99V as a fixed gain amplifier

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chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

99V as a fixed gain amplifier

Post by chrisp »

I need to amplify an unbalanced signal by 18db. Can anyone check me on the following design, please

Configuring a 99V as a non-inverting amp would mean -

(a) SigIn goes to 99V + through a DC compensating resistor, Rin.

(b) 99V Out goes to SigOut, and also to the feedback resistor Rf.

(c) The other side of the feedback resistor goes to 99V -, as well as to ground through the voltage divider resistor, Rg.

Now the gain of the opamp is Vout = Vin (1 + Rf / Rg), or

(1) Vout / Vin = 1 + Rf / Rg

We want gain to be 18db, so 18db = 10 * log (Vout / Vin), or

(2) Vout / Vin = 10^(18/10)

Putting (1) and (2) together means 10^(18/10)= 1 + Rf / Rg, so

(3) Rf = (10^(18/10) - 1) * Rg, or Rf = 62 * Rg

so if Rg is 1K, Rf is 62K.

Now Rin is (Rg*Rf) / (Rg + Rf) = 62000K/63K = 984R (=1K closest).

So Rin is 1K, Rf is 62K, Rg is 1K. Is this correct? Is there anything else I should be considering?
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

Hang on

Shouldn't (2) be Vout / Vin = 10^(18/20) (becuase 18db = 20 * log(Vout/Vin) )

That means if Rg=1k, Rf would be more like 7k (6K943) and Rin would be 870R (875R)

Plugging these real life numbers back in the equations,

db = 20 * log (1 + Rf/Rg) = 20 * log (1 + 7/1) = 18.06db. Close enough for rock and roll.
Chris P
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I do lots of things. I believe eclectic skills are best.

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Joe Malone
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Post by Joe Malone »

A DC compensating resistor on the way to the +in of the 99v isn't going to remove the 99v 1volt DC offset due to the -in being a emiter input. So is of little use. Even with 1% resistors you can get more than +/-0.5dB gain difference so I would make the -in to ground resistor a 25 turn trim pot in series with a cap so the DC offset is only x 1 and not x the AC gain.
Joe :-)
JLM Audio
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chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Post by chrisp »

Thanks for that, Joe - I was treating the 99V like a black box theoretical opamp when I should have simply been looking at how the 99V gain is done in the BA / BAD etc schematics. I see what you mean and stand educated.
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

chrisp
Posts: 184
Joined: Sun Mar 04, 2007 11:46 am
Location: Sydney, Australia

Re: 99V as a fixed gain amplifier

Post by chrisp »

OR .....

SIGIN to 99V +
99VOut to SIGOUT and Rfeedback=10K and Cf=47p
99V- to Rf, Cf and 100uf electro anode
100uf cathode to Rser = 1450 ohms = 1k3 and 150R in series, and then to ground.

My maths says that Gain = 1 + Rf/Rser = 1 + 10000/1450 = 7.8966
db = 20*log(gain) = 20 * 0.8974 = 17.95db (18db)

Can anyone out there please confirm?
Chris P
---------------------------------------------------------
I do lots of things. I believe eclectic skills are best.

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