# A unconventional use for a NPN BJT

Discussion in 'Electronic Basics' started by Gene, Sep 2, 2004.

1. ### GeneGuest

It is known that with a high value resistor, a capacitor and NPN transistor
is possible to build a relaxation oscillator. The trick is to "turn upside
down" the transistor, that is, the emitter is connected to the positive
terminal of the capacitor (charged by the resistor) while the collector is
connected to ground and the base is left floating. Above 12-15V the
transistor displays a region of negative resistance which is the source of
the oscillation.
What is the explanation of this behavior from solid state physics viewpoint?
Any ideas?
Gene

2. ### Bob MastaGuest

Boy, this takes me back a few decades! I recall making
these relaxation oscillators in a lab class in the late 1960s!
I think this is an avalanche effect, where the conduction electrons
knock loose others that increase the conduction further, so the
resistance goes negative. A lower voltages there isn't enough
energy to knock others loose, so resistance is positive.
At higher voltages the current is limited by bulk resistance
or something so the resistance is again positive.

Best regards,

Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com

3. ### John S. DysonGuest

One comment, there is also a mode (that worked with some 2n2369s) that
uses the transistor in 'forward' mode (where the collector is positive
and emitter grounded.) The circuit is like this:

V+
|
|
RC
| ---------
| |
Collector |
---Base |
Emitter Capacitor
| |
GND GND

Note that the base is 'open'. The transistor is partially damaged
when operated in this mode. The V+ would be fairly high (like 40volts).

John

4. ### Active8Guest

You're probably right. I remember learning to check trans with a VOM
and was told that some - like horizontal outs - would zener from
C-E, so I had to make sure I knew where the base lead was.

5. ### John LarkinGuest

The Zetex avalanche transistors are cool in this mode. 300-volt,
25-amp pulses, reliably, from a SOT-23 transistor.

John