The gain of the OP-AMP can be controlled if fedback is introduced in the circuit. That is, an output signal is fedback to the input either directly or via another network. If the signal fedback is of opposite or out phase by 180° with respect to the input signal, the feedback is called negative fedback.
An amplifier with negative fedback has a self-correcting ability of change in output voltage caused by changes in environmental conditions. It is also known as degenerative fedback because it reduces the output voltage and,in tern,reduces the voltage gain.
If the signal is fedback in phase with the input signal, the feedback is called positive feedback. In positive feedback the feedback signal aids the input signal. It is also known as regenerative feedback. Positive feedback is necessary in oscillator circuits.
The negative fedback stabilizes the gain, increases the bandwidth and changes, the input and output resistances. Other benefits are reduced distortion and reduced offset output voltage. It also reduces the effect of temperature and supply voltage variation on the output of an op-amp.
A closed loop amplifier can be represented by two blocks one for an OP-AMP and other for a feedback circuits. There are four following ways to connect these blocks. These connections are shown in Figure 4.
These connections are classified according to whether the voltage or current is feedback to the input in series or in parallel:
- Voltage – series feedback
- Voltage – shunt feedback
- Current – series feedback
- Current – shunt feedback
Figure 4
In all these circuits of Figure 4, the signal direction is from input to output for OP-AMP and output to input for feedback circuit. Only first two, feedback in circuits are important.
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