Korg 35 IC Q1?

[Magnus Danielson]

From: jhaible at t-online.de (jh.)
Subject: Re: Korg 35 IC Q1?
Date: Sat, 6 Jan 2001 00:48:23 +0100

> > Got a question about the Korg 35 circuit that Martin posted this morning:
> >
> > Shouldn't there be something connected to the emitter of Q1?  Is it being
> > used as a diode?
> >
> > Romeo
> >
> 
> 
> I am wondering about the very same thing, ever since I've seen the
> now dosclosed circuit.
> The 2-transistor buffer amp with voltage gain was no surprise - it's
> basically the same as in the PE-2000, only that there are two LDRs
> instead of Q2 and Q3.
> The Q2 and Q3 part is what makes this filter special, and this was
> disclosed by K. Tanaka some time ago. Even using a 3rd transistor
> (Q1) to control Q2 and Q3 is no surprise - I found it a "natural"
> way to use it as an emitter follower in my JH-720 VCF.
> But like Romeo I'm asking - why use Q1 as a simple diode, and
> why use the cb diode, when in most other chips I've seen the be
> diode is used to make a diode function of a transistor.
> Has anybody figured it out ?

Um... if one looks on a planar NPN transistor it kind of looks like
this:

     B     E          C
    xxxx  xxxx       xxxx
-----xx----xx---------xx---
     xx    xx         xx SiO2
-----xx----xx---------xx---
   |    |  n+  |    |
   |     \____/     |
   |  p             |
    \______________/

  n 


----------------------------

Now, the CB np-junction has much larger area than the EB np-junction.
Shouldn't that contribute to a lower bulk resistance?

I haven't dug into my semiconductor book for a while, I really should
do more of that any day now.

Anyway, I think it is benefitial in getting a good expo curve
conformance in this simple case, right?

For me, it is really the CB junctions that also comes to use in the Q2
and Q3 and yes, these are all the same transistor.

Also, the 2SC1623 and 2SA812 are complements to each other, migth be
nice to know.

Cheers,
Magnus