transistor switch

[Fred Bloggs]

John Larkin wrote:

On Tue, 25 Apr 2006 12:10:26 GMT, Fred Bloggs <nospam@nospam.com
wrote:


To improve my design skills I like to look at the schematics of
commercial stuff. In those designs I often see transistor switches
connected as below.

. VCC
. +
. |
. .-.
. | | RL
. | |
. '-'
. R1 |
. ___ |/
. Vi--|___|---o----|
. | |
. .-. |
. | |R2 |
. | | |
. '-' |
. | |
. '------o
. |
. ===
. GND
. (created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


I usually only use R1, why would I need R2?

I calculate my R1 this way: R1 = (Vi-Vbe)/(0.1*iLoad)
How would R2 affect this?

R2 parallels the base, so it has Vbe across it and Vbe/R2 current
through it. Therefore R1 should conduct Iload/10 + Vbe/R2 making
R1=(Vin-Vbe)/(0.1*Iload+Vbe/R2). There are various reasons for including
R2 such as speed-up, linearization, input voltage attenuation, and
shunting reverse CB current leakage. Most of the time it is included to
shunt the CB leakage current around the BE junction.


Is that habitual holdover from germanium days? Transistors don't
really leak any more, and R1 would slurp that up anyhow. Ironically,
big power mosfets can have serious d-s leakage nowadays.

John

So can silicon power running hot.