Author Topic: Op-amp max current output vs frequency  (Read 916 times)

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Offline ricko_ukTopic starter

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Op-amp max current output vs frequency
« on: January 22, 2020, 03:27:27 am »
Hi,
does op-amp current vary depending on frequency? I'm thinking most likely the max current output decreasing as frequency increases perhaps, due to losses and parassitics? I googled it and looked on few datasheets but could not find any such graph/parameter.

Thanks
 

Offline profdc9

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Re: Op-amp max current output vs frequency
« Reply #1 on: January 22, 2020, 03:37:26 am »
What you want to look at is slew rate.  Slew rate is the maximum change to the output of the op-amp in volts/second.  So for example, if the op-amp is driving a 100 ohm load, and the slew rate is 5 volts/microsecond, then the change in current is 50 mA/microsecond.  So for example, for an output at 1 MHz/(2 pi) or about 159 kHz, the maximum oscillation amplitude in current is going to be 50 mA.
 
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Offline magic

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Re: Op-amp max current output vs frequency
« Reply #2 on: January 22, 2020, 09:08:48 am »
There is a current slew rate too. If nothing else, the slew rate times output stage transconductance is the limit ;) and it may be worse due to input capacitance of the output transistors and limited drive current available from the gain stage.

But if it's not specified then it probably is not a problem in any reasonable application and the usual voltage slew rate is the limit, as outlined above. Unreasonable applications where this may not apply involve: driving very high current at very high frequency into very low impedance load.
 
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Offline Vovk_Z

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Re: Op-amp max current output vs frequency
« Reply #3 on: January 22, 2020, 12:03:09 pm »
does op-amp current vary depending on frequency? I'm thinking most likely the max current output decreasing as frequency increases perhaps,
Max output voltage usually will decrease with frequency, but current not so much. Actually we can think that it doesn't change.
 
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Offline David Hess

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Re: Op-amp max current output vs frequency
« Reply #4 on: January 22, 2020, 08:25:09 pm »
It would be interesting to test.  Output current slew rate will be primary limited by voltage slew rate into the output load.  Voltage slew rate is limited by the compensation.  Current gain of the output transistors drops with frequency however that is way beyond the other limits.
 
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Offline magic

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Re: Op-amp max current output vs frequency
« Reply #5 on: January 23, 2020, 08:33:08 am »
On second thought, there is a much more reliable and predictable limit to output stage transconductance than storage effect or parasitics: the output protection resistors.

Take the classic NE5532 with its 15Ω. You aren't getting more than 70mA/V out of that thing. Given 8V/µs voltage slew rate, that's 560mA/µs current slew rate into a dead short. So at 5MHz it's limited to 18mA peak into a short, even though normally it could do twice more. Add 15Ω of external load and the limit becomes 9mA at 5MHz or 18mA at 2.5MHz.

But the above example also shows that one needs to go to quite extreme lengths to encounter this problem :)

Or use RC4558. According to Raytheon, its output resistor is 120Ω :scared: But I'm not sure if it's not a typo and I felt like using such a lousy opamp for this example would be cheating :D
 
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