Voltage-Controlled Current Sink not responding to control voltage fast enough - Page 1
Forum section: electronics / beginners

author: Ryan OMalley
author_rank: Newbie
timestamp: March 15, 2020, 10:38:07 pm
content: Hi everyone, I'm currently having trouble with a voltage-controlled current sink circuit that i'm using as part of a vco. The problem i'm having is that when I change the control voltage quickly, the current doesn't change as fast as the control voltage. This produces an effect like a slew rate limiter circuit, which I don't want. the current that the circuit is sinking is 3-15 microamps at 5 volts. I want this circuit to operate as fast as possible, but I'm not exactly sure how to do this. a copy of the schematic will be included below. any suggestions will be greatly appreciated.

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2020, 10:43:21 pm
content: C1 has the incorrect polarity.The LM358 is a slow op-amp, so you shouldn't expect it to respond quickly. .

author: Ryan OMalley
author_rank: Newbie
timestamp: March 16, 2020, 02:22:45 am
content: Hi, thank you so much for your help. I will definitely swap the polarity of c1 to the correct way around. I have tried a tlo82 in the circuit as well, but it produced the same result even with c1 removed and no ac input connected. Do you have any recommendations for an op amp that will respond quickly?

author: Zero999
author_rank: Super Contributor
timestamp: March 16, 2020, 10:42:08 am
content: What do you mean by quickly? What's the rate of change in Volts per second on the input, vs the output current in Amps, mA or µA per second on the output?The op-amp's slew rate is a measure of how quickly its output voltage can change. The output voltage appears on Q1's base, which is roughly always 0.6V above the voltage on the sense resistor R5 to R8. The LM358 has a slew rate of 0.5V/μs which means its output voltage can't change any faster than 0.5V per microsecond, so it will take 2µs for the voltage to change from 0V to 1V or -2V to -3V. The current slew rate of your circuit can be calculated by using Ohm's law. I = V/R, so if the slew rate of the op-amp is 0.5V/μs and 10k is selected for the sense resistor, then the current slew rate is I = V/R = 0.5/10*103= 50*10-6A/µs or 50µA/µs or 50A/s.

author: Ryan OMalley
author_rank: Newbie
timestamp: March 21, 2020, 09:03:20 pm
content: Hi, sorry for the late reply, I took some measurements and I figured out the input voltage goes from 0-5v in 12 uS. the current changes from 0.5-361 uA in the same amount of time; this was with the 10k sense resistor. I was considering using 2 LM318's in place of the Tlo82, do you think the Lm318 is a good op amp for this circuit?

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 22, 2020, 02:34:39 am
content: What you might do is grab a circuit simulator and try different op amps in the simulator to see if they will do what you are needing to do.I'm not sure about other manufacturers but TI has pspice models for nearly all of their op amps. If you use TI's simulator, Tina-TI, (download page) then the spice model may already be included with the software. If it isn't then you can always get either the pspice model or the Tina-TI macro (easier to install and use) from the product's web page--look under theDesign and developmenttab.

author: magic
author_rank: Super Contributor
timestamp: March 22, 2020, 07:02:12 am
content: Maybe reduce the resistor to 1k and divide the input signal by five instead of doubling? This will reduce your slew rate requirement from 10V/12µs to 1V/12µs.Watch out for opamps whose input range doesn't extend to ground. LM358 is okay, TL082 is not. LM318 probably too.

author: Vovk_Z
author_rank: Super Contributor
timestamp: March 22, 2020, 02:03:34 pm
content: AD822 and AD823 both are dual opamps with input range from ground. While AD822 is slow but more precision, AD823 is fast opamp. It is about an order faster then LM258 and as precision as LM258. They are not as cheap as LM358. If you want something as cheap as 358 but fast then you have to use bipolar power. Because practically there isn't fast cheap opamps with rail-to-rail inputs (I don't know such).If you need as fast as possible load then you definitely have to use bipolar power (+-12..+-15V) and faster (high-speed) opamps. You can start with LM318.