You can use a bipolar "=FCpside down"with the collector & emitter reversed.
This results in a transistor with low beta - but I'm not sure if it is
less than 1.

I have used this arrangement in the past to reduce the sensitivity of
a "SCR" type circuit made up of an NPN and a PNP transistor. Otherwise
the leakage current through the input device can be enough to trigger
the output device at high temperatures.

Makes people scratch their heads when they check the circuit however.

RP

On 09/01/06, Russell McMahon <apptech@paradise.net.nz> wrote:
> Thought experiment for those with time on their hands :-)
>
> Bipolar transistors are current amplifiers, so it almost always makes
> sense to operate them with base current less than collector current -
> usually much less . Some high voltage applications have forced Betas
> not much more than 1 as the switched power (HV x Ic) is >> the
> switching power Ib x Vbe.But, just sometimes, in low voltage
> applications it can be useful to have base current > collector
> current.
>
> I have just designed a circuit with a small signal PNP bipolar
> transistor with a forced Beta of about 0.5.
>
> Suggest why?
>
> Supply voltage is about 12 Volt. Transistor current is around 0.5 mA
> (so base current is about 1 mA). (Final collector and base currents
> may be reduced but this works OK for now).
>
> The purpose of this 'challenge' is to
>
> - Point out that some strange configurations may sometimes be useful.
>
> - Point out one such.
>
> - See if anyone else comes up with anything interesting.
>
> I'll be away for a few days so may not comment further till early next
> week.
>
>
>        Russell McMahon
>
>
> --
> http://www.piclist.com PIC/SX FAQ & list archive
> View/change your membership options at
> http://mailman.mit.edu/mailman/listinfo/piclist
>

-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist