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 Post subject: Op Amp problems
PostPosted: Apr Sun 07, 2019 9:15 pm 
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I've been trying to learn about op amps for a project I'm working on and am having all kind of proplems.
I am trying to get this circuit to work using an LM358 op amp. I have tried 3 different opamps with the same result.
I have breadboarded it several times and cannot find where I'm making a mistake.
The problem is that I am not getting any gain, no filtering and if I turn off the power the output doesn't change.
I am using 12v for power.
I am using an oscope on the input and output with the vert. scales set the same to check operation.


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NON INVERTING LOW PASS FILTER.gif
NON INVERTING LOW PASS FILTER.gif [ 6.38 KiB | Viewed 832 times ]

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 Post subject: Re: Op Amp problems
PostPosted: Apr Sun 07, 2019 9:24 pm 
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I don't remember all my op amp lore that well, but it looks like you have it set up for unity gain with those resistor values. Try putting a potentiometer in to replace the 9k resistor ...

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 Post subject: Re: Op Amp problems
PostPosted: Apr Sun 07, 2019 9:57 pm 
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That should give a gain of 10. 1+(9K/1K)
I don't know what you are doing wrong.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Sun 07, 2019 10:27 pm 
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It's a single-supply op-amp, so you need to add a bias network to the input to bias it to 1/2 of Vcc, add a coupling cap to the input, and add a coupling cap to the output. And the gain-setting voltage divider from the output should be returned to 1/2 of Vcc, not ground. Using the other opamp in the LM358 as a buffer for the 1/2 Vcc voltage divider is a good idea, otherwise you need to use pretty low resistances in the voltage divider.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Sun 07, 2019 11:02 pm 
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What is not seen here is your power supply. Are you using a positive / negative supply referenced to 0V (common)? If you are using a single sided supply only there is nor reference point Normally a + / - supply is used, Both reference to common (0V) If this is being done it looks like you should have a gain of 10 based on the values provided.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 12:15 am 
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I am using a single supply. I will try it tomorrow with a dual supply and see what happens.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 12:36 am 
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Scott wrote:
That should give a gain of 10. 1+(9K/1K)
I don't know what you are doing wrong.


That's what I thought and what the book says. I'll figure it out and get on with my project.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 3:43 am 
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At what frequencies have you run tested the circuit?

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 4:26 am 
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That is a low pass circuit, so the frequency you are testing must be low, and as others said a split supply or correct biasing is required. Take the capacitor out if you want to pass any freq (up to op amp capability).

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 5:33 am 
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Just trying to learn here.
Wouldn't "not getting any gain [and] no filtering" be consistent with a shorted capacitor? (...or testing only at relatively higher frequencies?)

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 8:46 am 
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What is your input frequency? Assuming that your input is biased properly, at 3KHz your 0.11uF capacitor (weird value) has an impedance of ~480Ω. 480Ω in parallel with 9.1K is ~450Ω. So @3KHz the gain of the amp will be 0.45/1+1=1.45 only. If your input frequency is higher than 3KHz then the gain will approach unity, after all it is a low pass amplifier! Remove the capacitor and you will have a gain of 10 all the way to at least 100KHz.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 10:21 am 
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hmmm.... didn't someone note earlier that the components appeared to be producing unity gain? 8) I just omitted the calculations lol.

No, I do NOT recall all the details of Op Amp design. I have not done any design tinkering in decades, but as for fixing stuff.........

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 11:42 am 
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One thing that is confusing: The Op-amp is labelled as having a gain of 10. (which would make it the world's worst Op-amp.....;) )

If the op-amp is real, it has a gain more like 10,000--the crude rule being that the open-loop gain is large enough that you no longer care what it is......
If that criteria is applied to the circuit, then the OVERALL gain would in fact be 10. (Assuming of course that the power supply issue is handled correctly.......

