Elenco Motion Detector Kit - Manual (Page 8)

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Figure 9
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 its 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 V
i
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 V
o
would be equal to –10(V
i
). 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.
Figure 8
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).
FILTERS
Figure 10
Figure 11
Figure 12
R1
R2
V
i
2
3
1
V
o
V
o
= –(R2/R1) V
i
OR
V
o
= –G V
i