Page 32 - Op Amps Design, Applications, and Troubleshooting

Basic Characteristics of Ideal Op Amps 15

=1 Vx5=5 V

= 2Vx 5 = 10V
= 4Vx 5 = 20V [output limited to +15 volts]

Now let us do similar calculations with the same input voltages applied to
an ideal op amp. You can quickly realize that in all cases except 0.0 volts input, the
output will try to go beyond the output limit and will be restricted to ±15 volts.
For example, if 0.1 volts is applied

V 0 = 0.1 V x oo = TO y [output limited to +15 volts]

In the case of 0.0 volts at the input, we will have 0.0 volts in the output since 0.0
times anything will be zero. At this point you might well be asking, "So what good
is it if every voltage we apply causes the output to be driven to its limit?" Well,
review Section 1.1.4 of this text, which indicates the usefulness of the op amp in
general. In Chapter 2 you will become keenly aware of how the infinite gain can
be harnessed into a more usable value. For now, however, it is important for you
to remember that an ideal op amp has an infinite differential voltage gain.
1.3.2 Common-mode Voltage Gain

Common-mode voltage gain refers to the amplification given to signals that
appear on both inputs relative to the common (typically ground). You will recall
from a previous discussion that a differential amplifier is designed to amplify the
difference between the two voltages applied to its inputs. Thus, if both inputs had
+5 volts, for instance, with respect to ground, then the difference would be zero.
Similarly, the output would be zero. This defines ideal behavior and is a charac-
teristic of an ideal op amp. In a real op amp, common-mode voltages can receive
some amplification and thus depart from the desired behavior. Since we are cur-
rently defining ideal characteristics you should remember that an ideal op amp
has a common-mode voltage gain of zero. This means the output is unaffected by
voltages that are common to both inputs (i.e., no difference). Figure 1.13 further
illustrates the measurement of common-mode voltage gains.
1.3.3 Bandwidth

Bandwidth, as you might expect, refers to the range of frequencies that can be
amplified by the op amp. Most op amps respond to frequencies down to and
including DC. The upper limit depends on several things including the specific op
amp being considered. But in the case of an ideal op amp, we will consider the

FIGURE 1.13 The common-mode
voltage gain of an ideal op amp is 0.