Op amp gain inverting configuration

[raff5184]

Hi,
I got this op amp (the small one): http://www.analog.com/en/products/amplifiers/operational-amplifiers/jfet-input-amplifiers/ad825.html?doc=AD825.pdf#product-overview

and this eval board: http://www.analog.com/media/en/technical-documentation/user-guides/UG-755.pdf

These are the specs: http://www.analog.com/media/en/technical-documentation/data-sheets/AD825.pdf

I created a simple inverting configuration with R_F=100k and R₁= 1k

I am working at 700kHz, and suppy voltage +-15V, I am expecting an output voltage 100 times higher than the input, but I get a maximum of 30. Do you have any idea why? Is it possible to understand from the data sheet what I can expect at 700kHz?

[Rerouter]

Unity gain bandwidth is what is biting you, 26,000,000/700,000 = 37 so your numbers add up the the spec of 26MHz, for a gain of 100 you would need atleast 2 op amps amplifying at a gain of 10 in series to meet that spec with that op amp, or find an op amp with a unity bandwitch above 75MHz

[DaJMasta]

Not much more to add, when you're selecting the opamp for the application, you need to make sure that your desired gain times your signal frequency is less than or equal to the rated unity gain bandwidth.

If you've got a means of feeding a signal in (like a signal generator), start from 700kHz and start turning it down, you should see that around 260kHz (with typical rating, 230kHz minimum) you'll see the full gain on the output.  Conversely, if you swap the 100k for a 33k (actually slightly more, but it's an easy value), you should see the gain you expect on the output of your 700kHz signal.

[w2aew]

As stated above, gain-bandwidth product is biting you. Here's a video describing GBW:

[raff5184]

Not much more to add, when you're selecting the opamp for the application, you need to make sure that your desired gain times your signal frequency is less than or equal to the rated unity gain bandwidth.

If you've got a means of feeding a signal in (like a signal generator), start from 700kHz and start turning it down, you should see that around 260kHz (with typical rating, 230kHz minimum) you'll see the full gain on the output.  Conversely, if you swap the 100k for a 33k (actually slightly more, but it's an easy value), you should see the gain you expect on the output of your 700kHz signal.

true. I did the swipe and I saw the gain increasing at lower frequencies

[raff5184]

Unity gain bandwidth is what is biting you, 26,000,000/700,000 = 37 so your numbers add up the the spec of 26MHz

is 37 the maximum I can reach at 700k?

[DaJMasta]

Basically, yes.  The individual part you have may be a bit better, you can characterize it if you want, but it's not actually guaranteed to the 26MHz figure, it's guaranteed to 23MHz, so if you were making a mass market product with this opamp, you'd need to aim for just a bit under 33 gain to be certain you'll be getting it in every product.

Of course, adding a second opamp of the same kind in the same configuration would give you another 33 gain max, or well over 1000 between the two.  A higher spec opamp can get you more performance, but it's usually a better choice to just add another stage, as it gives you almost as much gain as you could want if you were even close with one stage.

[raff5184]

Perfect, thank you. Last question, I saw that around 500 mV peak-to-peak the signal starts to be distorted (the peaks are cut off because of saturation). If I want to use a 2 stage amplifier, wouldn't the final signal be distorted? Basically because the output of the first stage is too high for the second one

[Rerouter]

This comes down to the capacitance in your circuit, you will probably find with 2 stages each with a lower gain this will be lower.

What i assume is happening is the resistor divider node has some stray capacitance that is making the output lag the input more than usual, so the differential across the op amp pins exceed the forward voltage of its protection diodes (some op amps have them strapped across the inputs) they turn on and cause your distortion

Because a gain of 10 per stage uses lower value resistors the effect of the capacitance will be less.