schmitt trigger or regenerative comparator circuit
op amp as a buffer amplifiervoltage follower
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Difference (or differential) amplifiers
INSTRUMENTATION AMPLIFIER
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Feedback management in operational amplifiers
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Op amp Feedback Arangement
Positive feedback System :
Positive feedback control of the op-amp is achieved
by applying a small part of the output voltage signal at
Vout back to the non-inverting ( + ) input terminal
via the feedback resistor, RF.
If the input voltage Vin is positive, the op-amp
1
amplifies this positive signal and the output becomes
, more positive. Some of this output voltage is returned back
to the input by the feedback network.
Thus the input voltage becomes more positive, causing an even
larger output voltage and so on. Eventually the output becomes
saturated at its positive supply rail.
Likewise, if the input voltage Vin is negative, the reverse happens
and the op-amp saturates at its negative supply rail.
Then we can see that positive feedback does not allow the circuit
to function as an amplifier as the output voltage quickly saturates
to one supply rail or the other, because with positive feedback
loops “more leads to more” and “less leads to less”.
2
, ctd
Then if the loop gain is positive for any system the transfer function
will be:
Av = G / (1 – GH). Note that if GH = 1 the system gain Av =
infinity and the circuit will start to self-oscillate, after which no
input signal is needed to maintain oscillations, which is useful if you
want to make an oscillator.
Although often considered undesirable, this behaviour is used in
electronics to obtain a very fast switching response to a condition or
signal. One example of the use of positive feedback is hysteresis in
which a logic device or system maintains a given state until some
input crosses a preset threshold. This type of behaviour is called “bi-
stability” and is often associated with logic gates and digital
switching devices such as multivibrators.
3
, ctd
We have seen that positive or regenerative feedback increases the
gain and the possibility of instability in a system which may lead to
self-oscillation and as such, positive feedback is widely used in
oscillatory circuits such as Oscillators and Timing circuits.
Negative Feedback System
Negative feedback control of the amplifier is achieved by applying
a small part of the output voltage signal at Vout back to the
inverting ( – ) input terminal via the feedback resistor, Rf.
If the input voltage Vin is positive, the op-amp amplifies this
positive signal. Since it is connected to the inverting input of the
amplifier, the output becomes more negative. Some of this output
voltage is returned back to the input by the feedback network of
4 Rf.
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