EE305 - Negative Feedback Amplifier

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Title: Negative Feedback Amplifier

Aim: To verify the effect of negative feedback on gain.

Apparatus: See your laboratory manual for the apparatus.

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THEORY:

Negative Feedback, NFB is the most common form of feedback control used in process, micro-computer and amplifier systems. Feedback is the process by which a fraction of the output signal, either a voltage or a current, is used as an input. If this feedback fraction is opposite in value or phase (“anti-phase”) to the input signal, then the feedback is said to be Negative Feedback, or degenerative feedback.

Negative feedback opposes or subtracts from the input signals giving it many advantages in the design and stabilization of control systems. For example, if the systems output changes for any reason, then negative feedback affects the input in such a way as to counteract the change.
Feedback reduces the overall gain of a system with the degree of reduction being related to the systems open-loop gain. Negative feedback also has effects of reducing distortion, noise, and sensitivity to external changes as well as improving system bandwidth and input and output impedances.
Feedback in an Electronic System, whether negative feedback or positive feedback is unilateral in direction. Meaning that its signals flow one way only from the output to the input of the system. This then makes the loop gain, G of the system independent of the load and source impedances.
As feedback implies a closed-loop system it must therefore have a summing point. In a negative feedback system this summing point or junction at its input subtracts the feedback signal from the input signal to form an error signal, β which drives the system. If the system has a positive gain, the feedback signal must be subtracted from the input signal in order for the feedback to be negative as shown.

Negative Feedback Circuit

The circuit represents a system with positive gain, G and feedback, β. The summing junction at its input subtracts the feedback signal from the input signal to form the error signal Vin - βG, which drives the system.
Then using the basic closed-loop circuit above we can derive the general feedback equation as being:

Negative Feedback Equation

We see that the effect of the negative feedback is to reduce the gain by the factor of: 1 + βG. This factor is called the “feedback factor” or “amount of feedback” and is often specified in decibels (dB) by the relationship of 20 log (1+ βG).

Effects of Negative Feedback

If the open-loop gain, G is very large, then βG will be much greater than 1, so that the overall gain of the system is roughly equal to 1/β. If the open-loop gain decreases due to frequency or the effects of system ageing, providing that βG is still relatively large, the overall system gain does not change very much. So negative feedback tends to reduce the effects of gain change giving what is generally called “gain stability”.

Why NFB is needed in amplifiers

Transistors cannot be manufactured to have a closely controlled value of current gain h_fe therefore it should not be possible to build a number of examples of the same amplifier circuit, all having the same gain. In addition the gain of a transistor varies with temperature, and even has different gain at different frequencies. All of these factors would make transistor amplifiers totally unreliable and impossible to make in large numbers. The main reason that this situation does not exist, and transistor amplifiers have become the mainstay of the electronics industry is the introduction, very early in the transistor’s history, of negative feedback.

Principle of NFB

The principle of negative feedback is that a portion of the output signal is fed back to the input and combined with the input signal in such a way as to reduce it. This reduces the overall gain of the amplifier but also introduces a number of benefits, such as reducing distortion and noise, and widening the amplifier’s bandwidth.

Problems with NFB

Introducing feedback within a system can also introduce the possibility of instability; in amplifiers the signal will normally undergo a phase reversal of 180 degrees between input and output but reactive components such as capacitors and inductors, whether actual components or ‘stray’ capacitance and inductance, can introduce unwanted phase changes at particular (usually high) frequencies. If these additional changes add up to a further 180 degrees at any frequency where the transistor has a gain of more than 1, the application of negative feedback may become positive feedback. Instead of reducing gain this will increase it to the point where the amplifier will become an oscillator and produce unwanted signals. Negative feedback must therefore be designed to maximize the benefits mentioned above, without creating unwanted problems.

The Negative Feedback Amplifier in Closed Loop Mode

In a basic negative feedback arrangement the phase reversing amplifier has a fraction of its output (V_out) fed back and added to the input (V_in) so as to reduce the amplitude of the input signal. The arrows show the relative polarity of the signals and it can be seen that the output and the feedback signals are in anti-phase to the input signal. The fraction of the output signal to be fed back is controlled by the potential divider (β) and this fraction is added to the input signal in anti-phase, so that it is in effect, subtracted from the input signal (V_in) to give a combined signal (V_c) that is reduced in amplitude before being fed to the actual input of the amplifier.

The gain of the amplifier, excluding any feedback, is A_o so that, for example, every 1mV applied across the circuit’s input terminals, the amplifier will produce a phase-reversed signal of A_o x 1mV across the output terminals.

The feedback circuit comprising R1 and R2 will feed back a fraction (β) of output V_out which = A_o, so that A_o x βmV (Aβ) will be added in anti-phase to the 1mV signal to produce a reduced input signal of V_c.

The signal source V_in driving the amplifier must therefore deliver not 1mV but 1+AβmV to produce the same amplitude of output. Therefore the overall gain of the amplifier with negative feedback is reduced and is now called the closed loop gain (A_c).

The effect of NFB on amplifier Gain

This is of great significance because it means that, once negative feedback is applied, the closed loop gain A_c depends almost exclusively on β, which in turn depends on the ratio of the potential divider R1, R2.

For Procedure and Circuit Diagram, See your Electrical Laboratory Manual on page 3. 

 Fill Up Tables in Your Manual and Continue To….

PRECAUTIONS:

For Precautions, See General Electrical Lab Precautions

Answers to Questions:

No Questions in this experiment.

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About Stephen Djes

Stephen Djes is a passionate Graduate of Engineering from the University of Benin, and he is geared towards helping fellow engineering students in the great institution of UNIBEN to do better at academics.