Why is my op-amp frequency response not ideal?

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Discussion Overview

The discussion revolves around the frequency response of an instrumentation amplifier using LM324 op-amps, focusing on unexpected output behavior when both inputs are shorted and driven by a sine wave. Participants explore the implications of input coupling, gain configurations, and the effects of frequency on output characteristics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that shorting both inputs of the LM324 leads to unexpected output behavior, suggesting a potential relationship to the compensation capacitor in the device.
  • Another participant proposes configuring the device as a unity gain follower to better understand the output behavior.
  • There is a discussion about whether the inputs are AC or DC coupled, with implications for the observed bandpass response.
  • A participant questions the validity of the output given that both inputs are shorted, indicating that the input offset voltage should typically drive the output to the high or low rail.
  • Further clarification reveals that the original setup involved an instrumentation amplifier rather than a single op-amp, which may explain the output behavior.
  • Participants request additional details about the output graph, including amplitude and frequency characteristics, to better understand the response.
  • Concerns are raised about the balance and inaccuracies in the circuit affecting the output, particularly when the output is derived from the difference between two similar terms.

Areas of Agreement / Disagreement

Participants express differing views on the cause of the output behavior, with some attributing it to the configuration of the instrumentation amplifier and others questioning the coupling method and circuit balance. The discussion remains unresolved with multiple competing explanations presented.

Contextual Notes

Limitations include a lack of detailed data on the output graph and the specific coupling method used, which may affect the interpretation of results. The discussion also highlights the potential for inaccuracies in passive components and construction techniques impacting the circuit's performance.

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I am having a play about with the differential amplifier using a LM324 and have shorted both inputs and set them to 3V sine wave at 100Hz.
i understand that in a ideal op-amp the output should be 0, how ever i am unsure of what is happening at the output...

i increase the frequency every 1Khz and record the voltage on the output which is in the mV range. when i plot the results i can see that the gain slowly increases then all of a sudden the gain increase rapidly then as you further increase the frequency it rapidly decreases. The same type of repression you get with a band pass filter.

my only explanation i can see from reading books is that it is to do with the compensation capacitor in the device. could some one shed some light on the subject?
 
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If you shorted both inputs you are operating the LM324 as a comparator and are, in effect, measuring how the offset changes with respect to frequency. I suggest you configure the device as a unity gain follower. Connect the 3V sine wave to the + input of the op amp and feed the output back to the - input.

Are you ac or dc coupling the inputs? If you are ac coupled you will get a bandpass response.
 
looking back at the results i realized that the results were for a 3 op amp instrumentation amplifier that i was messing with rather than the single diff opamp. the inputs of the IA are shorted with a wire and the function generator is connected directly to the inputs

you mention about a bandpass response, this is what it looks like in the graph. what is causing this response?
 
Oh, so it isn't an LM324. Is the input to the instrumentation amplifier dc coupled or ac coupled? If there is a series capacitor then there is a zero at C times the input resistance of the IA. Typically an IA works down to dc, so either your setup is ac coupled for some reason (check the output of the function generator) or your data is wrong.
 
With both inputs shorted together, it's surprising that your getting any output as the input offset voltage would likely have the output at the high or low rail. If you did have a part with Vos=0, you'd be getting a lot of noise.
I'd suggest a useful experiment for the hobbyist. Simply make a gain circuit, i.e. 101, and check the output gain as you increase the input frequency. Try this again with a gain of 11.

What did you learn. Is it useful?

Mike In Plano
 
Mike_In_Plano said:
With both inputs shorted together, it's surprising that your getting any output as the input offset voltage would likely have the output at the high or low rail. If you did have a part with Vos=0, you'd be getting a lot of noise.

Mike In Plano

Mike, the OP didn't actually have the op amp connected that way. He or she was using an instrumentation amp, not a raw op amp. That's why the device wasn't railed.
 
Thanks for all the message guys, just to clear things up I have attached a picture of the 3 opamps I have connected together, what I want to know is why the output is behaving like it is in the graph.

Claire
 

Attachments

  • 400px-Op-Amp_Instrumentation_Amplifier_svg.png
    400px-Op-Amp_Instrumentation_Amplifier_svg.png
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Your graph is a little short on detail. o_O

The amplitude peak---is that something of the order 5dB or more like 50dB? What happens at 80kHz?---the graph takes a dive, or you didn't explore beyond 80kHz? What are your values of R2 and R3?

You monitored the output waveshape and can vouch for it being sinusoidal throughout? What input amplitude were you using?
 
Last edited:
I do apologise for the lack of detail, some one needed to use the test equipment urgently so that's all the data I could pick up.
The output from the sig gen is a sine wave with a amplitude of 1 PK-Pk the peak is at about 15KHz at about - 8dB ( input 1V output 350mV)
r3 = 27K and r2 = 1K
 
  • #10
If these are OP-AMPS, it seems that the pair of unity gain buffers achieve nothing much, leaving a single stage with ideal gain = 0. This leaves your circuit's response to be wholly determined by non-idealities, and passive element inaccuracies.

With the output being the difference between two similar terms, it's inaccuracies and imbalance which determines any non-zero result. Construction techniques need to observe balance so there are no unintended capacitances introduced to upset the balance.

The resistor arrangement around the first pair is not a potential divider, the ends connect to nominally-equal points, so it's just unity feedback.
 
Last edited:

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