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Can someone reccomend a good unity gain op amp

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panfilero

Member
I'm flipping through datasheets and it seems like most op amps boast high gain, bandwidth, low noise.... and so on and so forth.... but I can't figure out which would be good for my application....

I don't care about gain, I don't care about bandwidth....

I'm looking for an opamp that I can use as a unity gain buffer, with negative feedback (no feedback resistor though) and I will be inputting a DC voltage on it's non-inverting input, and I want to see that same DC voltage on the output...

but, I'm having two problems... first, I'm not exactly sure what criteria I need to be looking for, I've heard that op amps with high bandwidth may have trouble with unity gain... or that some op amps in general may have trouble with unity gain...

could someone tell me what criteria I need to be looking at? or can anyone recommend any op amp that can do this well?

much thanks!

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extra info:
I will be driving an RC circuit with my op amp.... if it helps any to see what I'm trying to do... the next two pics show my circuit... I'm basically trying to make a pos/neg voltage supply I can control with a 4-20mA current, and I need my output voltage to clamp at a low level and a high level....

PosNegVoltageSupply.JPG
PosNegVSupSimulation.JPG
 

dknguyen

Well-Known Member
Most Helpful Member
So it sounds like neither the gain accuracy, noise, stability, or offset really matter here.

If so, just choose any non-high bandwidth op-amp. ANd you probably want an op-amp with a rail-to-rail input and output since you are working at unity gain.

What bandwidth do you need? Is this such a low frequency that bandwidth of any modern op-amp just doesn't matter? What's the frequency of your RC circuit?
 
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panfilero

Member
I'm not really sure what my C value is going to be in my RC circuit... but I think it may be pretty high... like 500mF or more....

I'm only going to be putting a DC voltage to my op amps, so I guess my bandwidth... well, I guess I don't care about it... cause I really don't need any... I recently found this guy

http://www.electro-tech-online.com/custompdfs/2009/05/buf634.pdf

which says it's a buffer amplifier... I'm wondering if he would be a good choice?

thanks for the response
 

crutschow

Well-Known Member
Most Helpful Member
Are you concerned about DC offset between input and output? Some op amps have several mV offset while others have less than one.
 

Willbe

New Member
I'm looking for an opamp that I can use as a unity gain buffer, with negative feedback (no feedback resistor though) and I will be inputting a DC voltage on it's non-inverting input, and I want to see that same DC voltage on the output...
You want a Unity Gain Stable amp.
Walter Jung writes about how to drive capacitive loads.
 

Hero999

Banned
That will work but it's a bit overkill.

The cheapy LM358 will do the job.

By the way what are you driving?

For 18R you'll need an output buffer and what's with the 180R in series with the 500mF capacitor, is that 0.5F or 500µF?

Using mF is not a good idea because it's very ambiguous, it can stand for milli Farrads or micro Farads.
 

Hero999

Banned
I agree but that doesn't change the fact that plenty of old schematics and capacitors use m for micro Farads. This is why it's a bad idea to use mF, as a general rule for values < 100mF use µF and F for values ≥100mF.

Why are you using a 0.5F capacitor and 180R resistor?

Are you aware that it will take 270s for the output to reach 5% of the steady state value?
 
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panfilero

Member
Why are you using a 0.5F capacitor and 180R resistor?

Are you aware that it will take 270s for the output to reach 5% of the steady state value?
that's just the nature of my load, what I show here is a model of what my load is, which basically comes down to this RC circuit, yup my time constant is large, but these parameters are not something I have control of
 

Hero999

Banned
What's the maximum voltage across the capacitor?

If you're not worried about 800mV of saturation and only need a few thundered mA then a simple emitter follower booster will do.
 

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