Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!shadooby!yale!cmcl2!phri!news
From: roy@alanine.phri.nyu.edu (Roy Smith)
Newsgroups: sci.electronics
Subject: Need help solving circuit
Message-ID: <1990Jan4.191641.2967@phri.nyu.edu>
Date: 4 Jan 90 19:16:41 GMT
Sender: news@phri.nyu.edu (News System)
Organization: Public Health Research Institute, New York City
Lines: 63


	I'm stumped solving the following DC circuit to find the bias points
of all the transistors.  It's the final audio amp for one channel of a
Pioneer SX-770 reciever, after a few unimportant simplifications.  It's
apparantly a 2-stage push-pull, but I can only guess if it's running class A,
AB, or B.  The diode on the left marked 1.3V is a temperature compensator.
The ECG catalog lists a 1.3V forward drop for the replacement part; I think
it's probably safe to model it as a constant voltage source.  The 0.2V source
at the lower right is the thevenin equivalent of an earlier stage, looking
through the DC-coupled feedback connection.  The forward signal connection
from that stage is AC coupled, so doesn't show here.  If it matters, the 110k
and 25 ohm resistors marked with *s are variables; I've arbitrarily set them
to their midpoints.  They are actually 82-152k and 0-50 ohm, presumably used
to balance the gains of the two channels at the factory.

	I don't know how to attack this.  I could do a straight forward mesh
analysis, but I'd end up writing KVL around 12 loops and I don't have any
plans on solving 12x12 matricies manually, at least not in this life.  Are
there some tricks I can use to break this into several simplier circuits?  My
first thought was to slice it into 3 vertical sections and assume that the
base drive for section N+1 is negligable when solving section N, but I can't
get the voltage at point V1 without knowing the collector currents for the
later sections.  In practice, I can cheat because the schematic lists the
nominal V1 as 20.2V, but if I didn't know that, how would I solve for it,
other than building the circuit and measuring it with a voltmeter?

        +48
         |
         +-----------------+-------------+
         |                 |             |
        4.3k               |             |
         |               |/  NPN         |
         +---------------| B=100         |
         |               |\              |
    +   ---                v             |
   1.3V \_/                |           |/  NPN
    -    |                 +-----------| B=60
         |                 |           |\
         |                 |             v
         |                220            |
         |                 |            0.5
         |                 |             |
  V1 O---+-----+-----------+-------------+------------+
         |     |           |             |            |
         |     |           22            |            |
       *110k  *25          |             |            |
         |     |           v             |            |
         |     |         |/  PNP         |            |
         |     +---------| B=100         |           27k
         |   |/  NPN     |\              |            |
         +---|  B=100      |           |/  NPN        |
         |   |\            +-----------| B=60         |
         |     v           |           |\            --- + 0.2V
         |     |          220            v            -  -
        6.8k  150          |             |            |
         |     |           |            0.5           |
         |     |           |             |            |
         V     V           V             V            V
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,philabs,cmcl2,rutgers,hombre}!phri!roy
"My karma ran over my dogma"