fast amp gain tweak

Fast opamps tend to have low open-loop gains, like 60 dB maybe. So if one needs precision, and gain is set by available 0.1% resistors, some sort of secondary gain fine-trim is prudent.

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R22 can up the gain a little, as needed.

Sometimes, app notes and data sheets add a small resistance to the otherwise-grounded non-inverting input of fast amps. This has been explained to me as a "damping resistor", possibly to kill wire-bond resonances. As long as R15 is there, I may as well add R22 for the gain tweaker.

Actually, R22 *reduces* the gain, but it will work anyhow. I can pitch the gain high with the R14/R13 ratio or something, and then trim it down.

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John Larkin         Highland Technology, Inc 

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John Larkin
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What's the difference between pin 1 and 6? Just to make the output physically available closer to the inputs?

Reply to
bitrex

E2_Gain_Tweak.jpg

How stable is the "about 60dB"?

More importantly, after tweaking, how well does the thing hold to the desired gain over temperature and aging?

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Tim Wescott 
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Tim Wescott

Yes, a few fast opamps do that. It keeps the load current/wirebond inductance voltage drop from getting into the feedback path, and makes layout nicer.

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John Larkin         Highland Technology, Inc 

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John Larkin

Yes. At first it looked awesomely good, but I soon found the ADA4817's pin 1 connection to be a pain. For amplifiers with post-feedback resistor networks, and where you're relying on a low 0.1 or 0.15pF capacitance of the resistor, the close pin 1 proximity is presents unwelcome extra feedback. For a TIA using a pole-zero correction network (AoE III Fig 8.80.C) on the feedback resistor, it's a killer! I snip the lead off the so-8 package and use a special footprint without pin 1.

I kept a second version of the PCB with pin 1, but this looks to be valuable only for above 50 to 100MHz circuitry.

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    - Win
Reply to
Winfield Hill

Nice, thanks. I don't see anything about the series R(R15) in the data sheet,

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Well there is talk on page 16 fig's 47,48,49 of a series R with the SOIC pac, and Gain =1. Where did you learn this trick?

George H.

Reply to
George Herold

I've found ICs to be pretty consistent over time. The number and configuration of the transistors inside isn't going to change.

It's not as if I can buy a GHz opamp with 120 dB open-loop gain.

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John Larkin         Highland Technology, Inc 

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John Larkin

What I like about that amp is its basically zero bias current and noise current. Current-mode amps are terrible that way. I need its specs for my input stage, not shown, so I may as well use them in the dumb 100 MHz inverters too.

The 4817 looks like the best choice, because it's probably the only choice. The low open-loop gain is a mild annoyance.

A couple tenths of a pF of feedback C shouldn't hurt here.

I nabbed Phil's ADA4817 model, but it doesn't include distortion or output stage subtelties, so it's not much better than tweaking UniversalOpamp2.

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John Larkin         Highland Technology, Inc 

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John Larkin

I saw it sort of intermittently in a TI data sheet, and asked. The response was vague. I have used fast current-mode amps before, with the NI input hard grounded, without problems. It does make a nice way to tweak gain without involving the main feedback path.

So, sometimes people think this is a good idea.

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John Larkin         Highland Technology, Inc 

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John Larkin

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