Page 188 - Troubleshooting Analog Circuits

Back to Electronic Circuits . . . I75

Output oscillation is Check to see if output
intermittent? is ringing (see
Pease’s Principle).
Output distorts? Load too heavy? Check resistive and
reactive load.
Input is distorted? Check the input.
Slew rate distortion? Test with a lower input
frequency or size.
Bad gain? Resistors have bad tol- Check resistor markings
erance or wrong value? and tolerances.
Oscillations at various Check for oscillations
levels? across working range.
In general: Amplifier is suspected Swap in a known good
to be bad? amplifier.
Swapped amplifier is Swap “bad” amplifier
“bad,” too? into a good circuit.
“No output?” (Output Output shorted to ground? Amplifier gets hot.
is zero volts). Turn off power, measure
ohms.
Amplifier has low VOS Put in a signal thru R,
(a very good op amp)? see if output moves.
etc., etc.

Now, what is the best thing about this table? That it will solve all your op-amp
problems? Hell, no!! You can surely bump into circuits and problems that I have
never seen, that I have never even envisioned-circuits that need more help than this
table will give.
Well, is it because it gives you some general approaches that can be used for any
circuit? That is a good idea, and this is definitely of some value, but that is not the
most valuable thing.
Okay, what is the most valuable thing about this table? The most valuable thing is
that you can make up your own troubleshooting tables. You don’t have to be perfect,
or brilliant, or unemng. You don’t have to keep perfect notes. You don’t have to
make a plan of action and follow it exactly, one after another sequentially. You don’t
even have to write your plans down, although that is usually a good idea. You don’t
have to do any one thing, except to think occasionally. If you do some thinking, in a
skeptical way, you can guess solutions and tests and answers that would take me
forever. You have your own systems with which you are familiar, and your equip-
ment, and your friends. Together, you can solve problems that nobody else can. So, I
guess I’ll admit that some confidence would be a useful tool for you. And if there are
specialized techniques that you know, well, good for you. I never told you that I
know everything. But I bet some of the techniques in this book will be useful.
I will throw in a couple more scenarios for other basic circuits. They may not solve
every problem, but they will indicate the breadth and depth of thinking that may be
needed to solve tough circuit problems.
Examples:
Single-transistor amplifier Positive regulator (with LM317)
Negative regulator (with LM337) 723-type regulator
Absolute-value circuit Instrumentation Amplifier
Switching regulator using LM3524 Switching regulator using LM2575.