Page 116 - Troubleshooting Analog Circuits

Oscillations Do Occasionally Accompany Op Amps I03

where GBW is the gain-bandwidth product. In those cases in which the gain or
impedance is low, such as where

(1 + R,/R,,) 5 2dGBW x R, x C,,

use the following equation

c, = in
( + RF/ ‘in)

I won’t bore you with the math, but these equations did come from real analytical
approaches that have been around for 20 years-I championed them back at
Philbrick Researches. The value of CF that you compute is not that critical; it’s just a
starting point. You really must build and trim and test the circuit for overshoot,
ringing, and freedom from oscillation. If the equation said 1 pF and you get a clean
response only with 10 pF. you’d be suspicious of the formula. Note that when you go
from a breadboard to a PC board, the stray capacitances can change, so you must
recheck the value of CF. In some cases, you may not need a separate capacitor if you
build 0.5 pF into the board. In any case, you certainly don’t have to just trust my
equations-build up your breadboard and fool around with different values of CF and
check it out for yourself. See if you don’t agree.
My last recommendation is that when you think the circuit is okay-that is. free
from oscillation-test it anyway per Pease’s Principle to make sure it’s as fast or
stable as you expect. Be sure that your circuit isn’t ringing or oscillating at any ex-
pected operating condition or load or bias.

Noises, Theoretical and Otherwise

In addition to oscillatory behavior, another problem you might have when using op
amps is noise. Most op amps have fairly predictable noise. It’s often right down at
theoretical levels, especially at audio frequencies. There’s a pretty good treatment of
noise and its effects in various applications in Thomas Frederiksen’s book (Ref. 4).
Also, if you want to optimize the noise for any given source resistance or impedance.
National Semiconductor’s Linear Applications Note AN222 (Ref. 5) ha5 some good
advice, as does the article in Ref. 6.
You’ll have difficulty with noise when it’s unpredictable or when op amps of a
particular type have varying noise characteristics. This problem rarely happens at
audio frequencies but is likely to happen sporadically at low frequencies, such as 10
or 100 Hz or even lower frequencies. Every manufacturer of transistors and ampli-
fiers tries to keep the noise low, but occasionally some noisy parts are built.
Sometimes the manufacturer is able to add tests that screen out the noisy parts. But
these tests aren’t cheap if they take even one second of tester time, which may cost
three cents or more. We are trying to add some 0.3-second tests to some of our more
popular amplifiers, but it’s not trivially easy.
Here’s a tip-we find that true RMS testing for noise is a big waste of time, be-
cause amplifiers that are objectionably noisy have much worse p-p noise than you
would guess from their RMS noise data. So the p-p test is the best discriminator. We
get the best resolution with a bandwidth from around 30 Hz to 3 kHz, after we roll off