836 Chapter 25
A new term entered the audio design vocabulary:
compensation. Compensation is the brutal slowing down
of the amplifier in order to stop rampant, screaming
instability. Essentially it was accomplished by defining
the bandwidth of the overall loop around the amplifier or
a particular gain stage within the amplifier—or both.
And typically robbing the device of its promise.
Hot on the heels of the μA709 came the now much
loved and despised, but always revered μA741. Best
known in its plastic encapsulated eight-pin dual-in-line
configuration, it still took our industry many years to
catch on to the fact that here existed a seemingly almost
vice-free op-amp. Well, at least, it was free of some of
the 709’s vices. It was heavily internally compensated
to nominally guarantee stability, but the penalty for this
was rapidly disappearing open loop gain with increasing
frequency. There was just enough gain left to squeeze
20 dB of broadband gain safely over a 20 kHz band-
width. Some IC manufacturers came up with good 741s,
usably quiet and free of the grosser output offset voltage
problems that plagued earlier devices. The 741 was also
output-protected to the extent of being short-circuit
proof, a relief to all.
Subsequent generations of op-amps to the 709
included the 748 (the uncompensated sister to the 741)
and the 301, again, some versions being excellent for
this class of device. That the 748 and 301 were user
compensated did allow for more optimal parameter
setting and in most circuits only required one capacitor
to achieve this (as opposed to the necessary two
resistor-capacitor networks for the 709).
Although on the surface this appeared to be of great
convenience to the designer, it disguised the fact that far
superior bandwidth and phase-margin performance
could be obtained by carefully considering the nature of
the compensation network. Rather than just a simple
capacitor of sufficient value to hold the amplifier stable
(which also turned the internal compensated transistor
into a Miller integrator doing absolutely nothing for the
speed of the device), a more complex network such as a
two-pole resistance-capacitance network, Fig. 25-17E,
improved matters greatly.
External feed forward, while in use as an inverting or
virtual-earth mixing stage, also enabled a dramatic
increase in bandwidth and speed over the more conven-
tional compensation arrangements, as shown in
Fig. 25-17.
25.7.1 Slew-Rate Limitations
All these early devices had one great failing that has
been leaped on vigorously by the hi-fi fraternity and Figure 25-17. Various op-amp compensation techniques.
F. Feed forward considerably
improves speed.A. Naked op-amp.B. Typical 709 compensation.C. Simple single-C compensation reduces
slew rate.D. Single-C with limiting resistor
extends bandwidth.E. Two-pole compensation improves
stability and bandwidth.Output compensationOutput protectionInput compensationAD301AD301LM318
AD301