Strange op-amp behavior? - Page 1
Forum section: electronics / beginners

author: Peeps
author_rank: Contributor
timestamp: July 12, 2013, 12:27:50 am
content: I tapped into my car's stock head unit so I could drive my amplifier without needing an aftermarket stereo. I figured it would be a good idea to put a buffer between the head unit and the amp for driving the long cables and adjusting levels.My op-amp of choice was a TI TL084BCN. The only thing is, I get this awful looking square-like wave output which only gets uglier when adjusting the feedback pot. Meanwhile the circuit works perfectly with an OPA2132.These TL084BCN's are brand new too and I've tried a few of them. Can someone shed some light on this?Edit:I'm now thinking this is because the TL084's have a J-FET input stage rather than BJT...

author: w2aew
author_rank: Super Contributor
timestamp: July 12, 2013, 03:26:16 am
content: Both of those are FET input, so that's not it. The most likely problem is that you're violating the input common mode range of the op amp. In fact, if you check the data sheets, you'll see that your violating the common mode input range of both of them, but by a lesser amount with the OPA2132, so it's probably being more forgiving.

author: Peeps
author_rank: Contributor
timestamp: July 12, 2013, 03:38:58 am
content: But I'm only feeding it a 1Vp 1kHz sinewave with 1V of DC offset with my signal gen. I swept it through various voltages which caused it to change but it never normalized.Edit:Wait I see...with the TL084BCN I can only go to about 1V before bottoming out, while the OPA2132 goes to about 0.5V. I guess I assumed I could go closer to 0V with it.

author: AlfBaz
author_rank: Super Contributor
timestamp: July 12, 2013, 04:42:26 am
content: I looked at the datasheets when you first posted. I was under the impression the OP2132 had less of a common mode input range than the TL084 and as such found it odd that former worked but the latter didn't.I can't see what Vcc is in your schematic. Unless it's 2V I'd up the input signals DC offset to half ofVccVom and see how that goes

author: Peeps
author_rank: Contributor
timestamp: July 12, 2013, 04:47:09 am
content: Vcc is +12V. I'll have to reconfirm this, but the DAC in the head unit outputs ~1.5V DC with the small signal AC riding that. All I wanted to do was simply use a non-inverting buffer to boost the entire signal which is proving to be more complicated now that I see how the op-amps react when operated near their minimum input.

author: c4757p
author_rank: Super Contributor
timestamp: July 12, 2013, 04:51:16 am
content: AC couple it...

author: AlfBaz
author_rank: Super Contributor
timestamp: July 12, 2013, 04:59:03 am
content: Quote from: c4757p on July 12, 2013, 04:51:16 am...or use a +- supply

author: c4757p
author_rank: Super Contributor
timestamp: July 12, 2013, 05:34:11 am
content: The 12V supply in a car is too convenient for me to muck about with PSU circuitry for low current applications. Here's what I have done before: 12V -> slow fuse -> big ol' choke -> big ol' cap (100V rated) paralleled with chunky 48V TVS diode -> big ol' resistor -> 8-10V Zener diode. Simple to design and assemble and performs well. No negative rail, but AC couple in, AC couple out and Bob's your uncle.

author: AlfBaz
author_rank: Super Contributor
timestamp: July 12, 2013, 05:58:17 am
content: Yeah, forgot that it was in a car and for the signal here there's plenty of head room with 12V

author: Peeps
author_rank: Contributor
timestamp: July 12, 2013, 06:25:43 am
content: Quote from: c4757p on July 12, 2013, 05:34:11 amBut if I AC couple in the opamp will need a split rail so it can produce a negative waveform...or am I supposed to AC couple then manually set my own DC offset at say 6V so the opamp has like +-4 to work with. Probably should have done that in the first place. And I will add the inductor and diode to the board.

author: c4757p
author_rank: Super Contributor
timestamp: July 12, 2013, 06:31:52 am
content: You always set a DC bias when you AC couple something. It can be 0V, but you still set it.

author: w2aew
author_rank: Super Contributor
timestamp: July 12, 2013, 02:24:25 pm
content: Quote from: Peeps on July 12, 2013, 06:25:43 amThe input common mode range of the TL084BCx guaranteed only to within 3V of the rails, and that's with +/-15V supply, so it's probably worse with only 12V total supply (vs. 30). The OPA2132 is only a little better, guaranteed to +/-2.5V of the rails with +/-15V supply. To use these op amps in your single-supply 12V application, you'll have to create a rail-split reference (resistor divider), and AC couple both input and output.

author: Skimask
author_rank: Super Contributor
timestamp: July 12, 2013, 02:41:28 pm
content: Quote from: w2aew on July 12, 2013, 02:24:25 pmIt is worse.I just shot myself in the foot last week dealing with that trying to get it to pull within +/-2v of the rails at +/-10.5vdc.Not even close. Should've read the datasheet a bit harder.

author: Peeps
author_rank: Contributor
timestamp: July 12, 2013, 10:22:59 pm
content: Thanks for all the help everyone. I should have looked at the datasheets a little more carefully.