Page 36 - Op Amps Design, Applications, and Troubleshooting

Introduction to Practical Op Amps 19

Appendix 1 presents the manufacturer's specification sheets for a 741 op
amp, one of the most widely used devices. We will refer to these specifications in
the following paragraphs.

1.4.1 Differential Voltage Gain

You will recall that an ideal op amp has an infinite differential voltage gain. That
is, any nonzero input signal will cause the output to be driven to its limits. In the
case of a real op amp, the voltage gain is affected by several things including

1. The particular op amp being considered
2. The frequency of operation
3. The temperature
4. The value of supply voltage

For DC and very low-frequency applications the differential voltage gain
will generally be from 50,000 to 1,000,000. Although this is less than the infinite
value cited for ideal op amps, it is still a very high gain value. As the frequency
increases, the available gain decreases. The point at which this decreasing gain
becomes a problem is discussed briefly in a subsequent paragraph and more thor-
oughly in Chapter 10. For purposes of the present discussion, you should know
that the differential voltage gain of a typical nonideal op amp starts at several
hundred thousand and decreases as frequency increases.
Now let us determine the differential voltage gain for an actual 741 op amp
(refer to Appendix 1). In the specification sheet, the manufacturer calls this param-
eter the Large Signal Voltage Gain. The value is given as ranging from a low of
20,000 to a typical value of 200,000. No maximum value is given. You will also find
a number of graphs in Appendix 1. You should examine the graphs that present
open-loop voltage gain as a function of another quantity. The terms open and dosed
loops are used extensively when discussing op amps. If a portion of the ampli-
fier's output is returned to its input (i.e., feedback), then the amplifier is said to
have a closed loop. You can readily see from the graphs in Appendix 1 that the
gain of the op amp is not especially stable. Pay particular attention to the graph
showing open-loop voltage gain as a function of frequency. Notice that the gain
drops dramatically as the frequency increases.
In Chapter 2 you will learn that the gain of the op amp can be easily stabi-
lized with a few external components. In fact, the fluctuating gain characteristic
can be made insignificant in an actual op amp circuit.

1.4.2 Common-mode Voltage Gain

Although an ideal op amp has no response to voltages that are common to both
inputs (i.e., no difference voltage), a practical op amp may have some response to
such signals. Figure 1.18 shows how the common-mode voltage gain is measured.
In the ideal case, of course, there would be no output and the computed gain
would be zero. In the real case, there might be, for example, as much as 2 millivolts
generated with a 1 millivolt common-mode input signal. That is, the common-
mode voltage gain might be 2 in a typical case.