[sdiy] Linear current source homework problem

[Dave Manley]

Aaron Lanterman wrote:
>
> On Thu, 5 Oct 2006, Scott Gravenhorst wrote:
>
>> The FatMan at http://www.paia.com (schematic is available there) uses 
>> this method, though to reduce the error caused by current supplied to 
>> the summing node through the EB circuit of the transistor, a 
>> Darlington pair is used. It is also possible to get even better 
>> performance (accuracy) using a MOSFET instead of a BJT or Darlington. 
>> See the FatMan VCO for the Darlington method, the VCA and VCF also 
>> use a linear current source and they use the less accurate single BJT 
>> method.
>
> Interesting.
>
> Looking at the the VCA of the FatMan, I'm guessing C24 in the feedback 
> loop of the control amp amp is supposed to address parasitic 
> oscillations... but what is diode D9 doing??? For Q12 to be "on" I'd 
> expect D9 to be off...
>
> - Aaron
>
 From the FatMan pdf file (fatmwork.pdf):

"FatMan’s Voltage Controlled Amplifier uses one OTA from
IC18. The main components of the V/I that control this
element are IC13:c and Q12. This voltage to current
converter is unlike the others in that it must be stable for
zero control voltage (so the VCA can turn off completely).
Adding D9 to the circuit clamps the output of IC13:c and
keeps it from going negative and C24 provides frequency
compensation for the high loop-gain state that exists at
near-zero control voltages"

-Dave