[sdiy] Bob speaks (RAP... not Moog)

[harrybissell]

Hiya kids.  Here is a copy of Bob Pease's comments on the ARP
npn-pnp expo design question I posted last week.

I don't think my first forward got to the list.  This e-mail is in
response
to my e-mail to Bob Pease... commenting on the "RAP" design of the
ARP 4027 submodule.  He pleads "not guilty" to the design... but
graciously
agreed to comment on the subject anyway

<text follows>

**** Hello, Harry,

let me  see what  I can figure out  for  answers to your  questions...
I have worked with ARP  for many years, and I know how  he thinks....  /

rap

*** see at ***

harrybissell at prodigy.net wrote:

> Hello Bob.
>
> Your designs are often a topic for fond discussion on
> the synth-diy mailing list.
>
> we have some questions about a VCO you designed for ARP
> synthesizers...  The ARP4027 module
>
> The schematic (if you don't remember your designs from
> maybe thirty years ago) can be found at
> http://www.arptech.synth.net/pdf/4027.pdf
>
> note the "RAP" in the title block :^P

*** Yeah - but that was NOT my design - I never worked for ARP!
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:  Get  an NPN transistor whose Vbe is "typical"  for the middle

of the distributions of the NPNs  and PNPs  that will be used.   Tie its

base to ground, and run a suitable current in its emitter.  Perhaps  200

UA?   Compare this Vbe  to the Vbe of an NPN, - the DUT, plugged into a
nice socket -  running at the same current, and read the  DELTA Vbe  (
Ve1 - Ve2) on a  decent  DVM. Bin the NPNs  into bins 1 mv  wide.  Do
this  while a lot of air is moving  over them both, good room temp air
from a blower,  stirred in a big cardboard box.  Don't let them get
heated much by fingers.

I will soon send  directions  on how to find a couple  columns I have
written on this. See at the end at &&&&&

Grade out a whole bunch of NPNs  to see the Vbe graded into bins 1 mv
wide.  Then cover up all the NPN  bins with clear plastic.

Now  buffer the  emitter of the NPN  with a GOOD ( sub 20 UV) op-amp.
Tie  the base of a PNP there, using a nice socket.  Run the PNP with the

same  current  in its emitter , as the NPN.  Grade the delta  Vbe  of
the
PNP, also into bins 1 mV  wide.  I.E. read its Ve  compared to ground.
Make sure that a BIGGER Vbe  gets put in a bin adjacent  to the NPN that

has a BIGGER  Vbe.   Set these bins  next to the covered-up NPN bins.
Of
course, moving air past the NPN and PNP.

Grade out enough PNP's so the  number of transistors is as needed.

Grab one NPN  from the SAME  size bin as the PNP's - big, or small, same

number of millivolts. The matching will be very good.

Got the picture?  Additionally, you can take matched pairs of NPNs  from

the same  bin  of NPN's;  and matched pairs  of PNPs  from the PNP bins.

This is how ARP and I and Bob Malter did it, 40 years ago.

NOTE:  The  resistor  marked "3%"  - was it  3.87k? -  tied to the base
of the transistor in that bloc - the PNP, I recall - was  a Fenicoloy
resistor at + 3400 ppm per  degrees C, and it was there to provide
temperature compensation, for the GAIN of that bloc.   Don't  forget
that
!

*** Best regards. /rap

> sorry to bother you with these questions... but even our 'old
> timers' don't go back far enough to answer them. You didn't
> expect (as an engineer) that you would ever be free from supporting
> your old (VERY old) designs  ;^P

*** Like  I said  - I can  even support ARP's old designs!!/ rap

> Thanks for any assistance you can offer...
>
> harrybissell at prodigy.net

&&&&&&&&&&&&&&&&&&&&&&&&
Subject:    Vbe matching

  From:
       Robert A Pease x5613 <rap at galaxy.nsc.com>

Go to:

http://www.national.com/rap/Story/transistors.html

  and also: go to:   http://www.elecdesign.com/Index.cfm?Action=Pease

and  go back to Aug. 22, 1991.  to see:

http://www.elecdesign.com/Articles/Index.cfm?ArticleID=731

 for more details  on matching transistors with  moving air.