[sdiy] VCS3 oscillators

[Ian Fritz]

At 10:46 PM 10/3/2004, harrybissell wrote:
>The schematic of the 3080 shows Darlington transistors for the PNP
>mirrors... a sure sign that the PNP gain is low (ie lateral PNP structure)
>
>Who knows for the 3280... which does not show the details of the mirrors.
>Anyone have that info ???

Yesssss ... You need to refer to the Serge T. article in EN #113.

>I'm plating with a spice model (potential) for the 3280... assuming that it
>is
>similar to the 3080 in mirror construction.

Nooooo ... Similar, but some fairly significant differences.  Most 
important: the control current mirrors only reflect about 80%.  Thus the 
transconductance equation is roughly

gm = 16 Iabc, compared to

gm = 19.2 Iabc, for the 3080.

Also, if you are using the linearizing diode, twice the diode current is 
mirrored (compared to 1x for the 13600/13700).  And that is actually 
reduced to about 90% of the nominal.

Finally, the biasing scheme of the mirrors makes it impossible to use the 
W. Jung trick of driving the control current through the PS pin.  This will 
actually be the feature I will miss the most (although I bought 100 3080's 
recently).

   Ian