Determining Loop Gain

S MARIA ANTONYAssistant Professor/ ECE DeptKIT- Kalaignarkarunanidhi Institute of Technology

Feedback Amplifier

If an amplifier has feedback path exists from output to input then that amplifier is termed as Feedback amplifier.

Feed back is the parameter which quantifies the amount of feedback given in a feedback amplifier. 

TYPES OF FEEDBACK

If feedback in the amplifier decreases the overall gain, then the feedback is termed as negative feedback

If feedback in the amplifier increases the overall gain, then the feedback is termed as positive feedback

NEGATIVE FEEDBACK AMPLIFIER

It is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal.

The applied negative feedback improves performance (gain stability, linearity, frequency response, step response) and reduces sensitivity to parameter variations due to manufacturing or environment.

An idealized negative feedback amplifier is a system of three elements An amplifier with gain AOLA feedback network β, andA summing circuit

The General Feedback Structure

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

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 Equation

• Closed loop and open loop• Closed loop gain

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

AA

xxAs

of

1

Some Properties of Negative Feedback

• Gain desensitivity

• Bandwidth extension• Noise reduction• Reduction in nonlinear distortion

AdA

AAdA

f

f

11

The Four Basic Feedback Topologies

• Voltage amplifier---series-shunt feedbackvoltage mixing and voltage sampling

• Current amplifier---shunt-series feedbackCurrent mixing and current sampling

• Transconducatnce amplifier---series-series feedbackVoltage mixing and current sampling

• Transresistance amplifier---shunt-shunt feedbackCurrent mixing and voltage sampling

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 Series-Series Feedback Topologies

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

The Shunt-Shunt Feedback Topologies

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

the loop gain can be found with the following steps:

• Break the feedback loop anywhere • Zero out the input signal xs

• Apply a test signal to the input of the feedback circuit

• Solve for the resulting signal xo at the output If xo is a voltage signal, xtst is a voltage and measure the

open-circuit voltage If xo is a current signal, xtst is a current and measure the

short-circuit current

• The negative sign comes from the fact that we apply negative feedback

A

xs=0

xf

xi

xoxtst

Axx

AxAxAxx

xx

xx

tst

o

tstfio

fi

tstf

gain loop

0

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

These properties can be achieved by trading off gain