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NEGATIVE FEEDBACK
The open loop gain (or maximum gain) of a typical op-
amp is very high (usually greater than 100,000),
enabling a very small input voltage to drive the op-
amp output to it's extremes.
resistor is connected between the output and
inverting input terminals allowing a portion of the
output signal to be brought back and cancel part of
the input (Figure 8). This process is called Negative
Feedback. The signal being fed back is out of phase
with the input and thus subtracts from the input signal.
If the resistor was connected between the non-
inverting input and output terminals, it would be called
Positive Feedback. The closed loop gain (or gain
after feedback) from the input Vi to the output terminal
depends on the ratio of R2 to R1.
For example, if R2 = 100 and R1 = 10, the gain (G) =
R2/R1 = 100/10 = 10. Thus, the output voltage Vo
would be equal to –10(Vi). The (–) sign indicates that
the output and input voltages are of opposite polarity.
FILTERS
LOW PASS FILTER
A low pass filter attenuates (decreases) all signals
above a certain frequency and passes frequencies
below that frequency. An example of a low pass filter
is a simple RC network as shown in Figure 9. Low
frequencies are passed unharmed. As the frequency
rises the output is reduced (see Figure 10).
Figure 9
Figure 10
To prevent this, a
-7-
VOLTAGE COMPARATOR
Operational amplifiers can be used to compare the
amplitude of one voltage with another.
comparator, its function is to determine when an
input voltage exceeds a certain level. When used as
a comparator, the op-amp is used without feedback
and at maximum gain.
reference voltage and the other tied to the input
voltage.
R1
2
Vi
3
Figure 8
HIGH PASS FILTER
The high pass filter attenuates frequencies below a
certain frequency and passes frequencies above that
frequency. An example of a high pass filter is a simple
RC network as shown in Figure 11. Low frequencies
are reduced when passed through the filter while high
are passed unharmed (see Figure 12).
Figure 11
Figure 12
As a
One input is set to a
R2
1
Vo
Vo = –(R2/R1) Vi
OR
Vo = –G Vi
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