[SOLVED] Load Conditions of an Opamp

[rmanalo]

Hello everyone,

I recently read journals on opamp and I've noticed that they've tested the opamp at two load conditions (which was left out in some textbooks): 17pF||1Mohm and 50pF||3.3kohm. Now from what I've figured out is that the 17pF||1Mohm (light load) is from the loading effect of an oscillator's probe (15pF+2pF).

Can anyone tell me the significance of 50pF||3.3kohm (heavy load) and where does it come from?

 

[crutschow]

50pF may be the maximum stray capacitive load the op amp can drive and still remain stable.
3.3k ohm may be the smallest resistive load the op amp can drive to its maximum output voltage, worst-case.

 

[BradtheRad]

I attached a 1k resistor to the output of a 741 op amp, and the voltage dropped 5 percent. I was surprised although it turned out to be reasonable when I saw a schematic of 741 internals. The output stage is a half-bridge with neighboring resistor values 25 & 50 ohms. This doesn't sound like a lot but nevertheless it calculates as a few percent of a 1k ohm load.

 

[rmanalo]

Thank you for the reply @crutschow, I'd like to know what is their significance, like for example does that mean it's the maximum input capacitance of a PMOS pass transistor of an LDO? or the equivalent resistance of the next stage seen by the output of an opamp? If it's possible can you give some examples of what types of load are these? because in the article aside from the 17pF||1Mohm (light load/oscilloscope probe) they don't seem to describe what this 50pF||3.3kohm (heavy load) implies.



- - - Updated - - -

Thank you for the reply @BradtheRad. So the output load limits is dictated by the internal resistance of the opamp. (I guess this is expected). I assume that the standard limits (if there is a standard that exists) is that this heavy load (50pF||3.3kohm) is what most opamps are able to drive?

 

[vivekroy]

I found the PhD guy of the guy. Here it is: https://trace.tennessee.edu/cgi/viewcontent.cgi?article=2529&context=utk_graddiss
If its from a paper, always post the entire reference.


Page 85 of the above text describes what is meant by light load and heavy load.

This OpAmp is generally tested under two load conditions, namely, the light load and the heavy
load. The light load originates from the loading of an oscilloscope probe, where 15 pF capacitance and
1MOhm impedance can be found. The heavy load is created by connecting the output to a 35pF capacitor
(which adds up to 50 pF including the probe capacitance) and a 3.3 KOhm resistor. However, if an
oscilloscope probe is not used in certain measurements, physical capacitors and resistors are added to
mimic both loads.

You also did not mention the application. Looks like these are used for electrochemical cell measurements whose load variations are what the above mentioned corresponds to.

 

[rmanalo]

Thank you for the link @vivekroy. Your comments are noted, I attached the citation of the paper for future reference of this thread.

[ref] L. Zuo, S.K. I/s/l/a/m/, "Low-voltage bulk-driven operational amplifier with improved transconductance", IEEE Trans. Circuits Syst. I Regul. Pap., vol. 60, no. 8, pp. 2084-2091, 2013.

Edit: it's weird that words pertaining to religion is censored here.

 

[t4_v]

@rmanalo,
Very often opams (and other designs) are tested to verify the maximum conditions they are able to handle. When you know that, you may use your block many times in different places (what means different loads) in your design.