Current feedback op-amp and overload recovery - Page 1
Forum section: electronics / beginners

author: fmzambon
author_rank: Contributor
timestamp: April 29, 2023, 01:55:08 pm
content: Hello everyone,just a quick question: do current feedback op amps have an overload recovery time, same as "regular" voltage feedback ones? Or do they come out of saturation so quickly that they do not have such parameter?Specifically I'm looking at theLT1227. I can't find anything about "overload" or "saturation" in the datasheet (other than the saturation voltages). I tried googling, but nothing useful.Best regards,Andrea.

author: magic
author_rank: Super Contributor
timestamp: April 29, 2023, 07:51:28 pm
content: I have no practical experience but it would seem unlikely that they recover from clipping "instantaneously" (whatever that means). They use a similar pair of complementary common emitter transistors to generate output voltage swings and seem to have a similar chance of saturation problems there.The CFB input stage may potentially help with its high dynamic range - if the output is really far off from where it needs to be, the input stage feeds a relatively large current into the second stage current mirror, resulting in brief cross-conduction in the second stage and clearing the opposite mirror of stored charge faster. Maybe.

author: fmzambon
author_rank: Contributor
timestamp: April 30, 2023, 12:09:38 pm
content: Hello,yeah,somerecovery time is bound to be present. I mean, if inside the op amp there is even one BJT that has to come out of saturation before the amplifier can recover, then the storage time of that BJT would contribute to the recovery time of the whole op amp.What I'm not sure of is whether CFAs can have recovery times significantly slower than the storage time of a BJT. For example, do they generally have internal capacitors that have to charge/discharge slowly through current sources (I'm looking at you LM324!) or other arrangements like that?Best regards,Andrea

author: magic
author_rank: Super Contributor
timestamp: April 30, 2023, 12:21:53 pm
content: Most opamps are internally compensated with some sort of capacitors, or they have to deal with parasitic collector capacitance even in absence of explicit capacitors.Nevertheless, CFA tend to have very fast slew rate once the transistors get out of saturation and operate normally.LM324 is not a fast opamp. And its input stage runs out of steam with just tens of mV differential voltage - any larger difference won't make it slew faster, unlike a CFA.

author: Jwillis
author_rank: Super Contributor
timestamp: April 30, 2023, 08:29:55 pm
content: Quote from: fmzambon on April 29, 2023, 01:55:08 pmYour looking for the Over Drive Recovery. This is hard to predict because it depends things like the input transistor characteristics and the thermal characteristics of the die. There are ways to mitigate Over Drive by shunting overdrive current away from the input stage. How this is done I'm not sure.Maybe use a faster op amp?

author: David Hess
author_rank: Super Contributor
timestamp: May 05, 2023, 09:48:52 pm
content: Test it and find out.Current feedback operational amplifiers lack the "windup" mechanism of a Miller integrator in a compensated voltage feedback operational amplifier, and instead have bandwidth limited by their feedback resistance and internal parasitic capacitance.In voltage feedback operational amplifiers, thermal effects dominate over transistor recovery. The same seems likely to be the case with current feedback amplifiers, but their lower open loop gain may conceal it if they never settle as accurately.

author: MrAl
author_rank: Super Contributor
timestamp: May 05, 2023, 11:05:34 pm
content: Hello there,Yes the LM358 is subject to a delay after saturation and that i believe is due to that one capacitor that is used for unity gain compensation.Perhaps you can test the op amp you might want to use in a spice environment. There are some things that spice will not show though or will not show up the same as in real life. That's unfortunate, but you could at least try and also as someone mentioned get an op amp sample and test it yourself, which is always a good test anyway.

author: fmzambon
author_rank: Contributor
timestamp: May 17, 2023, 06:38:12 pm
content: Hello,I missed the last few replies, sorry.Yes, I already did several LTSpice simulations. I'm just not sure how much I can trust LTSpice when it comes to simulating op-amp overload recovery.I mean, I've seen LTSpice op-amp models that fail to model the amplifier's supply current correctly (I've seen a TL072 model where the supply current is simply modeled by putting a resistor across the supply pins. You can imagine what happens if you try to use the op-amp with supply voltages other than the one that the resistor was calculated for...), so I'm a bit hesitant to trust LTSpice blindly.I guess the only definitive answer is to built it and test itBest regards,Andrea

author: David Hess
author_rank: Super Contributor
timestamp: May 17, 2023, 08:16:18 pm
content: Quote from: fmzambon on May 17, 2023, 06:38:12 pmSpice will not model recovery caused by thermal effects, and most models lack the parasitic capacitance elements.

author: MrAl
author_rank: Super Contributor
timestamp: May 18, 2023, 06:27:42 am
content: Quote from: fmzambon on May 17, 2023, 06:38:12 pmHi,Yes it is a little tricky but some spice models do have the little cap in there, i think. If not, you may be able to reproduce the design transistor by transistor using an internal drawing. That means you will have to place that little cap in there. You can then also vary the cap value to see the effect on the recovery due to that cap.I will say it's pretty bad if the op amp pins to one rail. You may not even get 100Hz out of it.I went through this in the past a few times in real life and also had to deal with the input offset, as well as the output voltage range which spice does not seem to get right at all.

author: magic
author_rank: Super Contributor
timestamp: May 18, 2023, 09:30:40 am
content: Quote from: David Hess on May 05, 2023, 09:48:52 pmFor the record, the exact mechanism doesn't make much difference (and single stage conventional opamps like AD797 are also compensated by shunt capacitance to ground). In either case, input stage current (dis)charges some internal capacitance and this determines slew rate.The point of current feedback is higher dynamic range of the input stage, i.e. it can generate more current into said internal capacitor for faster slew rate, if differential input voltage gets large enough. (With CFA, by "differential input voltage" I mean voltage across the feedback resistor, because IN- is of course actively driven to equal IN+).I suppose that LTspice would model slew rate of this chip under various conditions reasonably well, but hell knows about recovery from internal transistor saturation, and surely no recovery from minor offset voltage shifts induced by internal power dissipation.