[sdiy] Odd transistor connection, why???

[Oren Leavitt]

David Ingebretsen wrote:
> As I am digging into my Steiner resurrection, studying schematics, and
> trying to simulate some of these circuits, I am hitting a problem with the
> SPICE simulations. I have every confidence the schematics are correct, and
> the circuits work (at least the few I actually built in the 70's), but the
> simulations are not working.
> 
> The common link between the ones which don't work is a weird NPN transistor
> connection. Nyle connected the base to the emitter or collector and then
> uses the transistor as a two pin device, like this:
> 
> 
>              ______
>             |      |
>           __|__    |
>           /  \     |
>     _____/    >____|_____ 
> 
> What does this do to the transistor? What is the point of doing this? He
> does it in his VCO and in the EG. I am becoming convinced it is this weird
> connection which is giving SPICE problems. I can see when it the base is
> connected to the collector, it should be a diode, but then why connect it to
> the collector at all? I have no idea what it does when connected to the
> emitter. In all the EG schematics, the base goes to the emitter. In the two
> different VCO schematics I have, it goes to the collector.
> 
> The Synthacon schematics for the VCO and EG are here: 
> 
> http://www.xmission.com/~dingebre/page9.html
> 
> (since they are on the net in other places, I don't think I'm violating any
> copyright. If I'm wrong, let me know immediately).
> 
> David
> 

In the larger schematic the base-emitter junction of Q8 is being used as 
a zener diode (~6V) when reverse biased (D5 conducting).
In the small schematic at the bottom they're probably using the 
base-collector junction of Q5 as a low reverse leakage diode - looks 
like it is serving as a discharge switch to C2.
My guesses.
- Oren