op amp specs question

Hello,

I need an op amp that can handle a 800mA current surge at it's output for a few mS.... it will be operating as a unity gain buffer, with negative feedback... it there a spec on the datasheets that tell you how much of an inrush current an op amp can handle at the output? I see that most the opamps can supply around 60mA but I can't tell about what kind of surge they can handle....

Reason for the surge is a capacitive load

Thanks!

Reply to
panfilero
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Sounds like you need a high current opamp. I like the OPA544 for

+/-15V and the OPA569 for 5V. If the output is single ended you could always buffer the low current opamp output with a transistor.

George Herold

Reply to
ggherold

Thanks George, so I'm worried about current surges in and out of my opamp output... does this mean I could handle this with a totem pole transistor set up at the output of my opamp?

Reply to
panfilero

They usually have current limiting built in. If you don't mind your cap taking a few tens of ms perhaps to charge the caps to some quiescent voltage at turn on then it's not a problem. What would be a problem is high desired dV/dt at the op-amp output.

Also most op-amp don't like heavy capacitive loads, it changes the phase response and at unity gain that's exactly what you don't want for stability.

You could also add a discrete output stage within the feedback loop..

Graham

Reply to
Eeyore

A schematic is needed. So many variables.

Graham

Reply to
Eeyore

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ok, here's a link to the schematic

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basically the point of my opamps is to sink and source current and protect my DAC.... so they are just voltage followers... I was thinking maybe I'm overdoing it with these two opamps and could just use two transistors in a totem pole arrangement to source and sink the current... but I've never done that and I'm not sure if it's a good solution

Thanks!

Reply to
panfilero

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Forget totem-pole, it's 'old-hat'

U1B is almost superflous unless you need to sink current into it ( I presume you do ).

What value of capacitor do you have in mind and why is it there ( what is its purpose, since being driven by voltage sources it will do nothing other than be a pain in the ass and limit dV/dt ) ? What's going to be connected to it ? Where did you get this circuit ?

Graham

Reply to
Eeyore

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So is this better than pursuing a two transistor solution like totem poles?

You are right I will be sinking into U1B, because my voltage across the load is bipolar

Its for a lab experiment at school, we don't have the test article yet... it's a piezo element which was described by the vendor as a capacitive load... but they couldn't give me any specific values for it (I don't know how to model it) so I'm trying to make my circuit for the worst case scenario... which would be a purely capacitive load... but I'm worried about my design cause I don't want to fry my opamps with the surge current from the cap...

I made up the circuit

Reply to
panfilero

Not sure what sort of piezo element you will be using, but the ones I have used require high voltages (several hundred to 1000 volts). You may want to look at the high voltage / high power op-amps by Apex.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

Reply to
Bob Masta

per manufacturer I need to send at the most -1V to 1V across it, and I need to be ready for a current surge...

I was thinking... if I put a resistor at the output of each opamp, before the load (and in series with the load) then... when the capacitor is first energized it would be like a short and basically there would be a resistaqnce between my opamp outputs... therefore if I have say 6V at one output and 5V at the other then the most current I will see (per ohms law) would be (6-5)/(R1+R2) where R1 and R2 are the resistors at the outputs of the opamps that I just mentioned... does this make sense? Would this solve my current surge issue?

Thanks!

Reply to
panfilero

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I'd use complementary emitter followers.

Right, now it makes sense. What size piezo and we can give you an estimate of the likely C.

Graham

Reply to
Eeyore

Putting an R in series with the C would be very effective but may slightly reduce the acoustic output.

Graham

Reply to
Eeyore

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If you want the results to be linear, you better off with op-amps. You haven't said much about the signal. The op-amp may have to handle some serious power if you are swinging a capacitive load up and down very much.

Reply to
MooseFET

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