Instruction manual

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. To prevent this, a 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.
VOLTAGE COMPARATOR
Operational amplifiers can be used to compare the amplitude of one voltage with another. As a 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. One input is set to a reference voltage and the other tied to the
input voltage.
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).
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).
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Figure 8
Figure 9 Figure 10
Figure 11 Figure 12