Page 130 - Troubleshooting Analog Circuits

Even Buffered Circuits Can Oscillate I17

ture. Most buffers don’t have any thermal shutdown feature, but the new LM632 1 s
and LM6325s show that a good op amp, at least, can have such features designed in.
When using buffers to drive remote loads, be sure that the transmission lines or ca-
bles have suitable termination resistances on both ends to prevent reflections and
ringing. If you can afford the voltage drop, it’s best to put about 50 l2 between the
buffer and its cable.
When your buffer provides a lot of extra voltage gain, you must make sure that the
gain rolls off in a well-engineered way at high frequencies, or the loop will be unstable.
If the buffer-amplifier has a positive gain, as in Figure 9Sb, you can use capacitive
feedback around the main amplifier. But when the buffer-amplifier has a gain of -10
(Figure 9.5c), you may want to apply a feedback capacitor from the input of the
buffer-amplifier (the output of the op amp) to the noninverting input of the op amp.
In some cases, you can achieve stability by putting a series RC damper from the non-
inverting input to ground to increase the noise gain. but this trick doesn’t always work.
Damping this loop is tricky, because there is so much gain stacked up in cascade. But
the feedback capacitor to the negative input makes this safe and easy.

Nasty Latches

A circuit that inadvertently latches up presents a problem exactly opposite that of an
oscillating circuit. Or. you could correctly say that a latched-up circuit is an oscillator

I I

- GAIN -0.95

p-w

OPTIONAL pi --+(---A GAIN=-10

- C C= 10,100, OR 1000 pF
-
AS SUITABLE

Figure 9.5. Depending on the gain of the buffer, you can use these three schemes to stabilize a buffered
amplifier.