Need help analyzing a circuit

From: James Howe (concentricnews_at_wingspread.imap-mail.com)
Date: 12/09/04 

Date: 08 Dec 2004 20:07:31 EST

I'm new to electronics so I'm not great at circuit analysis. I've been
reading some books and I understand some simple circuits, but the circuit
below has me somewhat confused. This circuit is used to convert an input
voltage into a constant current. The capacitor in the circuit is actually
hooked into a timer to periodically discharge it, but I'm not interested
in that part of the circuit at this time.

Basically what I'm trying to figure out is voltage and current at parts of
the circuit. I've tried my best to produce an ASCII version of the
circuit, please excuse my crude attempt. The circuit basically consists
of a handful of resistors, two 2N4126 PNP transistors, a .01uf capacitor
and an LM324 op amp. The positive voltage is a control voltage and can
range from approximately 0v to 5v. The negative voltage is fixed at -12v.

In my reading, I've seen descriptions on how to compute voltage and
current through NPN as well as PNP transitors. However, all the examples
have a positive (or negative for PNP) voltage going into the collector and
the emitter connected to ground. This circuit is different because the
collector is connected to negative voltage through a capacitor and the
emitter is connected to a positive voltage. I'm not sure the best way to
attack this circuit for the purpose of figuring out voltage and current
values. Any help or tips you can provide would be greatly appreciated.

Here is the schematic:

           56k 1k
+4v *--/\/\/\--+--/\/\/\----------------------+
                 | |
                 | / e
                 | /
                 | \ ------| Q1 (PNP)
                 | -|\ /e \
                 +------| \ / \ c
                        | \--/\/\/\-|Q2 (PNP) |
                 +------| / 47 \ |
                 | +|/ \c |
                 | / LM324 | |
0v *----------+ +--------+
                                                 |
                                                 |
                                                 |
                                                 |
              .01 uf 100 |
-12v *--------]|-------------------/\/\/\-------+

And if the picture is munged, here is a description:

A +4v supply is connected to a 56k and 1k resistor in series. Between the
56k and 1k resistor a connection is made to the LM324 OPAMP on the
inverting input. The non-inverting input is connected to ground. The
output of the OPAMP connects to a 47 ohm resistor and into the base of a
2N4196 PNP Transistor. The emitter is connected to another 2N4146 and the
emitter of the second transistor connects to the other end of the 1k
resistor. The collectors are tied together and are connected to a 100 ohm
resistor which is connected to a .01 uf capacitor and finally to a -12v
power supply. As I said earlier, in the full circuit, the capacitor is
periodically discharged.

Thanks. 

-- 
James Howe
Contact: http://public.xdi.org/=James.Howe

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