[sdiy] question on JFETS in vco of MS series

[ASSI]

On Saturday 27 August 2011, dan snazelle wrote:
> there is a JFET in the MS20 VCO which connects to the CV source...on the
> schematic it is called Q5
> the drain connects to ground and the source to a 10k resistor

Are you looking at the same schematic?  The drain of Q5 connects to resistor 
R15 and the source connects to one of the collectors of Q6 (pin 2, the 
output of the expo converter), which is not at ground (as it happens to be 
in many other VCO).  The other collector at pin 4 however is, by means of 
IC3.

> the odd thing (from my understanding) is that the gate is connected to
> GROUND.

As has already been mentioned, Q5 is a cascode transistor to keep the 
collector of Q6 at roughly the same potential while the current from the 
expo converter changes by several orders of magnitude and the drain 
potential of Q5 by several Volt.

> I thought that ground shut off a transistor. obviously this circuit
> works, so my guess is that ground is NOT in fact OFF, so it must be
> acting as a constant of some sort.

Q5 is an n-channel JFET.  At V_GS=0V it is actually fully conducting, you 
need a negative V_GS to pinch the channel off.  V_GS becomes negative 
because the collector of Q6 will be at some positive potential, depending on 
V_P of the JFET (a small pinch-off voltage is beneficial in this 
application).  If you wanted it to be a "virtual ground" you'd need to 
adjust the gate of Q5 to a negative voltage, which might improve things 
considerably if the V_P is large.


Regards,
Achim.
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