Korg 35 IC Q1?

[houshu at dsp.cl.nec.co.jp]

Hello List,

First of all, many thanks to Martin Czech for his effort on Korg35.

The usage of Q1 in Korg35 is not strange for me.
As you know, Q2 and Q3 must be saturated when they are used as
variable resistors, and DC current should not go through them.
DC current around them is only base-collector current of Q2.
For temparature compensatation of the BC junction, a diode using 
BC junction of the same transistor looks appropriate.  Because the 
current supplied to bases of Q2 and Q3 through R3 is very small,
emitter follower is not needed to isolate R3 from R1 and R2.

Regards,

> Date:    Sat, 06 Jan 2001 04:39:24 JST
> From:    Magnus Danielson <cfmd at swipnet.se>

> From: jhaible at t-online.de (jh.)
> > > 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

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 Osamu HOSHUYAMA                    NEC Corporation  Kawasaki JAPAN
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