On 10-Jan-02 Alan B. Pearce wrote:
>>If the transistor  has a beta (Ic/Ib) of 100, and the collector current is
>>300mA, then the base current is 3mA, NOT 30mA.   I'm not sure where the
> "10% of
>>the collector current" rule came from, but I think it's wrong.
>
> You will not get a transistor to saturate if you use the published beta
> value to get the base current. Transistors in saturation always have the
> base overdriven to force the device right into saturation. In the case of
> bipolar transistors this is known as "forced beta", as the common way of
> doing it is to drive the base with a current that is many times what is
> normally expected for the same collector current in linear operation.

I guess I've been doing too much analogue & RF design work !  Indeed for
saturation the base needs to be overdriven.  In this application I would
probably go for a darlington pair, but if we are going to use two transistors I
would use a NPN with emitter to ground and PNP with it's emitter to the +ve
supply and a couple of current limit resistors in each base.  This also gets
over the problem with PIC output clamp diodes if trying to switch a supply
greater that the PICs supply voltage.

Peter.

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