Author Topic: Strange op-amp behavior?  (Read 3582 times)

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Offline Peeps

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Strange op-amp behavior?
« on: July 12, 2013, 10:27:50 AM »
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...
« Last Edit: July 12, 2013, 11:34:46 AM by Peeps »
 

Offline w2aew

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Re: Strange op-amp behavior?
« Reply #1 on: July 12, 2013, 01:26:16 PM »
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.
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Offline Peeps

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Re: Strange op-amp behavior?
« Reply #2 on: July 12, 2013, 01:38:58 PM »
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.
« Last Edit: July 12, 2013, 02:03:42 PM by Peeps »
 

Offline AlfBaz

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Re: Strange op-amp behavior?
« Reply #3 on: July 12, 2013, 02:42:26 PM »
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 of Vcc Vom and see how that goes
« Last Edit: July 12, 2013, 02:44:03 PM by AlfBaz »
 

Offline Peeps

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Re: Strange op-amp behavior?
« Reply #4 on: July 12, 2013, 02:47:09 PM »
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.
 

Offline c4757p

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Re: Strange op-amp behavior?
« Reply #5 on: July 12, 2013, 02:51:16 PM »
AC couple it...
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Offline AlfBaz

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Re: Strange op-amp behavior?
« Reply #6 on: July 12, 2013, 02:59:03 PM »
AC couple it...
...or use a +- supply
 

Offline c4757p

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Re: Strange op-amp behavior?
« Reply #7 on: July 12, 2013, 03:34:11 PM »
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.
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Offline AlfBaz

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Re: Strange op-amp behavior?
« Reply #8 on: July 12, 2013, 03:58:17 PM »
Yeah, forgot that it was in a car and for the signal here there's plenty of head room with 12V
 

Offline Peeps

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Re: Strange op-amp behavior?
« Reply #9 on: July 12, 2013, 04:25:43 PM »
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.
But 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.
 

Offline c4757p

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Re: Strange op-amp behavior?
« Reply #10 on: July 12, 2013, 04:31:52 PM »
You always set a DC bias when you AC couple something. It can be 0V, but you still set it.
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Offline w2aew

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Re: Strange op-amp behavior?
« Reply #11 on: July 13, 2013, 12:24:25 AM »
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.
But 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.

The 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.
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Offline Skimask

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Re: Strange op-amp behavior?
« Reply #12 on: July 13, 2013, 12:41:28 AM »
The 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).
It 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.
I didn't take it apart.
I turned it on.

The only stupid question is, well, most of them...

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Offline Peeps

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Re: Strange op-amp behavior?
« Reply #13 on: July 13, 2013, 08:22:59 AM »
Thanks for all the help everyone.  I should have looked at the datasheets a little more carefully.
 


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