Transconductance Amp w. 741 - Page 1
Forum section: electronics / beginners

author: eev_carl
author_rank: Regular Contributor
timestamp: July 31, 2018, 12:05:44 pm
content: Hi,I have some OTAs coming but I'm trying to get a transconductance amp working in the meantime. I pulled this schematic from a Mims' scrapbook but can't seem to get it working. I don't get any amplification across R1 (Vin=Vout) and don't see any current. I was wondering if I was missing something that was implied in the schematic.Thanks in advance,Carl

author: Audioguru
author_rank: Super Contributor
timestamp: July 31, 2018, 12:40:33 pm
content: You did not attach the schematic from Mims scrapbook that did not work for you so we don't know what you are talking about.Then you linked to another schematic at "slideshare" but the link does not work. Please attach schematicshere at EEblogto your reply.Don 't you know that the 741 opamp design is 50 years old and today its performance is horrible compared to newer designs?

author: eev_carl
author_rank: Regular Contributor
timestamp: July 31, 2018, 12:57:27 pm
content: Image added to original post. I'm using R1=33k / R2=10k and R1=560 / R2 = 1k. I tried reversing these too.I also pulled out the Slideshare link.Yeah, I have plenty of op amps here like the OPA2132. I can try that, but I think there's something else wrong with the circuit besides the performance.

author: Cherenkov11
author_rank: Contributor
timestamp: July 31, 2018, 01:26:25 pm
content: I'm not an expert on Op Amps, but that looks to me a Non-inverting configuration, like the image. For what I know the R1 (RF) it is not the load, the load must be attached between out and ground. And the amplification is given by the next formula: Av = 1 + Rf/R2. (R1 = Rf)

author: eev_carl
author_rank: Regular Contributor
timestamp: July 31, 2018, 01:29:32 pm
content: Yeah it looks the same as the Non-Inverting Amplifier on the previous page.

author: Audioguru
author_rank: Super Contributor
timestamp: July 31, 2018, 02:33:48 pm
content: Now you forgot to tell us the supply voltages for the old 741 opamp that was designed to use only a +15V and -15V supply. You also did not tell us the input voltage or if it is AC or DC you used.I agree that R1 is not the load, it is part of the negative feedback that sets the voltage gain.

author: eev_carl
author_rank: Regular Contributor
timestamp: July 31, 2018, 02:39:33 pm
content: The supply voltage is +/-9V. I tried both DC (1-5V) and AC (1-5Vpp @ 100Hz). I'm getting 0uA when I put my MM in series with RL. I expected to see a current.

author: Wimberleytech
author_rank: Super Contributor
timestamp: July 31, 2018, 07:55:56 pm
content: Quote from: eev_carl on July 31, 2018, 02:39:33 pmWith +/- 9V supply. Tie Vin to ground. Measure and report voltages on all pins of the 741 opamp.Then we will tell you what is wrong :-)

author: eev_carl
author_rank: Regular Contributor
timestamp: July 31, 2018, 08:19:23 pm
content: I think my resistor values were too high (33k as a RL). I see a current when I use 560 / 1k. I've set this particular schematic aside and am using a JEFT as a variable resistor to control the gain on an inverting configuration. (Making an op amp VCA while waiting for some OTAs.)