Page 91 - Understanding Automotive Electronics

2735 | CH 3 Page 78 Tuesday, March 10, 1998 11:03 AM

3 ELECTRONICS FUNDAMENTALS

Use of Feedback in Op Amps

The op amp is normally not operated at maximum gain, but feedback
techniques can be used to adjust the gain to the value desired, as shown in Figure
3.4b. Some of the output is connected to the input through circuit elements
(resistors, capacitors, etc.) to oppose the input changes. In the example of Figure
3.4b, the feedback path consists of resistor R . The gain is adjusted by the ratio of
f
the two resistors and is calculated by the following equation:

---- =
--------
A v = – R f V out
R i V in
As indicated by applying the signal to the (–) terminal, the minus sign in the
equation means signal inversion from input to output; that is, if the input goes
positive, the output goes negative.
Since the op amp amplifies the voltage difference between its two inputs,
it can be used as a differential amplifier as well as a single-input amplifier.
It is interesting to consider the inputs to the op amp in the inverting
mode configuration (Figure 3.4b). The output voltage v is given by
o
v = A (v – v )
o
2
1
where
v is the noninverting input voltage
1
v is the inverting input voltage
2
or
v 1 – v 2 = v o
----
A
For an ideal op amp, the open-loop gain should approach infinity. In this case,
the input voltages should become equal. That is, v – v = 0 for the ideal case, or
2
1
v = v .
2
1
The inverting amplifier of Figure 3.4b has the noninverting input at
ground potential, that is, v = 0. Consequently, the inverting input is (ideally)
1
also at ground potential. In fact, the feedback path provides a current that holds
the inverting input at so-called virtual ground. Furthermore, as the inverting
input is held at ground potential, the input impedance of the op amp circuit of
Figure 3.4b presented to input voltage V is the resistance of R :
i
in
V in
------ = R i
i in
By contrast, the input impedance presented to the input voltage V by the
in
noninverting op amp configuration (Figure 3.4c) is ideally infinite. This very

78 UNDERSTANDING AUTOMOTIVE ELECTRONICS