Agilent 6060b electronic load, why it's better than Dave's - Page 1
Forum section: electronics / projects

author: baoshi
author_rank: Regular Contributor
timestamp: April 06, 2013, 08:27:02 am
content: I'm reading the schematic of Agilent 6060b electronic load. Attached is the power stage.It uses inverting configuration instead of Dave's dummy load design. I tried to simulate it. I can see better loop stability when the integration capacitor C71 value is small, hence this design has better dynamic response.However I cannot understand what's the function of R162, C1 and C49.Can someone help me?ThanksBob

author: BravoV
author_rank: Super Contributor
timestamp: April 06, 2013, 09:29:26 am
content: Interesting finding, what did you use to simulate that ? If its LTSpice, share it here please.

author: baoshi
author_rank: Regular Contributor
timestamp: April 06, 2013, 10:10:38 am
content: Quote from: BravoV on April 06, 2013, 09:29:26 amI upload both ltspice simulations here.Dave's design starts oscillation at about 5A. To make this design stable I need to increase C1 to 1000p. At the expense of losing dynamic response.Interesting finding is that if I remove R162, C1 and C49 from 6060 design, it oscillates at 3A+Update:It seems that C1 is the key. Adding this capacitor to Dave's design resulting in similar performance as Agilent's

author: jpb
author_rank: Super Contributor
timestamp: April 06, 2013, 10:20:00 am
content: I would guess that R162 is to provide a fixed dc gain for U1 - without it you'd be left with the open loop gain which is probably pretty variable from device to device.I think that C1 and C49 provide some negative feedback between the two op amps at high frequency (they capacitively couple the positive input of one op-amp to the negative input of the other via a resistor - perhaps this helps with phase-margin, I'm no expert.)

author: Harvs
author_rank: Super Contributor
timestamp: April 06, 2013, 11:58:10 am
content: Keep in mind in your simulations you're using different parts. In particular the poles caused by the output impedance of the op-amps driving the capacitance of the FETs. If that's not the same, then your stability assessment can be way off.In the 6060B, they use the MC34072p which one of its key selling points is the ability to drive up to 10nf. That's pretty significant for an opamp, and while I can't find anything specific in the LT1012 datasheet (I haven't looked too hard), the fact that it's designed for precision measurements makes me doubt it will have that much drive capability. If the opamp doesn't have that output drive capability, the output stage will saturate against the 22.5ohm load and it'll take a lot of compensation capacitance to get any stability margin back.

author: baoshi
author_rank: Regular Contributor
timestamp: April 06, 2013, 01:36:45 pm
content: Quote from: Harvs on April 06, 2013, 11:58:10 amHi Harvs,Great point.Although I found a LT design note (http://cds.linear.com/docs/en/design-note/dn83.pdf) claiming LT1012 is capable of driving up to 10n capacitive load, I did simulation with MC33072 (cousin of MC34072 with only temperature rating difference) and receive much better result. There are some overshot but generally it seems stable.However if I replace my 6600b simulation with MC33072 I get superb result, no overshot at all. It seems that the 1800p capacitor is still the key.I attach new simulation files below.Bob

author: free_electron
author_rank: Super Contributor
timestamp: April 06, 2013, 03:21:33 pm
content: Don't substitute parts in that circuitry... The opamps are carefully chosen.... Like the capability to drive the capacitance.The resistor limits the opamps gain.The two caps act as feed-forward . For a rapid changing signal they decrease the loop gain.So if this would want to oscillate the gain would go down and it would stop all by itself.Now, there is something else in this beastie that makes it difficult. You cannot simulate its behavior with just a single loadbloack. In the 6060 there are 4 or 8 of these in parallel....This has an impact as well. If one node goes out of whack the other 7 pull in the reins... So they essentially discipline each other...

author: baoshi
author_rank: Regular Contributor
timestamp: April 09, 2013, 06:54:52 am
content: Thanks @free_electron for the explanation. The two caps are really nice touch. I'm trying to figure out why Agilent choose to use inverting configuration and now the feed-forward caps seems to be the answer. (I cannot simulate the same behavior in non-inverting configuration)