Gain Stage Range Shift Toggle

[benny_profane]

I'm working on an op amp gain stage and wanted to have a toggle switch to shift the range of GAIN knob. I'd like to keep the dBV range constant while shifting the gain window. For example, a gain stage with a delta of 18dB has a 'Low' range of -6dB to +12dB and a 'High' range of +8dB to +26dB.

What would be the best architecture to accomplish this (i.e., inverting or non-inverting)? I think it should be possible to use one potentiometer while adding/removing a resistor—but I'm getting tripped up.

Any input would be appreciated!

Cheers

 

[Feral Feline]

I don't think it would matter inverting or non-inverting (I may be wrong), but definitely think you're on the right track with switchin' in/out a resistor in series with the pot.

Dr Sci's The Elements has that feature.

Check out the build doc to GCI's The Brutalist JR, it describes this very thing of swapping a resistor for a gain bump, put it on a switch.

 

[benny_profane]

Here're some equations.

Inverting op amp gain:
Av=Rf/Rin
20(log(|Av|)=dBV

Low Setting (min):
20(log(x)) = -6
log(x) = -0.3
10^-0.3 = 0.5

Low Setting (max):
20(log(x)) = 12
log(x) = 0.6
10^0.6 = 4

Op Amp Values (Low Setting):
Av = (Rf + Rseries)/Rin

Av_Low(min) = (Rf + Rseries)/Rin
0.5 = (0 + Rseries)/Rin
Rseries = 0.5(Rin)

Av_Low(max) = (Rf + Rseries)/Rin
4 = (Rf + (0.5(Rin))/Rin
4(Rin) = Rf + 0.5(Rin)
3.5(Rin) = Rf

Low Setting values:
Rin = 28k
Rf = B100k potentiometer
Rseries = 15k

---

High Setting (min):
20(log(x)) = 8
log(x) = 0.4
10^0.4 = 2.5

High Setting (max):
20(log(x)) = 26
log(x) = 1.3
10^1.3 = 20

Op Amp Values (High Setting):
Av = (Rf + Rseries)/Rin

Av_Hi(min) = (Rf + Rseries)/Rin
2.5 = (0 + Rseries)/Rin
Rseries = 2.5(Rin)

Av_Hi(max) = (Rf + Rseries)/Rin
20 = (Rf + (2.5(Rin))/Rin
20(Rin) = Rf + 2.5(Rin)
17.5(Rin) = Rf

High Setting values:
Rin = 5.7k
Rf = B100k potentiometer
Rseries = 14k

---

So, it looks like this could work by changing the Rin value from 28k (Low) to 5.7k (High) and still maintain the GAIN pot value and the series feedback resistor. I'm not sure if this is the best way to accomplish this or if it should be with a non-inverting stage. Another option would be a transistor boost stage following the op amp gain stage. But, I'm not sure if that would react linearly and provide a static +18dBV gain.

 

[Feral Feline]

That's all way beyond my ken.

I wish I could do stuff like that, to help my builds and understanding, but — alas I'm a mathophobe.

 

[fig]

Edit: Doh, that's not a schematic. Sorry, thought I was onto something. That probably should have been a dead giveaway