When I present seminars,
I often ask the members
of the audience to hold
up their hands if they
think bipolar op amps have better gain and
linearity than CMOS. I get a good majority of
hands. But neither is bad!
The good-old LM301A (well over 30 years
old) has a good gain of 260,000 at no load,
with just 75 µV p-p of gain error while its output
is swinging 20 V p-p (Fig. 1). What happens
when we put on a load? With its rated 2k
load, the gain falls and even reverses a little and
becomes nonlinear in its error.
The lower curve shown here (with a 1k
load to exaggerate the error) shows that the
LM301A’s nonlinearity becomes as big as 48
µV p-p. This isn’t due to low “gain” or gM, but
to thermal feedback from the output transistors
to the input transistors, which is caused
by imperfect layout of the temp-sensitive
input transistors.
Well, this does look lousy, but there are mitigating
factors. This thermal effect is most obvious
at 0.2 to 20 Hz. At frequencies above 150
Hz, this effect tends to go away and smear out,
and at 1 kHz, you can hardly see it. So, this isn’t
a big problem for audio amplifiers.
Also, if the amplifier isn’t driving heavy load
currents, this thermal problem shrinks proportionately.
At light loads, it’s negligible. Even
with a 4k load, an LM301 can make a unitygain
inverter with a nonlinearity of 1.2 ppm. At
lighter loads, it’s even better.
Where can you learn more about this thermal
crosstalk? At Application Note AN-A, written
by Jim Solomon. If you go to www.national.com/rap and type AN-1485 in the search space, it
explains this thermal feedback and also tells you
how to find AN-A. So, even bipolar amplifiers
with “low” output impedance can have imperfect
gain and linearity due to thermal feedback from
the output transistors to the input stages.
ON THE BENCH
Now let’s look at some CMOS amplifiers
with ~ rail-to-rail outputs. The LMC662 is
typical—one of our first CMOS amplifiers
(Fig. 2). Like the LM301A, its gain degrades
when overloaded with 1 kO. But the nonlinearity
(deviation from best straight line) is only
18 µV p-p. That’s not bad for an 8-V, 8-mA
p-p swing.
This isn’t thermal cross-talk, just a matter of
honest gain. The LMC662 has four honest gain
stages to source current, but just three stages to
sink load current. Still, its high output impedance
causes the gain to rise a lot when lightly
loaded. How high does it rise? Well, it seems to
rise higher than 4 million, but the error is down
in the noise. As with the LM301A, its distortion
when driving a 4k load is about 1.2 ppm.
So, it really is possible to get low distortion
with ordinary op amps. And, it’s easy to
get exquisite linearity with good, inexpensive
amplifiers. For example, the LMC6022 does
better than 0.3 ppm (Fig. 3). That’s pretty good
for a micropower op amp. Its open-loop ZOUT
is above a megohm, but its closed-loop ZOUT is
below a milliohm!
NEWER AND BETTER
Now, I’ve shown you some of our worst and
oldest amplifiers with the worst distortion and
the poorest gain. If you want to see some of our
newer and better amplifiers, with better distortion
down to 0.03 ppm, go to www.national.com/an/AN/AN-1485.pdf.
In this app note, I explain the gain curves of
dozens of op amps. Be sure to look up the lowvoltage
LMC6042 and the full-voltage
LME49720, for example. They are better than
0.3 ppm nonlinear! Appendix A on pages
22-23 of that app note lists many op amps,
using both CMOS and bipolar transistors, with
nonlinearities from 2 ppm down to 0.3 and
even 0.03 ppm. All were tested using the gain
test circuits shown.
Comments invited! rap@galaxy.nsc.com —or:
Mail Stop d2597a, national Semiconductor
P.O. Box 58090, Santa clara, ca 95052-8090