Stability of opamp buffer with 200nF output capacitance? - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: November 30, 2019, 07:20:31 pm
content: Hi,We are investigating getting a fixed 8V3 rail with a cheap opamp as in the attached. (it must source and sink…the opamp’s output current is as in the attached)Can you confirm that the 4R7 resistor at the output of this MIC6211 opamp makes it stable?*........................*............................*..............................*.......................*.....................................*.........................Clearly the 200nF output capacitance adds a pole at 1/(2.pi.RO.CL)Where:Ro = opamp output resistance = 125RCL = Capacitive load = 200nF (Ceramic X7R capacitor)….This pole is at 63.7kHz.I would have thought a zero is needed to cancel this pole….hence the 4R7 was added.The 4R7 (=Rz) adds a zero at 1/(2.pi.Rz.CL) = 169kHz(The load resistance = 8V3/0.0085A = 976R = RL)Clearly , there is also a load pole at 1/(2.pi.RL.CL) = 815HzThere is also an unknown pole added inside the opamp’s internal compensation circuitry. This is going to make it difficult to make out the Bode plot.When I have all the poles and zeros, I can make out the bode plot and check the gain and phase margin with the 4R7 compensation resistor added.The pole transfer function is T(s) = 1 / { 1+ j(w/w0)}The zero transfer function is T(s) = {j(w/w0)} / { 1+ j(w/w0)}But what are all the pole and zero frequencies?MIC6211 datasheethttp://ww1.microchip.com/downloads/en/DeviceDoc/mic6211.pdf(incidentally, the actual voltage across the 200nF output capacitor bank is stable in the prototype circuit we have in the lab…in fact, it is even stable if we increase the output capacitor bank to 51.7uF of ceramic X7R capacitance!!….but we want to be sure)

author: Kleinstein
author_rank: Super Contributor
timestamp: November 30, 2019, 07:30:52 pm
content: In the circuit as shown the extra resistor does not help with stability, it is more like an additional problem.The right way would be to also separate the feedback with the low frequency feedback from the output, but the high frequency feedback directly from the OPs output. That is the standard way of driving capacitive loads.

author: jimmc
author_rank: Frequent Contributor
timestamp: November 30, 2019, 10:15:31 pm
content: This may be of help to you, particularly the 'In-loop compensation' section. (As Kleinstein has suggested.)https://www.analog.com/en/analog-dialogue/articles/ask-the-applications-engineer-25.html#Jim

author: ocset
author_rank: Super Contributor
timestamp: November 30, 2019, 10:26:52 pm
content: Thanks, sorry ive just finished punching myself in the head a few times....i posted the wrong schem...the right one is as attached.The schem in the top post is what we accidentally did on the first prototype.....strangley, even with the feedback taken from the wrong place like in the top post, it stays stable even if the ceramic output capacitance is increased to 51.7uF.We appreciate that the schem in the top post is wrong (feedback taken from wrong place)...though the fact that it is stable even when capacitance is increased to 51.7uF seems to suggest that this may be workable anyway. I like the in-loop-compensation method , but its more components, and if just adding a compensation resistor can do it like in this post, then we'd be happy with that.....the output capacitance can be reduced to 20nF if need be.Thanks Jimmc for the "In loop compensation" link...i noticed they are bringing the feedback back to the non-inv input....i believe that this is a mistake?

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 01, 2019, 07:04:58 am
content: Quote from: jimmc on November 30, 2019, 10:15:31 pmit looks like the swapped the the +/- inputs of the last schematic diagram

author: gamalot
author_rank: Super Contributor
timestamp: December 01, 2019, 08:10:21 am
content: Maybe you would like to try something like this

author: jimmc
author_rank: Frequent Contributor
timestamp: December 01, 2019, 03:22:45 pm
content: Quote from: treez on November 30, 2019, 10:26:52 pmYou are both correct, that is a mistake. Sorry I didn't spot it.Jim