Electronic Circuits and Systems | Report

ELECTRONIC CIRCUITS AND SYSTEMS 1
ELECTRONIC CIRCUITS AND SYSTEMS
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ELECTRONIC CIRCUITS AND SYSTEMS 2
P3
In analogue electronics, there two types of feedbacks which are employed, these are the negative feedback
and the positive feedback and both the feedbacks have closed-loop gains.
Negative feedback Op amp
This is a type of amplifier ́s feedback where part of the Vout ( output signal) is fed to the input at the
inverting terminal (both the feedback signal and the input are both on the negative terminal / inverting
terminal) via the Rf ( which is feedback resistor) as illustrated in the following figure;
Figure 1: Showing a negative feedback amplifier (Mesias 120)
The effect of negative feedback on an amplifier
The effect of this feedback results to a closed-loop circuit to the operational amplifier leading to amplifier ́s
gain referred to as closed-loop gain. This feedback gives the negative terminal input different signal to the
real input signal since the actual input signal will be the input signal plus the negative feedback signal at the
summing pint. It is thus prudent to isolate the actual input signal from the inverting signal input by the use of
an input resistor, Rin. Since this is negative feedback, the summing point is assumed to have a zero voltage,
therefore, referred to as a virtual ground as illustrated in the following diagram;

ELECTRONIC CIRCUITS AND SYSTEMS 3
Figure 2: Showing the summing point / virtual ground (Mesias 121)
The following are some of the summarized effects of negative feedback on the performance of an amplifier;
Negative feedback increases the bandwidth: Whilst the open-loop amplifiers have higher voltage gain they
normally have low bandwidth. The cut-off frequencies are always achieved when open-loop gain reduces to
the equivalent of 1/B, then f2 (CL) will occur when;
Av= 1/B = ACL ................................................................................................ 1
Therefore, for an instant, the cut-off frequencies of this feedback which is designed for a closed-loop gain
of 100 will only occur if the open-loop goes to 100. For negative feedback, the upper cut off frequency is
usually more than 20 kHz while the lower is about 100Hz or even less, thus f2≫ f 1, thus,
BW = f2−f 1 ≈ f 2 ............................................................................................ 2
This implies that the negative feedback ́s bandwidth is equal to the upper cut-off frequency.
Negative feedback decreases the output noise: The circuit noise which is produced in the feedback loop of
an op-amp is reduced by a factor of (1+AVB). It is worth noting that it is only the noise generated in the
feedback loop is reduced by the feedback. Thus the noise which is produced by the bias resistors outside the
feedback loop is not affected by the feedback.