how to optimize for multistage gain selection

How do you deal with overloading the gain stage? Is the attenuator ahead of the first stage?

--

  Rick C. 

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When amplifying a signal with a great variety, one has to consider the amplifier stage noise as well as overload issues.

For best weak signal performance, keep the first stage gain at maximum and adjust only subsequent stage gains. This way the total noise is determined by the first stage only, masking the subsequent stage noise,

Only when the signal becomes so strong that it risks overloading the first stage gain, start reducing first stage gain. At such high signal levels, the amplifier noise is no longer an issue.

There was a lot of gain. You can switch in x10 gain stages up front, The attenuator in the ~middle and then fixed gain at the end.

George H.

Hmmm... I'm not getting it. The idea is to have high gain in the amps at all settings so as to minimize the bandwidth??? Sounds counter productive.

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  Rick C. 

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  -+ Tesla referral code - https://ts.la/richard11209

ote:

and I would like those stages to be configurable, but I also need to have the overall gain vs. configuration curve to be such that every new step is ~5% increase w.r.t. the previous.

for the second, how would I design the set of values those gains need to b e at? Is there an algorithm to do that?

ethod to do so.

y a bad

nd adds

head of the first stage?

t all settings so as to minimize the bandwidth??? Sounds counter productiv e. Ya know Rick, if you were inquisitive and polite, rather than argumentative and insulting, I would be happy to share details. It's not anything amazing. But we sell them without complaints. Gain from 10 to 10^4 and BW from DC to 1 MHz. (opamps are amazing.)

GH

ote:

e:

ut and I would like those stages to be configurable, but I also need to hav e the overall gain vs. configuration curve to be such that every new step i s ~5% increase w.r.t. the previous.

4 for the second, how would I design the set of values those gains need to be at? Is there an algorithm to do that?

method to do so.

lly a bad

and adds

ahead of the first stage?

at all settings so as to minimize the bandwidth??? Sounds counter product ive.

ve

Sorry if I come across as argumentative... (maybe not, isn't that the poin t to discuss/argue a point?) but I don't mean to be rude or insulting.

After I hit send I realized I misread the post and thought you were saying attenuator - fixed gain - attenuator - fixed gain. But you are saying vari able gain - attenuator - fixed gain.

Gain limits bandwidth. So any gain you are including that then gets reduce d by attenuation is tossing away bandwidth, no?

I don't get the purpose of variable gain in the first stage. When set to t he highest gain setting the bandwidth is reduced. When set to the lowest g ain setting the noise is maximized. Wouldn't it give better results to hav e the fixed gain in the first stage? That would give highest bandwidth and lowest noise, no?

But then that comes back to the issue of overloading the first stage. Actu ally, I'm liking the attenuator - fixed gain - attenuator - fixed gain for noise and range, but it provides the worst bandwidth, no?

I don't think you said what your product is. If you only want 1 MHz of ban dwidth I suppose that is not important in the above analysis. 1 MHz should be pretty easy to achieve, no? I guess it's lowest noise that is the hard goal.

--

  Rick C. 

  +- Get a 1,000 miles of free Supercharging 
  +- Tesla referral code - https://ts.la/richard11209

So the gain range would be 20 to 80 dB and the maximum gain would be about 35 and 45 dB/stage, this at 1 MHz. For proper feedback action, the open loop gain would have to be, say, 20 dB better, so with standard compensation, the open loop gain would drop to unity around 1 GHz, so an amazing op-amp is needed. With more aggressive compensation, a lower open loop limit would suffice, but since this is a variable gain stage, the stability at lower gain settings must be analyzed.

It's also useful to set the rolloff with a filter and not just let the amp roll off. Getting anywhere near the op amp's maximum slew rate forces it to run with a large differential input voltage, which can make a mess.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC 
Optics, Electro-optics, Photonics, Analog Electronics 

160 North State Road #203 
Briarcliff Manor NY 10510 

hobbs at electrooptical dot net 
http://electrooptical.net

On Friday, April 19, 2019 at 11:57:53 AM UTC-4, snipped-for-privacy@downunder.com wrot e:

rote:

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out and I would like those stages to be configurable, but I also need to ha ve the overall gain vs. configuration curve to be such that every new step is ~5% increase w.r.t. the previous.

d 4 for the second, how would I design the set of values those gains need t o be at? Is there an algorithm to do that?

l method to do so.

ally a bad

s and adds

r ahead of the first stage?

,

s at all settings so as to minimize the bandwidth??? Sounds counter produc tive.

ive

Here's a schematic.

George H.

Thanks, this clarifies some things.

Hopefully the signal is AC, since switching in an inverting stage will reverse the polarity.

I calculated 1.5 bits for the two first stages and 3 bits for the attenuator. From previous posts I assumed that there would be 3+4 control bits.

Anyway, all those op-amps are really marginal for 1 MHz bandwidth. The full voltage swing stops at about 200 kHz and at 1 MHz, the swing is only a few volts.

The open and close loop gain are marginal, some of which drops a few dB at 1 MHz. In order to have, say the -3 dB full chain response at 1 MHz, the gain of each individual stage should drop by less than -1 dB,

Better op-amps may help.

On Friday, April 19, 2019 at 4:52:15 PM UTC-4, snipped-for-privacy@downunder.com wrote :

rote:

eadout and I would like those stages to be configurable, but I also need to have the overall gain vs. configuration curve to be such that every new st ep is ~5% increase w.r.t. the previous.

and 4 for the second, how would I design the set of values those gains nee d to be at? Is there an algorithm to do that?

rmal method to do so.

usually a bad

hens and adds

ator ahead of the first stage?

ont,

amps at all settings so as to minimize the bandwidth??? Sounds counter pro ductive.

tative

Yeah, noise amplifier.

Oh I'm not the OP. I was just posting a schematic of a gain with switched attenutators after..

Oh yes! This circuit is from ~10 years ago. Before putting new opamps in the gain stage, I could use faster opamps in the 100kHz filter stage. (not shown.)

In this circuit, amplifying noise, you start to get clipping of the voltage before hitting the slew rate limit of the output opamps. Vrms