Negative Feedback in an Amplifier
A most important thing ever invented in the history of analog amplifier is the
implementation of negative feedback. A small portion of the output signal is fed back to
the input negatively, to form a controlled amplification of the input.
Although negative feedback is not always utilized in small signal amplifications, the use
of the technique should never be left behind. People who design circuits as not using
negative feedback when they should may think that it is not that important, they need to
save space on the board and budget on the whole product. But they are missing out many
things only for a very small benefit.
Let us take an example of a simple one or two transistor amplifier.
A one transistor amplifier without negative feedback:
Without negative feedback, a bigger gain is achievable with less component count and
less cost. Here, the total gain is determined by R1/R2, or in this case it’s 10 times. But the
actual gain varies over the entire frequency domain of the amplifier. The transistor also
has noise factors and temperature dependence that determines the actual gain.
A one transistor amplifier with negative feedback:
With negative feedback only a portion of the open loop gain can be achieved, but with a
totally different performance, i.e. stability, frequency response, noise and temperature
independence, among others. Here, the open loop gain is around R4/R5 or 100 times, but
the closed loop formed by the negative feedback resistor R7 and R6 determines the
overall gain to become only almost 10 times, or R7/R6. But all of the impurities of the
transistor mentioned above in the non-feedback amplifier is reduced to only a fraction of
the total gain formed by the negative feedback. In this case, the noise, temperature
dependence and frequency response variations are reduced to only around (closed loop
gain)/(open loop gain) or a tenth of them, resulting in a much more stable and reliable
amplifier.
The only drawback in the negative feedback circuit compared to the one without it is the
component count. In this example, the additional R7 is the added component. In larger
and more sophisticated circuitry the number of added component count may be more
significant than this.
As a designer, you’ll know which to choose based on the advantages and disadvantages
we discuss here.