[sdiy] Transistor help

[Peter Forrest]

What a fascinating RAP (not that RAP) rap about ARP PNP /NPNs!
Shows the degree of fiddly work required to put together those early 
commercial synths.  I love the word 'Easy'.
Peter
----- Original Message ----- 
From: "harrybissell" <harrybissell at prodigy.net>
To: "James Patchell" <patchell at cox.net>
Cc: "Rob Keeble" <rob at emulatorarchive.com>; <synth-diy at dropmix.xs4all.nl>
Sent: Thursday, April 28, 2005 2:08 AM
Subject: Re: [sdiy] Transistor help


> Puzzled by a set of initials on an early ARP VCO schematic that read "RAP"
> I asked Bob Pease about it... it was a *different* RAP but he
> kindly agreed to advise on the questions I had. Text follows...   H^) 
> harry
>
> I never saw this  sawtooth oscillator before - but I recognize how ARP
> did things!
>
>> The questions involve the expo converter design...
>
>>> 1) what are the advantages / disadvantages of the NPN / PNP
>> type converter vs. the dual NPN / opamp servo design ???
>
> **** I am sure  the answer is: The NPN/PNP  logger is -  GOOD ENOUGH!!
>
> *** The servo version is BETTER - slightly more precise -but it's more
> complicated, and is bulkier, and costs  more --  so there's no need to do
> that, if the other one is GOOD ENOUGH!
>
>> 2) What parameters are selected for in the NPN / PNP
>> transistors....
>
> *** (A)  fairly high beta, 150 min;   and
> *** (B)  close Vbe match, within 1 or 2 mV, Max.
>
>> 3) How are the NPN / PNP transistors matched ?
>
> *** Easy: