Feedback Amplifiers

Outline

• Introduction
• The general feedback structure
• Some properties of negative feedback
• The four basic feedback topologies
• The series-shunt feedback amplifier
• The series-series feedback amplifier
• The shunt-shunt and shunt-series feedback amplifier

• The stability problem
• Stability study using bode plot
• Frequency compensation

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Introduction

• It’s impossible to think of electronic circuits without some forms of feedback.
• Negative feedback
• Desensitize the gain
• Reduce nonlinear distortion
• Reduce the effect of noise
• Control the input and output impedance
• Extend the bandwidth of the amplifier

• The basic idea of negative feedback is to trade off gain for other desirable properties.
• Positive feedback will cause the amplifier oscillation.

The General Feedback Structure

This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

The General Feedback Equation

• Closed loop and open loop
• Closed loop gain

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• Feedback factor β
• Loop gain Aβ
• Amount of feedback (1+ Aβ)

Some Properties of Negative Feedback

• Gain desensitivity

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• Bandwidth extension
• Noise reduction
• Reduction in nonlinear distortion

The Four Basic Feedback Topologies

• Voltage amplifier---series-shunt feedback

voltage mixing and voltage sampling

• Current amplifier---shunt-series feedback

Current mixing and current sampling

• Transconducatnce amplifier---series-series feedback

Voltage mixing and current sampling

• Transresistance amplifier---shunt-shunt feedback

Current mixing and voltage sampling

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The Series-Shunt Feedback Topologies

voltage-mixing voltage-sampling (series–shunt) topology

The Amplifier with Series-Shunt Feedback

voltage-mixing voltage-sampling (series–shunt) topology

The Shunt-Series Feedback Topologies

current-mixing current-sampling (shunt–series) topology

The Amplifier with Shunt-Series Feedback

current-mixing current-sampling (shunt–series) topology

The Series-Series Feedback Topologies

voltage-mixing current-sampling (series–series) topology

The Amplifier with Series-Series Feedback

voltage-mixing current-sampling (series–series) topology

The Shunt-Shunt Feedback Topologies

current-mixing voltage-sampling (shunt–shunt) topology

The OP Amplifier with Shunt-Shunt Feedback

current-mixing voltage-sampling (shunt–shunt) topology

The Series-Shunt Feedback Amplifier

• The ideal situation
• The practical situation
• summary

The Ideal Situation

• A unilateral open-loop amplifier (A circuit).
• An ideal voltage mixing voltage sampling feedback network (β circuit).
• Assumption that the source and load resistance have been included inside the A circuit.

The Ideal Situation

Equivalent circuit.

R_if and R_of denote the input and output resistance with feedback.

Input and Output Resistance with Feedback

• Input resistance

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In this case, the negative feedback increases the input resistance by a factor equal to the amount of feedback.

• Output resistance

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In this case, the negative feedback reduces the output resistance by a factor equal to the amount of feedback.

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The Practical Situation

• Block diagram of a practical series–shunt feedback amplifier.
• Feedback network is not ideal and load the basic amplifier thus affect the values of gain, input resistance and output resistance.

The Practical Situation

The circuit in (a) with the feedback network represented by its h parameters.

The Practical Situation

The circuit in (b) with h₂₁ neglected.

The Practical Situation

• The load effect of the feedback network on the basic amplifier is represented by the components h₁₁ and h_22.
• The loading effect is found by looking into the appropriate port of the feedback network while the port is open-circuit or short-circuit so as to destroy the feedback.

• If the connection is a shunt one, short-circuit the port.
• If the connection is a series one, open-circuit the port.
• Determine the β.

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Summary

• R_i and R_o are the input and output resistances, respectively, of the A circuit.
• R_if and R_of are the input and output resistances, respectively, of the feedback amplifier, including R_s and R_L.
• The actual input and output resistances exclude R_s and R_L.

Example of Series-Shunt Feedback Amplifier

Example of Series-Shunt Feedback Amplifier

• Op amplifier connected in noninverting configuration with the open-loop gain μ, R_id and r_o
• Find expression for A, β, the closed-loop gain V_o/V_i , the input resistance R_in and the output resistance R_out
• Find numerical values

Example of Series-Shunt Feedback Amplifier

Example of Series-Shunt Feedback Amplifier