PNP transistor as a high side switch (2N3906) problem

From: Paladiamors (u_at_t.com)
Date: 08/27/04 

Date: Fri, 27 Aug 2004 06:20:45 GMT

Hi there,

I have a problem using a PNP 2N3906 transistor as a high side switch. I'm
plugging a 18V line to the emitter, with a 3.3k pull up resistor from the
18V line from 2 9V batteries to the base of the transistor. The collector of
the transistor is going to a 12V power regulator (so the transistor is
acting as a high side switch). The base is eventually going to be pulled
towards ground through a 1k resistor using a switch (later it's going to be
connected to the serial port of a computer using an extra transistor to pull
the base down).

The problem I'm facing is that the base is some how being pulled down (I'm
seeing a Vbe of about -0.5V) and there is about a 9V signal on the
collector. When I short the base and the emitter together, I still see a 9V
signal on the collector. When I pull the base down for a Vbe of about 0.8V I
get the full 18 V on the collector. Has anyone experienced this problem
before? Am I using bad transistor model (I tested 2 different transistors
and got the same problem)? Is my circuit flawed?

I've modeled the circuit in circuit maker and circuit maker is telling me
that the theory is correct, that the transistor should work in this
situation. Any advice would be appreciated.

Thanks

Paladiamors.

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