Page 423 - Op Amps Design, Applications, and Troubleshooting

Nonideal AC Characteristics 399

FIGURE 10.11 Adding a
compensation capacitor increases
stability but reduces the bandwidth
of an amplifier.

The internal capacitance, called a lag capacitor, can be connected to any one of
several points within the op amp. Since larger values of capacitance are required
for lower impedance points, it is common to connect the lag capacitor at a high-
impedance point in the device. Additionally, by inserting the capacitor in one of
the earlier stages rather than in the output stage, it has less of a slowing effect on
the slew rate. Probably the most common value of internal compensating capaci-
tance is 30 picofarad. It can be readily identified on the simplified schematic of the
standard 741 included in Appendix 1.

External Frequency Compensation. Although the inclusion of an internal
compensating capacitor greatly simplifies the use of an op amp and makes it less
sensitive to sloppy designs, it does cause an unnecessarily severe reduction in
the bandwidth of the circuit. Alternatively, the manufacturer may elect to bring
out one or more pins for the connection of an external compensating capacitor.
The value of the capacitor can be tailored by the designer for a specific applica-
tion.
The extreme case, of course, is to put heavy compensation on the device to
make it stable all the way down to unity gain. This makes the externally compen-
sated op amp equivalent to the internally compensated one. However, many appli-
cations do not require unity gain. In these cases, we can use a smaller compensating
capacitor, which directly increases the bandwidth. So long as the closed-loop gain
curve intersects the open-loop gain curve with a net slope of less than 40 dB, we will
generally have a stable circuit.
The LM301A op amp is an externally compensated, general-purpose op
amp. Its data sheet includes a graph that illustrates the effect on open-loop fre-
quency response for compensating capacitors of 3 and 30 picofarads. A second
graph shows the dramatic increase in large-signal frequency response obtained by
using a 3-picofarad capacitor instead of a 30-picofarad. With a 30-picofarad capac-
itor, the full-power bandwidth is limited to about 7.5 kilohertz (nearly the same as
a standard 741). By using 3-picofarad, however, the full-power bandwidth goes
up to about 100 kilohertz. This can be attributed to an increased slew rate. It
should be noted that 3 picofarads is a very small capacitance. This value can easily
be obtained or even exceeded by stray wiring capacitance.