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are pull ups/downs needed on bjt's?

do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?
 
E

Eeyore

Jan 1, 1970
0
do I need to put a pull down on the base of your garden variety
2n2222?

It depends what you're doing with it !

if the base is left floating, no base current = transistor
off, right?

Aside from leakage currents and storage issues (tiny for a 2N2222) yes.

Graham
 
T

Tim Wescott

Jan 1, 1970
0
do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?
No base current = transistor off, yes.

If you charge up the base, then leave it floating, the base capacitance
must discharge through the base, which takes time. It will be faster if
it discharges through the current-limiting resistor that you may find on
the output of a CMOS logic device, but it'll discharge faster yet if you
put in that resistor to ground.

If your base is sitting there at 0V with lots of capacitance to the
world (or just the wrong conductor) your transistor may turn on from
EMI. The resistor to ground will help prevent this.

If you're running the thing close to it's rated voltage, a resistor from
base to ground will help keep it from leaking excessive collector current.

There are more caveats yet, but these are the biggest ones that I can
think of.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
J

John Popelish

Jan 1, 1970
0
do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?
It depends on how completely off you need, and how quickly
it has to achieve off.

Any transistor with collector to emitter voltage has some
(possibly very small) current leaking into the base from the
collector. The open base transistor amplifies that leakage
current with its current gain. When a turn on base current
goes to zero, the charge stored in the base can take quite a
while to drain away while the transistor slowly turns off.
 
S

Stanislaw Flatto

Jan 1, 1970
0
do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?
Maybe?!
BJT's ARE controlled devices so depending on quality of _control_ you
get the responses.
They are not telepatic and don't read your mind.

HTH

Stanislaw
 
M

Mike Monett

Jan 1, 1970
0
do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?

This is a Bad Idea. It can make your circuit sensitive to stray charges and
temperature effects. There are several extremely sensitive circuits on the
web that use transistors with floating base.

Never leave inputs floating. Always use the appropriate resistance or
recommended termination method on unused inputs. CMOS is especially
sensitive due to the high input impedance.

Many op amps share internal bias supplies. If you use only one section of a
dual or quad, the other sections can upset the bias supply and make the one
being used behave strangely. To prevent this, tie the postive inputs of
unused sections to the appropriate voltage, and connect the negative inut
to the output pin.

Always check the manufacturer's recommendation on floating and unused
inputs to prevent hours wasted troubleshooting silly problems.

Regards,

Mike Monett
 
W

whit3rd

Jan 1, 1970
0
do I need to put a pull down on the base of your garden variety
2n2222? if the base is left floating, no base current = transistor
off, right?

Maybe.
The base wire is one source of base current. Three others
are:

(1) dV/dT on the collector induces current into the base through
the CB stray capacitance (Miller effect depends on this)

(2) avalanche breakdown of the collector/base junction (this is
the normal operation of an avalanche transistor)

(3) photoelectric effect can cause current in the depeletion region
of the
transistor (yes, the 2N2222A is in a metal opaque case, but cosmic
rays
and X-rays can still do it).
 
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