Active load circuit has low slew rate -- why? - Page 1
Forum section: electronics / projects

author: guymo
author_rank: Regular Contributor
timestamp: October 06, 2021, 07:33:13 am
content: I built up a version of the Arachnid labs "Re-Load 2" constant current DC load --http://www.arachnidlabs.com/reload-2/index.htmland schematic snippet attached. I had the idea to replace the adjustment pot with an input to connect to a function generator, to generate load transients.This seems to work, but the resulting slew rate is very slow: slewing from 0 to 200mA seems to take 30ms. I measured this using the Digitise function of a Keithley DMM6500, with the load attached to my HP bench PSU.My build uses a BTS141 for the FET.Looking at the circuit and the BTS141 datasheet I can't see any reason for such slow slewing. I know the BTS141 is not exactly a standard FET device so my guess is that there's some aspect of its behaviour that means it is not appropriate for this application. Can anyone help explain? And if that is the reason, can you recommend an alternative part which could be adequately driven by this circuit?

author: Thomas
author_rank: Regular Contributor
timestamp: October 06, 2021, 10:07:10 am
content: Try reducing C4 to 1nF.

author: magic
author_rank: Super Contributor
timestamp: October 06, 2021, 12:58:41 pm
content: Can you get some scope and post the exact waveforms at the input, opamp output and the shunt resistor?Quote from: Thomas on October 06, 2021, 10:07:10 amNot sure if that's the problem; time constant of C4·R8 is only 1ms.

author: Kleinstein
author_rank: Super Contributor
timestamp: October 06, 2021, 01:14:58 pm
content: The rgulator still has to provide gain to control the gate. So the time constant ofr the loop is C4*R8 * R1*S , where S is the transconductance of the FET.For low currents the transconductance is not that high, maybe 0.2 A/V. So the R1*S part may give you an extra factor of 100 or so.

author: magic
author_rank: Super Contributor
timestamp: October 06, 2021, 01:46:48 pm
content: OK, I think you are right.Furthermore, if OUT is initially zero, applying 10mV to IN+ will cause OUT and IN- to jump to 10mV at first.Then R8 will sink 1µA from C4, resulting in the output ramping up at 10V/s before the FET even turns oneditI would try with a darnlington instead of a FET.Remove C4 completely and see if the loop can be stabilized solely by increasing R7.That should have better transient response than those FET loads, I think.BTW, I'm not sure if this is a great choice of opamp for 0.05Ω shunt (what about error due to offset voltage?)Also, not sure if 0.05Ω shunt is a great choice for 200mA (it could probably be more).edit editI'm not necessarily saying that reducing C4 woludn't help too. Maybe it will work.Maybe I'm just a darlington fanboy

author: mikerj
author_rank: Super Contributor
timestamp: October 06, 2021, 06:52:31 pm
content: Quote from: magic on October 06, 2021, 01:46:48 pmC4 & R8 are totally dominating the bandwidth of this circuit. I suspect it oscillated and the author threw capacitance at it until it stopped.

author: magic
author_rank: Super Contributor
timestamp: October 06, 2021, 07:01:50 pm
content: Well, there is hope that the author included the components because everybody else also does and picked the values more or less at random160Hz is frankly a quite low a bandwidth. Maybe it could be sped up some 10~100x before it goes bonkers.

author: guymo
author_rank: Regular Contributor
timestamp: October 07, 2021, 12:07:16 pm
content: Thanks everybody. This was very helpful in getting me to analyse the loop properly.I had been thinking that C4 introduced a pole at 1/(C4*R8) and a zero at 1/(C4* R8||Rin) where Rin is the input impedance of the op amp. In my head these were very close together so would largely cancel out.If I have it right, this is the key:Quote from: Kleinstein on October 06, 2021, 01:14:58 pmThe FET stage is nothing like a follower and in fact attenuates by a significant factor. That has the effect of moving the pole up in frequency by the attenuation, so -- as everybody is saying here -- the C4-R8 pole does get to pull the gain down for a couple of decades after all, which I had not realised before.

author: Hamelec
author_rank: Regular Contributor
timestamp: October 08, 2021, 09:44:32 am
content: .. please report your changes / results. Thanks.

author: Vovk_Z
author_rank: Super Contributor
timestamp: October 08, 2021, 10:19:52 pm
content: C4 = 0.1 uF is typical silly value. This value is everywhere on the internet (possibly because people need fast active loads very seldom so they use random values). But if somebody really investigated this circuit he knows that it has to be 0.01 uF max (and of cause the lower - the faster).