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 1:24 pm 
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stevebyan wrote:
It's a single-supply op-amp, so you need to add a bias network to the input to bias it to 1/2 of Vcc, add a coupling cap to the input, and add a coupling cap to the output. And the gain-setting voltage divider from the output should be returned to 1/2 of Vcc, not ground. Using the other opamp in the LM358 as a buffer for the 1/2 Vcc voltage divider is a good idea, otherwise you need to use pretty low resistances in the voltage divider.

I'm not sure what you mean about "single-supply"....it has the expected V+ and V- supply terminals. Any op-amp can be connected to a single supply as long as you are not trying to make it work with DC signals.

Datasheet with MANY words:
http://www.ti.com/lit/ds/symlink/lm358.pdf

Somewhere in there is info on how close input and output terminals can be to the rails......

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 2:54 pm 
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SteiniteFan wrote:
At what frequencies have you run tested the circuit?


10 hz to over 100 khz.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 3:59 pm 
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Dennis H. wrote:
I am using a single supply. I will try it tomorrow with a dual supply and see what happens.

??? What happened?....Is it working now?

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 4:00 pm 
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Barry H Bennett wrote:
hmmm.... didn't someone note earlier that the components appeared to be producing unity gain? 8).

NO, unless they don't understand how Op Amp works and their frequency response.

One more thing to pick on, the schematic name is "NON INVERTING LOW PASS FILTER". It is not a low pass filter, it is an amplifier with a very low frequency response.


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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 5:08 pm 
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pixellany wrote:
stevebyan wrote:
It's a single-supply op-amp, so you need to add a bias network to the input to bias it to 1/2 of Vcc, add a coupling cap to the input, and add a coupling cap to the output. And the gain-setting voltage divider from the output should be returned to 1/2 of Vcc, not ground. Using the other opamp in the LM358 as a buffer for the 1/2 Vcc voltage divider is a good idea, otherwise you need to use pretty low resistances in the voltage divider.

I'm not sure what you mean about "single-supply"....it has the expected V+ and V- supply terminals. Any op-amp can be connected to a single supply as long as you are not trying to make it work with DC signals.

Yeah, that was a bit of a non sequitur. The OP said he was feeding it 12 volts, implying a non-split single supply voltage.

Single-supply op-amps usually have two important characteristics: common-mode input range including ground (not pertinent to this application, as you say), and an output range including the negative supply (ground, in a single-supply configuration) and approaching the positive supply. This latter characteristic is important when running with low power supply voltages, low meaning less than ±15 volts. A traditional opamp running on a split ±6 volt supply would only have a guaranteed output range of about 2 volts peak-to-peak.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 7:00 pm 
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upsss wrote:
Barry H Bennett wrote:
hmmm.... didn't someone note earlier that the components appeared to be producing unity gain? 8).

NO, unless they don't understand how Op Amp works and their frequency response.

One more thing to pick on, the schematic name is "NON INVERTING LOW PASS FILTER". It is not a low pass filter, it is an amplifier with a very low frequency response.


I didn't give it that name. If you google "low pass filter" this is what you get.

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 Post subject: Re: Op Amp problems
PostPosted: Apr Mon 08, 2019 7:39 pm 
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I simulated the OP's version of the circuit in LTspice along with a single supply with bias version and a dual supply version:
Attachment:
OP-AMP CIRCUITS.jpg
OP-AMP CIRCUITS.jpg [ 44.8 KiB | Viewed 627 times ]

Here are the output waveforms (green trace is the OP's circuit, Yellow is the single supply with bias version and Red is the dual supply version):
Attachment:
OP-AMP CIRCUITS WAVEFORMS.jpg
OP-AMP CIRCUITS WAVEFORMS.jpg [ 58.82 KiB | Viewed 627 times ]

And here is the frequency response of the three (same color coding):
Attachment:
OP-AMP CIRCUITS FREQ RESP.jpg
OP-AMP CIRCUITS FREQ RESP.jpg [ 67.5 KiB | Viewed 627 times ]

Note that for the bias version, the low end response depends upon the values of C3 and C5. I chose the values shown for roughly a 1Hz. rolloff.
John

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