Capacitance in Test Gear? - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: August 05, 2018, 02:43:33 pm
content: Hi,I have an inverting OTA showing in the attached scope screenshot. The amp circuit schematic is also attached.I was modifying the circuit by swapping resistors and capacitors, changing the Ibias, and powering down the circuit. At some point, the circuit became very noisy and the input from my function generator added a negative DC offset. The output became very noisy and powering down everything and back up didn't bring the circuit back to the noise-less scope picture. (Unfortunately I don't have a screenshot of the noisy picture.)I powered down for about an hour and came back to the same noisy circuit and everything looked better as in the attached screenshot.Is there something in my test gear -- scope, function generator, power supply -- that needed to discharge? If so, is there a way for me to force a reset of this without powering down for a long time?Thanks,Carl

author: Wimberleytech
author_rank: Super Contributor
timestamp: August 05, 2018, 03:08:27 pm
content: Can you show ALL connections to the OTA? Please identify pin numbers as well. Where is pin 1 connected? Pin 2 (is that connected to the 33k?

author: eev_carl
author_rank: Regular Contributor
timestamp: August 06, 2018, 10:57:39 am
content: Thanks for looking at this. Here's the updated schematic with the pin connections.

author: Wimberleytech
author_rank: Super Contributor
timestamp: August 06, 2018, 12:45:17 pm
content: There is no negative feedback to stabilize dc conditions. Looking at the datasheet, I see that you CAN run this OTA open loop, but you must provide input-offset adjustment to do so. You were probably lucky to get it working at all. Adjusting signal generator offset was charging up the input cap enough to get the OTA in the linear range but then as the cap discharges, it went off the rails.Add dc negative feedback.For example, connect the 18K that is from pin 4 and ground and connect it between pin 4 and pin 5.

author: eev_carl
author_rank: Regular Contributor
timestamp: August 06, 2018, 01:02:31 pm
content: The circuit seems to be working fine for me. I hooked an AR envelope circuit up to the OTA VCA and am getting attack and release based tones from my VCA.I didn't see any of the strange behavior on my scope today or yesterday. I think the problem was with my test gear as the capacitors in my circuit are pretty small and would discharge quickly. I was surprised to see my scope go haywire, still go haywire after powering the circuit down, then shutting of the circuit and coming back an hour later to a working circuit.Regarding the OTA in closed loop mode...I read that closed loop operation would make the voltage gain independent of Ibias. Isn't this this opposite of what I want? I may have misunderstood. Here's my source link referencing "CLOSED-LOOP OPERATION".http://www.nutsvolts.com/uploads/magazine_downloads/11/April%202003%20Ray%20Marston%20-%20Understanding%20and%20Using%20OTA%20Op-Amps.pdfThanks,Carl

author: Wimberleytech
author_rank: Super Contributor
timestamp: August 06, 2018, 02:14:27 pm
content: Quote from: eev_carl on August 06, 2018, 01:02:31 pmOK, I agree. I did not do the calculations, but based on the reference, you have a gain of 100. With a 5mV offset voltage, you are still OK. Also, I did not understand you goal. Yes, if you want a variable-gain solution, you must operate it as a transconductance (no FB).Happy amplifying