Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!phri!roy
From: roy@phri.UUCP (Roy Smith)
Newsgroups: sci.electronics
Subject: Tx with no collector connection?
Message-ID: <3834@phri.UUCP>
Date: 5 Jul 89 00:40:22 GMT
Organization: Public Health Research Institute, NYC, NY
Lines: 48


	Sigh, it's amazing how much you can forget after college.  I was
browsing through one of those TAB books (The Humungus Book of Electronic
Circuits, or something like that) looking for RF mixer ideas.  I found one
that looked useful and sat down to analyize it.  First, the DC model: open
all the capacitors, short the inductors, and see what's left.  For one of
the stages, I got:


                  o +6V
                  |                          [key to ascii art: < and > are
                  >                          resistors, V are ground points,
                  < 10k       C              and the transistor is NPN]
                  >         /              
                  |     B |/
                  +-------|
                  |       |\
                  >         \ E
                  < 2.2k     |
                  >          <
                  |          > 1k
                  V          <
                             |
                             V

	There are, of course, connections to the collecter, but all of them
use a capacitor as a series element, so there is no DC collector connection
at all.  How does one go about analysing a DC circuit like this?

	After taking the thevinin equivalent of the 10k/22k voltage divider
you get 1.1V through 1.8 k.  Assuming Q is active, you use KVL to write

	1.8V = 1.8k*Ib + 0.7V + 1k*Beta*Ib

and solve for Ib.  This gives you an open-circuited current source of
(Beta+1)*Ib at the collector.  Obviously, with Ic=0, Q is cutoff, but
that's not very useful.  Am I missing something?

	If it helps, the AC model sees the modulated signal fed in via a
series capacitor at the base terminal, and the Local Oscillator fed in at
the emitter, above the 1k resistor.  The down-converted signal is picked
off the collector.  Presumably this uses product detection, but I don't see
where the product comes from either.
-- 
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
{allegra,philabs,cmcl2,rutgers,hombre}!phri!roy -or- roy@alanine.phri.nyu.edu
"The connector is the network"