LM301A Op-Amp: Why Does 20logA Increase at Unity Gain With 10pF Compensation?

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Why does my LM301A amplifier’s measured 20logA increase at unity gain when I use a 10 pF compensation capacitor in parallel with Rf, while 20 pF and 30 pF capacitors make the gain decrease?

The most likely cause is peaking from oscilloscope probe capacitive loading interacting with the op-amp compensation, not the capacitor value by itself [#21669888] An op-amp’s frequency response can peak with a capacitive load, and the probe capacitance can interact with the 10 pF feedback capacitor differently than with 20 pF or 30 pF [#21669888] To test this, insert about a 100 ohm resistor between the op-amp output and the oscilloscope probe and repeat the measurement; this should help isolate the op-amp from the probe capacitance [#21669888]

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#1 21669880 25 Sep 2013 13:24

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #1
21669880 25 Sep 2013 13:24

Are you placing the compensation capacitor in parallel with Rf?

Are you measuring the gain well below the corner frequency (the frequency at which the capacitor in parallel with Rf) begins to roll off the gain?

Are you operating the LM301A well below it's GBP (Gain Bandwidth Product) of 1 MHz?

Are you operating the circuit in the small signal region (well below an output swing of 10 V/us)?
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Unity-gain compensation capacitor increases high-frequency gain
#2 21669879 25 Sep 2013 13:32

Hatef Abdollahi Hatef Abdollahi

Anonymous

Unity-gain compensation capacitor increases high-frequency gain
Post #2 21669879 25 Sep 2013 13:32

I am applying three compensation capacitors for my op-amp to observe and analyze the characteristic changes in the behavior of the amplifier by setting 10 points of frequency in order to analyze the frequency response and compensation of the op amp. first two capacitors are 30pF and 20pF with two gains of 1 and 50 for each of them. their changes are similar to each other as both capacitors at both gain have decreased in value of their 20logA when A=gain( 50 and 1). but for 10pF compensation capacitor the value of 20logA only decreases when the gain is 50 but when the circuit has unity gain which is 1 the value of 20logA over Log(omega) increases which is totally the opposite of other two capacitors. can anyone tell me why is this happening ??? I am placing the compensation capacitor in parallel to Rf.
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#3 21669881 25 Sep 2013 13:43

Hatef Abdollahi Hatef Abdollahi

Anonymous

Post #3
21669881 25 Sep 2013 13:43

yes i am placing the capacitor in parallel with Rf
i have attached my readings table from the measurement below.
#4 21669882 25 Sep 2013 13:51

Hatef Abdollahi Hatef Abdollahi

Anonymous

Post #4
21669882 25 Sep 2013 13:51

and my question is that why with other two capacitors ( 20pF and 30pF ) when the op-amp has unity gain , the output voltage decreases but with this 10pF capacitor the output voltage increases ?
#5 21669883 25 Sep 2013 15:32

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #5
21669883 25 Sep 2013 15:32

I don't see the 10, 20, and 30 pF cases in the table.

Is your output voltage measurement simulated or measured? And if measured, what is the capacitive loading of the oscilloscope probe?
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#6 21669884 25 Sep 2013 15:44

Hatef Abdollahi Hatef Abdollahi

Anonymous

Post #6
21669884 25 Sep 2013 15:44

as you can see from the attachments below , my question is why the behavior of 10pF is different than 20pF and 30pF at unity gain ???
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#7 21669885 25 Sep 2013 15:51

Hatef Abdollahi Hatef Abdollahi

Anonymous

Post #7
21669885 25 Sep 2013 15:51

the X axis is log(omega) and the Y axis is 20logA
#8 21669886 25 Sep 2013 16:45

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #8
21669886 25 Sep 2013 16:45

Are your graphs measured or simulated?
#9 21669887 25 Sep 2013 16:46

Hatef Abdollahi Hatef Abdollahi

Anonymous

Post #9
21669887 25 Sep 2013 16:46

measured. do you know why is this happening ???
#10 21669888 25 Sep 2013 17:18

DAVID CUTHBERT DAVID CUTHBERT

Anonymous

Post #10
21669888 25 Sep 2013 17:18

I suspect the peaking is due to oscilloscope probe capacitive loading.

Op Amp frequency response can peak with a capacitive load and the compensation can interact.

An experiment to try is to place a 100 ohm resistor between the Op Amp output and the oscilloscope probe and repeat the measurements. The resistor will tend to isolate the Op Amp from the probe capacitance.
#11 21669889 25 Sep 2013 17:39

Hatef Abdollahi Hatef Abdollahi

Anonymous

Post #11
21669889 25 Sep 2013 17:39

thanks a lot, you really helped me, and thanks for you time.
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Topic summary

The discussion addresses an observed anomaly in the frequency response of an LM301A operational amplifier compensated with different capacitor values (10pF, 20pF, 30pF) placed in parallel with the feedback resistor (Rf). While 20pF and 30pF capacitors show a decrease in 20logA at unity gain, the 10pF capacitor causes an unexpected increase in 20logA over log(ω) at unity gain, opposite to the other cases. Measurements were taken at multiple frequency points to analyze the compensation effects. The output voltage behavior differs notably with the 10pF capacitor, showing an increase rather than a decrease at unity gain. The measurements were confirmed to be experimental rather than simulated. A suggested explanation involves the capacitive loading effect of the oscilloscope probe, which can cause peaking in the op-amp frequency response due to interaction with the compensation capacitor. To mitigate this, it was recommended to insert a 100-ohm resistor between the op-amp output and the oscilloscope probe to isolate the probe capacitance and obtain more accurate frequency response measurements.
AI summary based on the discussion. May contain errors.

LM301A Op-Amp: Why Does 20logA Increase at Unity Gain With 10pF Compensation?

Why does my LM301A amplifier’s measured 20logA increase at unity gain when I use a 10 pF compensation capacitor in parallel with Rf, while 20 pF and 30 pF capacitors make the gain decrease?

Didn't find an answer? Ask Artificial Intelligence (AI)

Topic summary