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?- posted
17 years ago
are pull ups/downs needed on bjt's?
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- Vote on answer
- posted
17 years ago
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
- Vote on answer
- posted
17 years ago
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.
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- posted
17 years ago
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
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- posted
17 years ago
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
- Vote on answer
- posted
17 years ago
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).