OP Amp Oscillations/Feedback

[FS]

When employing an op amp in a negative feedback configuration, phase
shifts due to the feedback loop, and the op amp itself can lead to
oscillation. I am experiencing something like this with the 741. Of
course I can play some tricks with capacitors, but my first question
is: Where can I find a simple resource describing the differential
equation for this scenario? Perhaps I have overlooked one on the web,
or maybe a good textbook will do (Horowitz and the Jung "Cookbook"
didnt'help here)

Thanks for any tips
Fritz

 

[Gene the Skeptic]

What kind of "feedback" are you using? If it is just resistive it is
unlikely that the circuit becomes unstable because there is only one pole,
created by the OPAMP transfer function characteristics (-6dB/oct slope) and
the resistor in the feedback.
Gene

 

[Ban]

FS wrote:

When employing an op amp in a negative feedback configuration, phase
shifts due to the feedback loop, and the op amp itself can lead to
oscillation. I am experiencing something like this with the 741. Of
course I can play some tricks with capacitors, but my first question
is: Where can I find a simple resource describing the differential
equation for this scenario? Perhaps I have overlooked one on the web,
or maybe a good textbook will do (Horowitz and the Jung "Cookbook"
didnt'help here)

Thanks for any tips
Fritz

Google for "opamp stability criterion" or similar. You can do that by
analyzing the actual circuit inside the opamp together with stray
capacitances from the PCB layout, or by the bode-plot resulting from it.
Whenever the open loop gain is greater than 1 *and* the phase-shift gets
180°, the amp will oscillate.
When you use reactive compensation elements, these are not dirty tricks, but
scientifically chosen components which tailor the transfer function in such
a way to insure stability.
--
ciao Ban
Bordighera, Italy

 

[FS]

Thanks for your response Gene-
I have the inverter-input connected to ground. The non-inverter is
connected to a photodiode- a little tricky here-the photo diode
receives light from a laser on the op-amp output to create the loop
optically. When I put the scope on either the diode or laser I get
oscillations-I think around 1 micro-second but I need to check when I
get back to the lab. I presume that the output, when attempting to
re-zero the difference on the inputs, overshoots, resulting in
oscillation. Further, I would expect laser and photo diodes to have
some inductance and capacitance (they are in close proximity so there
is no lightspeed lag here).
I can play with a decade capacitor box but my main interest is
setting up the diff-E for this loop so I can estimate the required
compenstation analytically. I thought perhaps this is described in a
textbook somewhere-I tried to do it with taylor series but didn't come
out the way I expected, and I am sure there is more to the picture
that a good text would add.

Thanks
Fritz

 

[FS]

Thanks for your reply Ban-
as I mentioned I am trying to analytically calculate the oscillation
rather than resort to "tricks" or trial-and-error here. I did not find
anything on the web which describes negative feedback in terms of a
second order differential equation however I presume the electronics
terminlogy here is "transfer function" and maybe I am looking to much
at the mathematical approach to this kind of problem. I will head out
to the library and see what I can find on feedback loops.

Thanks
Fritz

 

[Gene the Skeptic]

Hi Fritz,

the discipline that studies this topic is called "Control Theory" and it is
quite complex. A book I could recommend is "Automatic Control Engineering"
by Francis Raven. In any case, a loop that is unstable( oscillations) is
indicative of excessive phase lag and gain above unity. I do not know if you
have the capability to perform an open-loop Bode-plot, but if you were it
would tell you the degree of instability of the loop and the way to
stabilize it. Adding capacitors will not solve the problem. Considering your
lack of knowledge in this field I recommend the following:
1. Reduce the gain of the OPAMP until the loop has good stability.
2. Is the performance accettable? if yes, leave it. If not go to the next.
3. Is there a tracking error problem? If yes you could try to to put a
capacitor in SERIES with the feedback resistor that sets the OPAMP gain.
Start with a big one and step by step decrease its value until the loop
become unstable. Mark the capacitor value when this happens. Intall a cap
that has double of the capacitance. Now the loop should be stable, with
virtually zero tracking error.
4. If you have dynamic tracking problem, then it is problematic to improve
the situation without having control theory knowledge.

Das Glueck ist dem Kuehnen hold!

Gene

 

[Gene the Skeptic]

I agree 100%. The theory can lead to the "ballpark", but then good lab
investigation and analysis finalizes the design.
Gene

 

[Ban]

FS wrote:

Thanks for your reply Ban-
as I mentioned I am trying to analytically calculate the oscillation
rather than resort to "tricks" or trial-and-error here. I did not find
anything on the web which describes negative feedback in terms of a
second order differential equation however I presume the electronics
terminlogy here is "transfer function" and maybe I am looking to much
at the mathematical approach to this kind of problem. I will head out
to the library and see what I can find on feedback loops.

Thanks
Fritz

Fritz, the EEs do not use differential equations for their transfer
functions, but the Laplace transforms with the frequency operator S= j
f/f_nominal. This way the whole thing can be solved with simple algebra and
can be overlooked easily.
--
ciao Ban
Bordighera, Italy

 

[john jardine]

"Ban" <bansuri@web.de> wrote in message
news:qgw1d.249435$OR2.11281411@news3.tin.it...

Whenever the open loop gain is greater than 1 *and* the phase-shift gets
180°, the amp will oscillate.
When you use reactive compensation elements, these are not dirty tricks,
but
scientifically chosen components which tailor the transfer function in
such
a way to insure stability.
--
ciao Ban
Bordighera, Italy


I don't know about the rest of the world out there, but before spice

simulators, I never ever found a circuit that could be nicely stabilised by
the application of cold theoretical methods.
Disregarding of course, those trivial textbook examples, or the cop-out of
slugging the response to that of treacle, or having available a roomfull of
gain/phase measuring equipment, or having a pocketfull of those nice,
dominant poles that everyone except me seems to have in their toolkit.
The impossible to define parasitics seem always a major factor. The
semiconductor data sheets give too little info. The source and load Z's are
usually nothing like those estimated.
That 20nH of unseen, inductive cross-coupling may be impossible to analyse,
measure, estimate, simulate or (sometimes) even understand but is resulting
in 200megs oscillation, massive current drain and signal distortions, random
device failures and exciting artefacts in the preceding electronics.
Any solution must come from an in-the-flesh-on-the-bench-suck-it-and-see
approach. With experience it gets easier to home in on the sweet spots but
it's still ad-hoc and unscientific.

As the best *fixes* usually seem to involve the odd strategic R or C (or
god forbid, an occasional L), to me it still most definitely feels as if I'm
cheating or playing a dirty trick on the circuit, when just by adding that
critical 2p cap' the response instantly changes from a monstrous nightmare,
to that of pure sweetness and light.

Unless others know better

regards
john

 

[FS]

OK and I thank you all.
I checked back and I see 50 microsecond Oscillations. Again the
feedback is optical (e.g. there is no feedback resistor). I found that
placing a .04 uf capacitor from the output to input removed the
osciallations and I am now able to modulate the laser as needed.
I appreciate the book reference from Gene and will get a copy (I
have D'azzio but I presume it is somewhat dated).

Viele Dank,
Fritz
Woods Hole Oceanographic