Opamp + buffer, how to bypass buffer at HF for unity gain?
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 28th October 2003, 12:17 PM #1 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden Opamp + buffer, how to bypass buffer at HF for unity gain? Consider the standard setup, noninverting op amp followed by buffer, buffer inside the feedback loop. It is usually recommended to close the feedback loop around the opamp for HF, by a capacitor from the op amp output to the point between the feedback and gain resistors, ie. the non-inverting input. That is straightforward and makes sense. My question then is, what if we have unity gain, that is the feedback is a straight wire (well, not so straight, perhaps )? Surely the capacitor won't hurt, but will it make much sense? The inductance in the feedback path from the buffer might cause HF problems, of course, which motivates the capacitor. I would suspect, although I confess I have not tried to calculate on this, that the inductance is usually so small that we would need quite a large capacitor to get a crossover frequency about the same as for reasonable values of feedback resistor in the other case. A very large capacitor at the op amp output seems not a good idea, especially since we buffered it in the first place to avoid such problems. The context of this question is that I will have said setup, but the option to switch between unity gain and a higher gain by shortcutting the feedback resistor with a jumper (for the unity gain case, of course). Should I just calculate a suitable capacitor for the high gain case, and then use that also for the unity gain case, or should I have jumpers to also switch in a larger capacitor, and if so how to choose the value?
 28th October 2003, 12:50 PM #2 peranders   Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden If you plan to use BUF634 I image you just follow the instructions at the end of the datasheet. You will get more trouble at low gain than high, so concentrate your efforts there. I have tested OPA134, OPA627 and AD8610 together with a BUF634 with excellent results, but I don't mention where. I'll guess everybody know it by now? __________________ /Per-Anders (my first name) or P-A as my friends call me Group buy: DCT03 DC trap for big toroidal transformers. Sign up for interest HERE. 41 pcb's in interest.
 28th October 2003, 01:26 PM #3 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden In this case I will use a discrete diamond buffer. Walt Jung recommends the capacitor, but it is obvious from the formula he states that he assumes a non-zero feedback resisitor, and does not say anything about the unity gain case. One might, perhaps, go by the recommendations for BUF634, but they seem to suggest the capacitor is usually not necessary, which may apply for compact layout. For certain reasons my feedback paths will not be so straight as one would like, and will pass at least one switch. I do thus assume a capacitor is very wise in my case, in the high gain case. The problem with uníty gain case is not really mentioned in the datasheet for BUF634. It is very cook-bookish, suggesting a value of 200pF for certain opamps, not even suggesting the value be adjusted according to feedback resistor value!! Besides their stability discussions seems also to assume one does not isolate the feedback path from the load with a resistor as Jung suggests. It seems to me the recommendations in the BUF634 datasheet are too simplified to be very reliable even for the BUF634.
ppl
diyAudio Member

Join Date: Apr 2001
Location: Seattle, WA U.S.A.
Maybe this is what you had in mind. SW1 allows the gain to be changed from 2 to a higher value and still retain your compensation capacitor. if you just want to compensate the Buffer only you can place a small cap from the output to the input of the buffer. See the Circuit at the top of pg. 10 of this app note. thay call it the capacitor bootsrap method.
http://www.intersil.com/data/an/an548.pdf
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 28th October 2003, 01:53 PM #5 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden PPL, It is similar to what I have in mind, but for certain reasons I want unity gain, not a gain of 2, so I can see no other solution than shorting the feedback resistor. In the worst case, I could consider using a gain of 2 and a voltage divider at the input, but I would like to avoid that. I looked at that linked document. Maybe I am missing something, but I don't quite see how it solves the problem. I suppose, without having really thought about it, that it is intended to compensate for phase shift in the buffer. In my case, I am more worried about my not so straight feedback path, and inductance in that, which may be harder to compensate for.
 28th October 2003, 03:15 PM #6 jcx   diyAudio Member   Join Date: Feb 2003 Location: .. using PPL's drawing above, why not elimiante R2, with S1 open you have unity gain and don't have to switch R3 R3*C1 form an integrator with respect to the ouput, the idea is to have the integral time constant below the frequency that you get excessive phase shift from the buffer+load (especially useful with some modern op amps which are designed to have only 30 degree phase margin to push up the advertized bandwidth in "unity gain stable" mode)
 28th October 2003, 03:29 PM #7 peranders   Electrons are yellow and more is better! diyAudio Member     Join Date: Apr 2002 Location: Göteborg, Sweden Christer, this capacitor value is important only if you want the most out of the amp+buffer combination at very high frequencies. I have tested without this capacitor and it still works. This capacitor needs to be trimmed only if the opamp is too fast for the buffer. __________________ /Per-Anders (my first name) or P-A as my friends call me Group buy: DCT03 DC trap for big toroidal transformers. Sign up for interest HERE. 41 pcb's in interest.
 28th October 2003, 03:33 PM #8 Christer   diyAudio Member   Join Date: Sep 2002 Location: Sweden Thanks jcx, Of course that's the way to do it. Stupid of me not to think of that. That's even the standard way to do unity gain feedback. Must be some mental blocking that remains from some earlier phase in the design when things looked different. Or maybe it is just that I have a bad head ache today? Or maybe I am just trying to blame my stupidity on someting?
Christer
diyAudio Member

Join Date: Sep 2002
Location: Sweden
Quote:
 Originally posted by peranders I have tested without this capacitor and it still works.
On your PCB and with your combination of op amp and buffer
yes.

Quote:
 This capacitor needs to be trimmed only if the opamp is too fast for the buffer.
And what if it is?

Anyway, Jcx seems to have solved it for me.

ppl
diyAudio Member

Join Date: Apr 2001
Location: Seattle, WA U.S.A.
Thanks John for the note regarding making my circuit unity gain.

Christer> It is true the compensation method described in the app note I linked to is to compensate the Buffer only. If you noticed the illustration circuit it was used in had the Buffer driving a Converter input. This is almost a purely capacitive load with an almost non existent parallel resistance. Emitter followers do not like this very well and can peak or become unstable driving purely capacitive loads. Several methods are used to deal with this, with the most common being adding a small resistor in series with the output of the Buffer to isolate the load capacitance. This method has the drawback of higher output impedance. An alternative method I use a lot is to put a ferrite bead in parallel with this output isolating resistor so as to bring down the output impedance at lower frequencies while still letting the impedance rise at higher frequencies to maintain capacitive load isolation. This is similar to the R/C network on the output of loudspeaker Amps. Also in addition to C1 you can put another small cap across R3 to compensate the overall loop including the Buffer. Finally you could additionally add what I call Noise gain compensation. This is a series RC/ network connected from the Inverting input to the non inverting input of the op amp. I must point out that noise gain compensation requires at least C1 in the first schematic illustration I posted. I don’t think you will have to go that far however and Noise gain compensation can get real tricky under some situations.

The Attached Image is of the AD825 op amp. notice the R/C network on the output stage. this is similer to the capacitor placed between the Buffers output to input as depicted in the app note i linked to in my first reply.
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 ad 825 schematic optim.gif (23.3 KB, 517 views)

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