Quick and Dirty Op-Amp Test Circuit

[PerroGrande]

I'm *sure* this circuit isn't original, so I'm not going to pretend to take credit for it.      It is what the title infers -- a very quick and dirty way to check an op-amp. 

This version shows both sides of a dual op-amp.  It would be a trivial matter to change the pins to test a single or a quad.



How it works and what one should expect to see:

R1 and R2 form a voltage divider.  Because they are the same value, the voltage between point A and ground should be very near the midpoint of the supply/battery voltage.

This voltage is applied to the non-inverting input of both op-amps.  Both amplifiers are wired as gain=1 buffers. 

By virtue of the "golden rules of op amps" the output at points D & E should show the same voltage as we see at point A -- about half the supply voltage.  (This circuit is commonly used to provide the Vref voltage in pedal circuits.)

So, for quick and dirty testing (all voltage measurements are with respect to ground):

* Measure the voltage at point A with lines B & C disconnected. 

* Connect line B to the midpoint of the divider and re-measure point A.  If the voltage has changed noticeably, you *probably* have a shorted or otherwise bad input transistor(s) on the first half of your op amp.  I've seen a defective 4558 pull the voltage divider down to below 2v.

* If no sagging occurred when B was connected, compare the voltage at point D to the voltage at point A.  The voltage at point D should be very close to that at point A.  If you see a substantial deviation, which typically takes the form of the op-amp stuck at or near one of the rails, you probably have a bad op-amp.

* Disconnect line B and repeat the process for Line C and point E to test the second half of the dual amp.

Disclaimer:  This is a VERY crude test, and is advertised to do nothing more than provide a quick check for "is this op amp totally fried."  It will not detect some of the subtleties that can crop into op-amps (as described by R.G. in his article "When Good Op Amps Go Bad" over at Geofex). 

This is one of the quick and dirty circuits I keep tucked away on the corner of a breadboard (along with R.G's circuit to measure germanium gain and leakage and the FET matcher).  I know it isn't the most imaginative thing in the world, but if it helps someone troubleshoot a problem, then it has done well.