Opamp stability question

I believe that they are saying the same thing, just differently. An op amp in the inverting configuration inherently has a -180 deg phase shift. Hence, per stability criterion, if the loop gain has adds an (additional) phase shift of -180 degrees (or a total phase shift of

-360, 0, or 360 whatever you want to call it) at the 0db gain (db as 20

• log V/V) or a gain of 1, then you have an unstable system.

They are both right. The first assumes that at low=20 frequencies, there is a 180 degree phase shift (signal=20 inversion) to start with, and an additional 180 degrees, as=20 frequency rises, brings the total phase shift around the=20 loop to 360. The total, including the normal low frequency=20 inversion, is described in the second one.

Thank you folks!

...Just an additional question.

In reference to this IC:

MAX4469

Regarding the phase and gain charts given on page 3, at what opamp input is the test signal normally applied?

I'm curious because if I input a signal at the inverting input, then I'm technically adding a 180 degree phase shift instantly (right?). If that's the case, then at 100kHz I would already get a -180 - 120 = -360 degree phase?

Thanks!

Stability has little to do with where you inject an input signal. It has everything to do with a signal loop that can regenerate an echo.

The graphs on page 3 show the open loop gain and phase shift, if you apply a signal between the + and - input (with the phase referenced to the + input), magically balanced to produce an average output voltage of Vcc/2 (with a 100k load resistor terminated to Vcc/2, no load capacitive load and with 100 pF capacitive load).

Note the broad frequency band that produces roughly -90 degrees phase shift as the gain falls from a dominant roll off pole. Note also that the phase shift is still somewhere between -90 and -180 degrees phase shift as the frequency that produces 0 db gain is reached.

The negative feedback would normally go to the - input, which adds another -180 degrees to the phase shift shown.

Ok, I'll remember this in the future. Thanks, John!

Ahh, I see. So at audio frequencies, I should still have a stable condition.

By the way, is phase affected by a change in gain? I'm curious because if I change the gain of my opamp to 20V/V, its bandwidth decreases to about 10kHz. I'm not sure if that affects the phase, too.

At closed loop gains all the way to 0 db (unity gain follower) they should be stable. Closed loop gains of 10 or more (20db or greater) where the phase shift at the frequency where the closed loop gain goes through 0db (including the -20 db or more of attenuation in the feedback that gets added to the open loop gain) is 90 degrees or less will not only be stable, but not produce a frequency peak (tendency to ring) at any frequency. But gains higher than

20 db will produce a flat response band less than 20 kHz, so for the full 20 kHz audio range, 20 db is as high as you can go with these.

To find the phase shift at the point where the closed loop gain hits 0 db, add the open loop gain of the amp with the attenuation of the feedback network. If you divide the output signal by a factor of 10 (-20db) to close the loop, then then you subtract that 20 db from the open loop gain (move the gain curve down 20 db) and see where this shifted curve (amplifier gain factor times feedback attenuation factor to complete the loop or amplifier gain in db plus attenuation in negative db) passes through 0 db. Then you look directly below, at that frequency, to see what the amplifier phase shift is at this reduced o db closed loop operating point.

Sorry, John, you lost me. I guess I'll need to sleep on this..... Where are you getting the -20dB attenuation from the feedback? Where are you getting this information: "But gains higher than 20 db will produce a flat response band less than 20 kHz, so for the full 20 kHz audio range, 20 db is as high as you can go with these." I'd really like to know how you figure this out.

...And this, too. I think this is the time where I would need an alphabet book with pictures. I still have a lot more texts to go to before I can catch up.

To program the amplifier for a gain of 10 (20 db) you need to divide the output voltage by a factor of 10 and connect the divided output signal to the - input. This was a hypothetical case.

Look at the gain curves on page 3. If you apply one of these amplifiers as a gain 10 block (20 db) at the frequency where the gain curve falls below 20 db, the feedback will completely lose control of the amplifier, because beyond that, the amplifier is not capable of a gain if 20 db. If you program it for a closed loop gain of 100 (40 db), it will go open loop and the gain will start to follow the open loop curve down, beyond the frequency where the curve falls below 40 db, which is about a decade lower than the one where it falls below 20 db.

You are a lot closer than you think.

Thanks again, John!

Hello again:

Just another follow up question regarding op amp stability. Referring to the MAX4469 again, on the plot on page 3 of the datasheet, it shows that the 0db gain is at 200kHz. Now, when I go down to the the phase plot, it shows that the phase at this frequency is -140 degrees. So my phase would only need to change by 40 degrees to get to -180 degrees. Why is the phase margin in the Electrical Characteristics table on the same page showing 70 degrees?

Thanks!

Sorry, I don't have an answer for you. Normally, the phase margin and gain margin specs refer to the follower configuration, but they don't have to. Sometimes specs are made to look better than they are by including some note that a particular test circuit is used. But I don't see that, here. The non inverting waveforms also do not correspond to a simple follower based on the gain and phase curves. There should be some ringing or at least overshoot in the edges. I suspect they used a tweaked feedback network, and 'forgot' to include that information on the data sheet.

Thanks again, John! Your input is very much appreciated. :)