[sdiy] LM13600 Gain Question

[Scott Gravenhorst]

Ok, I get this.  JH's comment makes sense too.  Tailoring that actual range of
gain means messing with input and/or output resistors and that depends on the
max input expected and max required output.  Bottom line for me means rough
calcs and throwing it on the SBB to fine it out.

Thanks guys, this is what I needed.

Magnus Danielson <cfmd at bredband.net> wrote:
>From: Scott Gravenhorst <music.maker at gte.net>
>Subject: [sdiy] LM13600 Gain Question
>Date: Fri, 23 Jan 2004 10:24:23 -0700
>Message-ID: <200401231824.i0NIONK28361 at linux6.lan>
>
>> I'm looking at the LM13600 data sheet and fig. 4 (stereo
>> volume control) has a formula to calculate the gain given
>> Iabc as:
>> 
>> Vout
>> ---- = 940 X Iabc
>> Vin
>> 
>> Why 940?
>
>Look at page 6. Equation 5 there is the basic equation:
>
>       q*I
>          ABC        
>I    = ------ V
> out    2kT    in
>
>where CODATA-86 gives these values:
>
>                       -19 
>q = 1.60217733(49) * 10    C
>
>                     -23
>k = 1.380658(12) * 10    J/K
>
>and let's assume we are running at about 27 degrees, that is
>
>T = 300 K
>
>Then this formula becomes
>
>                       4 
>        1.60217733 * 10
>I    = ------------------ * I    * V
> out   2 * 1.380658 * 300    abc    in
>
>giving
>
>I    = 19.3407 * I    * V
> out              abc    in
>
>We also notice that the references figure 2 has the following equations hidden
>in it:
>
>        0.5
>V   = -------- V
> in   30 + 0.5  1
>
>V    = 30k * I
> out          out
>
>Resulting in 
>
>                               0.5
>V    = 30k * 19.3407 * I    * ---- * V
> out                    abc   30.5    1
>
>Tossing these together I get
>
>V    = 9511.82 * I    * V
> out              abc    1
>
>I am off by a decade and maybe I just fluke it... but it seems to fit
>together when I redo it.
>
>That at least how I should do it more or less. Maybe they made the misstake,
>I don't know. Hopefully you get a large enought hint by seeing the above
>calculations. 
>
>> Also, with a maximum Iabc of 2 ma. applied to one OTA, this
>> circuit can achieve a maximum gain of only 1.88.  So through
>> most of it's Iabc range, it's more of a voltage controlled
>> attenuator.  Am I whacked here?
>
>Change the output-resistor!
>
>V    = R    * I
> out    out    out
>
>Cheers,
>Magnus - time to go home 
>

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