[sdiy] LT1228 Lm13600 crap

[Ian Fritz]

Hi Rene!

At 07:56 PM 5/21/05, René Schmitz wrote:

>I would guess the reason for this difference is that there is current 
>stolen from going to the diffamp, which is used for the variable bias in 
>the output stage.

Thanks for pointing that out.  Looking at the 13700 vs the 13600 
schematics, it appears that they just erased Q3!  Do you think the current 
theft could be eliminated by tying the buffer input to the negative 
rail?  I'm not sure how simplified these schematics are.

I guess I'll try that -- wouldn't bother me a bit to burn a few of these 
units out.  :-)

Also, I should get a couple of 13700's and measure the Iabc linearity, as 
per Harry's excellent suggestion.

One further question, though.  Isn't Q3 just an additional current-mirror 
leg?  I would have thought that any error would just be Q3's (small) base 
current.  Also, shouldn't this be a linear error to first order, assuming 
beta doesn't change a lot with current.  How does Q3 produce a >5% 
nonlinearity?

   Ian