Page 113 - Troubleshooting Analog Circuits

100 8. Operational Amplifiers-The Supreme Activators

they really shouldn’t trust your parts to always be better than average. Maybe in Lake
Wobegon, all the kids are better than average, but you can’t go shopping for op amps
and complain when they are not all “better than average.”

Oscillations Do Occasionally Accompany Op Amps

One of the most troublesome problems you can have with op amps is oscillation. Just
as you can build an oscillator out of any gain block, then you must admit that any
gain block can also oscillate when you don’t want it to. Op amps are no exception.
Fortunately, most op amps these days are well behaved, and you only need to take
four basic precautions to avoid oscillations.
First, always use some power-supply bypass capacitors on each supply and install
them near the op amp. For high-frequency op amps, the bypass capacitors should be
very close to the device for best results. In high-frequency designs, you often need
ceramic and tantalum bypass capacitors. Using bypass capacitors isn’t just a rule of
thumb, but a matter of good engineering and optimization.
Second, avoid unnecessary capacitive loads; they can cause an op amp to develop
additional phase shift, which makes the op-amp circuit ring or oscillate. These effects
are especially noticeable when you connect a 1 X scope probe or add a coaxial cable
or other shielded wire to an op amp, to convey its output to another circuit. Such
connections can add a lot of capacitance to the output. Unless you’re able to prove
that the op amp will be stable driving that load, you’d better add some stabilizing
circuits. It doesn’t take a lot of work to bang the op amp with a square wave or a
pulse and see if its output rings badly or not. You should check the op amp’s
response with both positive and negative output voltages because many op amps with
pnp-follower outputs are less stable when VOut is negative or the output is sinking
current. Refer to the box, “Pease’s Principle.”
I’ve seen pages of analysis that claim to predict capacitive-loading effects when
the op amp’s output is resistive, but as far as I am concerned, they’re a complete
waste of time: The output impedance of an op amp is usually not purely resistive.
And if the impedance is low at audio frequencies, it often starts to rise inductively at
high frequencies, just when you need it low. Conversely, some op amps (such as the
NSC LM6361) have a high output impedance at low frequency, which falls at high
frequencies-a capacitive output characteristic, so when you add more capacitance
on the output, the op amp just slows down a little and doesn’t change its phase very
much. But if an op amp is driving a remote, low-resistive load that has the same
impedance as the cable, the terminated cable will look resistive at all frequencies and
capacitive loading may not be a problem. (But you still have to be able to drive that
low-impedance 7542 load!)
You can decouple an inverter’s and integrator’s capacitive load as shown in Figure
8.9. If you choose the components well, any op amp can drive any capacitive load
from 100 pF to 100 pF. The DC and low-frequency gain is perfectly controlled, but
when the load capacitor gets big, the op amp will slow down and will eventually just
have trouble slewing the heavy load. Good starting-point component values are R1 =
47 to 470 i2 and CF = 100 pF. These values usually work well for capacitive loads
from 100 pF to 20,000 pF. If you have to make an integrator or a follower, you’ll
need an additional 4.7-kQ resistor as indicated in Figure 8.9(d).
In some cases, as with an LM110 voltage follower, the feedback path from the
output to the inverting input is internally connected and thus unavailable for tailoring.
In this case, we can pull another trick out of our bag: The tailoring of noise gain.
Noise gain is defined as I$, where p is the attenuation of an op amp’s feedback