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I'm planning to use an AD817 op amp in the output stage of my function generator (datasheet). I'm worried about how the part would stand up to a short, so I ran some simulations. The simulation below sources 70mA through R3 when the output (probe tip) is shorted. This makes sense and tallies somewhat with the datasheet which gives a max current of 100mA @ 25degC, but less at higher temperatures. Since the part (according to spice) is dissipating 800mW it's likely to be hot. Although 800mW is within the max spec for this device (around 1.4W @ 25degC), I'm not comfortable with 800mW of dissipation. Which brings me to the question.

Is there a best practice way to limit the output current of an op amp under short circuit conditions?

Basic Amplifier: enter image description here

Here are some options I've tried, with comments:

1) Push-pull Amplifier to Relieve Op Amp Output Stage

Works well to limit op amp output current but could deliver 10V/47 = 200mA through R3 @ 1W. The transistors would need to dissipate 500mW each.

enter image description here

2) BJT Based Current Limiter

BJT based current limiter based on some sources I came across (see here and here). This is more like what I'm looking for, but I couldn't make it work!. Am I correct in thinking that the current diverted away by Q3/Q4 ends up at the base of R3?

enter image description here

Other options:

3) Reduce the gain. Reduced signal amplitude isn't desirable.

4) Limit the power supply current. Would (needlessly?) effect other parts of the circuit.

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EDIT - IGNORE C1 - unfortunately this is the only circuit that I could find with the Rout component shown within the feedback loop so, please ignore C1 - it is not meant to be there.

This technique is used (with care) a fair amount: -

enter image description here

Ignore C1 and just analyse how much current the op-amp can supply to a load in the place of C1. If the maximum output of the op-amp is (say) 4 volts (+/-5 volts supply), there can be no more than 53mA delivered into a short circuit.

On normal loads, the op-amp has to work a bit harder because the 75 ohm forms a potential divider BUT, the good side is the feedback loop compensates for this.

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  • \$\begingroup\$ I'm afraid I'm a bit lost. It's an inverting amplifier with a gain slightly south of -1. If I put a positive signal of 10V at Vin the op amp would attempt to put -10V odd across Rout and sink 143mA. Have I got that right? I'm not seeing the light (not unusual). Would you use this in front of or instead of the AD817? The AD817 is fast and I need the gain (and the capacitive load drive). \$\endgroup\$ – Buck8pe Sep 1 '15 at 12:49
  • \$\begingroup\$ Insert the extra resistor in series with the op-amp output of your circuit but move the negative feedback loop to after the resistor. That's all I'm saying - basically I'm turning an op-amp that can supply maybe 100mA into an op-amp that can only supply 50mA (or less because it depends on the value of Rout). \$\endgroup\$ – Andy aka Sep 1 '15 at 12:52
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    \$\begingroup\$ As shown this will be unstable - you should add a small capacitor directly from the output of the opamp to the inverting input. see maximintegrated.com/en/app-notes/index.mvp/id/5597 figure 10. \$\endgroup\$ – Kevin White Sep 1 '15 at 15:54
  • \$\begingroup\$ Apologies, it took me a while to twig this. I thought you'd simply moved R3(47ohm) into the feedback loop and I wondered what the difference was. Nice solution. I'll run all the suggestions through a bit and come back when I figure what works best. \$\endgroup\$ – Buck8pe Sep 1 '15 at 16:14
  • \$\begingroup\$ @KevinWhite I asked the OP to disregard the output capacitor - it was the only circuit I could find that was available on-line. Sorry for the confusion. \$\endgroup\$ – Andy aka Sep 1 '15 at 16:57
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You can use your method, but just add some more op-amps in parallel and increase the resistors. For example, use 100R resistor to get an output Z of 50 ohms.

I believe this method is used in Keysight (née Agilent, née HP) arb function generator output stages.

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ I like it, it's opulent and if I wasn't such a skinflint it would be top of the list;-) \$\endgroup\$ – Buck8pe Sep 1 '15 at 16:02
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The 50 ohms output impedance helps as it will limit the output current. There are opamps that can drive 50 ohms (to ground) BUT the output voltage will be limited to a couple of volts. Usually too little for a function generator especially if you would also like to be able to shift the DC level at the output up or down (this adds to the voltage).

The easiest and most common choice is to implement and amplifier like in option 1), the amplifier should be designed to drive any voltage into 50 ohms. Current limiting is not needed as the 50 ohms resistor will limit the current.

Ad option 2) I think the output current never reaches its limiting value because you have the 50 ohms at the output. Anyway, if you insist on current limiting I would suggest to use a more traditional solution with current sensing resistors in series with the emitters like your first link. Many audio amplifiers use this type of protection.

Another observation: your amplifiers appear to be class B amplifiers read this if that does not ring a bell. For a function generator you want low crossover distortion so I would suggest at least class AB or even better: class A. But it all depends on your needs and how efficient you want your design to be. Oldfashioned analog function generators get warm, they use a significant amount of power, that is (partly) because their endstage operates in class A !

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  • \$\begingroup\$ I agree, option 1 is attractive. The only issue is that the AD817 could produce +/-10V output, so I'd need to rate R3 and the transistors accordingly. Regarding option 2, I'm not sure the circuit really works. I included to show the way I was thinking, but in the sim R3 has 100mA across it, so Q3/Q4 are definitely drawing current...but then, I don't want that excess current appearing across R3 anyway. I'll have a look at some non-push/pull amplifiers to see how they work out, thanks for the tip. \$\endgroup\$ – Buck8pe Sep 1 '15 at 13:07
  • \$\begingroup\$ Yes, do investigate the simple non-push-pull amplifiers first and when you understand how these work move to push-pull. These will be a lot easier to understand once you master the simple amplifiers (as most push-pull amps are just a single transistor amplifier combined with a mirrored copy ;-) ). \$\endgroup\$ – Bimpelrekkie Sep 1 '15 at 13:15

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