback structure
General Feedback Structure

A : Open Loop Gain

A = Vo / V

 : feedback factor

 = Vf / Vo

EE3110 Feedback Amplifiers

negative feedback properties
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
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 topologies
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

feedback structure series shunt
Feedback Structure (Series-Shunt)

Voltage Gain Calculation:

  • Voltage amplifier voltage-controlled voltage source
  • Requires high input impedance, low output impedance
  • Voltage-voltage feedback

EE3110 Feedback Amplifiers

input output resistance series shunt
Input/Output Resistance (Series-Shunt)

Input Resistance:

Output Resistance

(Closed loop output resistance with zero input voltage)

EE3110 Feedback Amplifiers

h parameter modeling

Only uni-lateral case

  • will be considered :
  • NO reverse dependent signal found in the amplifier network. |h12a| = 0
  • NO reverse dependent signal found in the feedback network. |h21f| = 0
h-parameter Modeling

EE3110 Feedback Amplifiers

uni lateral
Uni-lateral

EE3110 Feedback Amplifiers

series shunt example
Series-Shunt Example
  • It is observed that:
  • Series connection in input ports
  • Shunt connection in output ports
  •  Series-Shunt connection
  • h-parameter should be used.

Equivalent circuit

EE3110 Feedback Amplifiers

h parameter analysis
h-parameter analysis

1

EE3110 Feedback Amplifiers

feedback structure series series
Feedback Structure (Series-Series)

EE3110 Feedback Amplifiers

input output resistance series series
Input/Output Resistance (Series-Series)

Input Resistance:

Output Resistance

(Closed loop output resistance with zero input voltage)

EE3110 Feedback Amplifiers

series series example
Series-Series Example

CE amplifier with an un-bypassed emitter

ac small signal equivalent circuit

EE3110 Feedback Amplifiers

feedback network with z parameter
Feedback Network with z-parameter

Reduce equivalent circuit

EE3110 Feedback Amplifiers

close loop analysis
Close loop analysis

EE3110 Feedback Amplifiers

final r in and r out
Final Rin and Rout

EE3110 Feedback Amplifiers

feedback structure shunt shunt
Feedback Structure (Shunt-Shunt)

EE3110 Feedback Amplifiers

input output resistance shunt shunt
Input/Output Resistance (Shunt-Shunt)

Input Resistance:

Output Resistance

(Closed loop output resistance with zero input voltage)

EE3110 Feedback Amplifiers

shunt shunt example
Shunt-Shunt Example

CE amplifier

ac small signal equivalent circuit

Shunt-Shunt connection found!  y-parameter

EE3110 Feedback Amplifiers

slide29

Feedback Network

y-parameter modeling

EE3110 Feedback Amplifiers

feedback structure shunt series
Feedback Structure (Shunt-Series)

EE3110 Feedback Amplifiers

input output resistance shunt series
Input/Output Resistance (Shunt-Series)

Input Resistance:

Output Resistance

(Closed loop output resistance with zero input voltage)

EE3110 Feedback Amplifiers

summary
Summary

EE3110 Feedback Amplifiers

supplementary
Supplementary

Find the input and output resistance from

- Two port network, and

- Circuit theory

EE3110 Feedback Amplifiers

circuit theory
Circuit Theory

EE3110 Feedback Amplifiers

two port network

Two Port Network

EE3110 Feedback Amplifiers