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jaks
Joined: 02 Feb 2008
Posts: 10
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 02 Feb 2008 13:16 voltage follower oscillation |
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Hi,
I have an op amp (tl071) which I use as a voltage follower. At the output I have a parallel capacitance (and nothing else, see picture).
Sometimes it works, sometimes the op amp goes into saturation (even when there is no input signal at all). Why doesn't it work always? Connections are checked, there's no loose connection.
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02 Feb 2008 13:16 Ads |
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FvM
Joined: 22 Jan 2008
Posts: 5154
Helped: 766
Location: Bochum, Germany
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02 Feb 2008 15:53 opamp capacitive load |
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Hello,
don't see, what you mean exactly with "saturation". The said capacitive output load will cause extensive "ringing" or permanent oscillation. To increase so called phase margin to an appropriate value, removing ringing or oscillation, a series resistor between voltage follower output and load is necessary, a few ohms may be enough. In the TL071 datsheet, no data for capacitive load are given. You could calculate minimum values for different capacitive load from datasheet parameters, but that's a bit difficult and probably not necessary here.
If you mean constant DC level at the output, this wouldn't be related to capacitive load but to exceeding the usable input voltage range (common mode voltage range).
Regards,
Frank
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jaks
Joined: 02 Feb 2008
Posts: 10
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02 Feb 2008 17:53 capacitance in opamp |
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| FvM wrote: |
| don't see, what you mean exactly with "saturation". The said capacitive output load will cause extensive "ringing" or permanent oscillation. |
Sorry, my english and my knowledge of op amps is not that good so that I mixed up something - I probably meant what you described as oscillation!
| FvM wrote: |
| To increase so called phase margin to an appropriate value, removing ringing or oscillation, a series resistor between voltage follower output and load is necessary, a few ohms may be enough. |
OK, I think I understand - so using an op amp with a larger output impedance could also help in my case? (Though, I have found no info about the TL071 output impedance in the datasheet...)
The stange thing actually is, that it sometimes works the way I have drawn it on the picture, but when I switch on the next time it suddenly does not work, then I switch off and on again and it works again - could that mean I am in a "border area" where (as you said) a resistor of a few ohms or also decreasing the parallel capacitance might make it "stable"?
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Audioguru
Joined: 19 Jan 2008
Posts: 1147
Helped: 95
Location: Toronto area of Canada
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05 Feb 2008 3:11 input capacitor of op |
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| I connect a 100 ohm resistor to the output of a TL07x opamp that needs to feed the high capacitance of a shielded cable. Then it does not oscillate.
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subharpe
Joined: 09 Jan 2008
Posts: 244
Helped: 34
Location: Bangalore,India
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05 Feb 2008 8:36 opamp stabilize output capacitor |
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The circuit itself without the resistor is not stable. Suppose presently there is a voltage of Vin = 1V atthe non-inverting input of the OPAMP. Then at the steady state the output voltage across the capacitor will b 1V. Now the input voltage changes to 1.2V. It has to chaarge the capacitor to 1.2 through the large input resistance of the OPAMP. Therefore there will be some delay during which there will be substantial voltage difference between the two terminals of the OPAMP. SO the outut will try to reach positive saturation. As it tries to reach positive saturation the voltage at the inverting terminal becomes higher than the voltage at the non-inverting termnal leading to negative saturation a t the output. So the circuit is not stable and depending on the finite slew rate with negative feedback the voltage should oscillate between ±Vsat. I fact it can be unpredictable too, as there is no stability in the circuit.
Therefore to stabilize it one must isolate the output and the load capacitor. Under this condition the change in voltage at the non-onverting terminal is reflected at the output terminal immediately without forcing the capacitor to change the voltage across it. Under this condition the change in voltage at the input changes the load voltage with certain delay depending on the time constant of the RC combination at teh output.
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