Feedback amplifiers

What is feedback amplifier?

If a 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. Feedback factor = Feedback signal/input signal.

What is negative and positive feedback?

If feedback from output to input is given in such in a way that the introduction of feedback in the amplifier decreases the overall gain, then the feedback is termed as negative feedback.

If feedback from output to input is given in such in a way that the introduction of feedback in the amplifier increases the overall gain, then the feedback is termed as positive feedback.

What are Advantages and disadvantages of Negative feedback?

Some of the Advantages of negative feedback:

a)      Input resistance increases

b)      Output resistance decreases

c)      Bandwidth increases

d)     Non linear distortion decreases

e)      Frequency distortion decreases

f)       Sensitivity will be decreased

g)      Gain stability

The main disadvantage of negative feedback is decrease in overall gain. The gain and feedback factor in an amplifier are often functions of frequency, so the feedback may lead to positive feedback.

What is de-sensitivity factor?

Desensitivity factor is defined as the factor with which the feedback desensitizes the gain. It is also called as return difference.

            Desensitivity factor = (dAfb/Afb)/(dA/A) = 1/(1+A*β)

Where Afb is gain with feedback

A is gain without feedback

β is feedback factor.

What is a loop gain in amplifier?

 Loop gain in amplifier is defined as the gain of feedback path from output to input. It is the product of loop gain and feedback factor in positive feedback amplifiers. In negative feedback amplifiers it is the negative product of loop gain and feedback factor.

What is the effect of negative feedback on voltage gain, BW, Noise, nonlinear distortion, Ri, Ro of a voltage amplifier?

The voltage gain, bandwidth, noise, nonlinear distortion, input resistance, output resistance are

Voltage gain with feedback = Voltage gain without feedback* desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Voltage gain without feedback < gain without feedback).

Band width with feedback = Band width without feedback/ desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Band width without feedback > Band width without feedback).

Noise with feedback = Noise without feedback* desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Noise without feedback > Noise without feedback).

Non linear distortion with feedback = Non linear distortion without feedback* desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Non linear distortion without feedback > Non linear distortion without feedback).

Non linear distortion with feedback = Non linear distortion without feedback* desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Non linear distortion without feedback > Non linear distortion without feedback).

Input resistance with feedback = Input resistance without feedback/ desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Input resistance without feedback > Input resistance without feedback).

Output resistance with feedback = Output resistance without feedback* desensitivity factor (since desensitivity factor << 1 in negative feedback, hence Output resistance without feedback > Output resistance without feedback)

What are the different types of feedback topologies?

There are four different types of feedback topologies based on type of output signal and feedback signal (voltage or current signal). Voltage feedback is taken in series with the load and current feedback is taken in shunt with the load. They are

a)Voltage-series: Output signal is voltage signal, feedback signal is voltage signal. Also called as series-series feedback. It is employed in voltage amplifiers.

b)Current series: Output signal is current signal, feedback signal is voltage signal. Also called as shunt-series feedback. It is employed in Transconductance amplifiers.

c)Current shunt: Output signal is current signal, feedback signal is current signal. Also called as shunt-shunt feedback. It is employed in current amplifiers.

d)Voltage shunt: Output signal is voltage signal, feedback signal is current signal. Also called as shunt-shunt feedback. It is employed in current amplifiers.

The first word indicates the type of output signal and the second word indicates the manner in which feedback signal is taken whether it is taken in series or shunt with the load.

What is the effect of each feedback network topologies on input and output resistance?

Feedback topology/ Parameter Voltage series   Current series Current shunt Voltage shunt
Input resistance

Increases

Rif = Ri*(1+A*β)

 

Increases

Rif = Ri*(1+A*β)

Decreases

Rif = Ri/(1+A*β)

decreases

Rif = Ri*(1+A*β)

Output resistance

Decreases

Rof = Ro/(1+A*β)

 

Increases

Rof = Ro*(1+A*β)

Increases

Rof = Ro*(1+A*β)

Decreases

Rof = Ro/(1+A*β)

What is a nyquist criterion to differentiate the feedback in amplifiers?

If the nyquist plot of amplifiers encircles (-1, 0) point in nyquist domain, the amplifier is unstable and has positive feedback.

If the nyquist plot of amplifiers does not encircle (-1, 0) point in nyquist domain, the amplifier is stable and has negative feedback.

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