Page 416 - Op Amps Design, Applications, and Troubleshooting

392 NONIDEAL OP AMP CHARACTERISTICS

the actual output voltage will show very little loading. In practice, this condition is
generally easy to satisfy. In most cases, the maximum output current of the op
amp will be exceeded before the output resistance becomes a problem.

10.2 NONIDEAL AC CHARACTERISTICS

The effects of limited bandwidth have been discussed in several of the earlier chap-
ters with reference to specific circuits. In general, the open-loop DC gain of an op
amp is extremely high (typically well over 200,000). However, as the frequency of
operation increases, the gain begins to fall off. At some point, the open-loop gain
reaches 1. We call this the unity gain frequency, which is also referred to as the gain
bandwidth product.
When the op amp is provided with negative feedback, the closed-loop gain
is less than the open-loop gain. As long as the closed-loop gain is substantially less
than the open-loop gain (at a given frequency), then the circuit is relatively un-
affected by the reduced open-loop gain. However, at frequencies that cause the
open-loop gain to approach the expected closed-loop gain, the actual closed-loop
gain also begins to fall off.
We can estimate the highest operating frequency for a particular closed-loop
gain as follows:

uni ain
ty 8
closed -loop gain

This is adequate for many, if not most, applications, but it ignores the fact that the
closed-loop gain falls off more rapidly as it approaches the open-loop curve. In
fact, when the circuit is operated at the frequency computed, the response will be
about 3 dB below the ideal voltage gain. If this 3-dB drop is unacceptable for a par-
ticular application, then the gain must be reduced (or an op amp with a higher
gain bandwidth product used).

10.2.2 Slew Rale
In order to provide high-frequency stability, op amps have one or more capacitors
connected to an internal stage. The capacitor may be internal to the op amp (inter-
nally compensated), or it may be added externally by the designer (externally
compensated) (Section 10.2.4 discusses compensation in greater detail). In either
case, this compensating capacitance limits the maximum rate of change that can
occur at the output of the op amp. That is, the output voltage can only change as
fast as this capacitance can be charged and discharged. The charging rate is deter-
mined by two factors:

1. Charging current
2. Size of the capacitor