# Transistor Question

Discussion in 'Electrical Engineering' started by Peter, Apr 30, 2005.

1. ### PeterGuest

If you have say 15volts going through a 1k resistor into the base of a
transistor, the emitter has an emitter resistor as well... say 500ohms and
the collector is tied directly to Vcc of 25 volts.

How do you figure out the max current and voltages through the collector,
emitter and base???

The collector can draw as much as it wants because it's tied to Vcc, you
don't know the voltage at the base because of the Re. Normally you'd have
the emitter tied to ground, so your base is approx. 0.7v and you figure out
your base current that way, then multiply it by beta, etc.... But the
emitter resistor is confusing me.

Does anyone know???

2. ### DwayneGuest

Yes I do.
I'm going to assume that the base-emmitter drop is 0.7 Volts (can be changed
at anytime).
Vb=Ve+0.7
ib=(15-Vb)/1k
ie=(Ve-0)/0.5k
ie=(Beta+1)*ib
therefore
(Ve)/0.5k=(Beta+1)*((15-Vb)/1k)
(Vb-0.7)/0.5k=(Beta)*((15-Vb)/1k)
Where
Vb = base voltage
Ve = emitter voltage
ib = base current
ie = emitter current
Beta = internal gain of Xsistor

And the rest is trivial.
np

Dwayne

3. ### DwayneGuest

This line should be
(Vb-0.7)/0.5k=(Beta+1)*((15-Vb)/1k)

4. ### Dave DGuest

Not with an emitter resistor it can't. In a standard common emitter
configuration, if the circuit in question has an emitter resistor then the
collector current is limited by that. Strictly speaking, this isn't an
electrical engineering question, it's electronics. You might find a lot of
interesting and relevent electronics discussion in one of the electronics
newsgroups.

Dave

5. ### PeterGuest

Sorry Dave. I tend to ask circuit questions in the engineering group and
repair related to regular electronics newsgroups. Not that I am trying to
offend anyone at all.

Thanks for the help everyone!

6. ### J. B. WoodGuest

Hello, and "strictly" speaking it is an EE question. Unless you think
that the practice of EE is restricted to power generation and
distribution. Most universities in the US don't think so. "Electronics"
or "electronics engineering" is but a subset of EE practice. My .02
worth. Sincerely,

John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

7. ### Dave DGuest

I was merely trying to direct Peter to an electronic-specific newsgroup
where he could find advice from people specifically trained in electronics,
I didn't set out to offend anyone.

Dave

8. ### Dave DGuest

Peter, I'm sorry if I gave you the wrong impression! It simply occurred to
me that an electronics group might be more suited to your question. There's
nothing wrong with posting such questions here, and I certainly wasn't
having a go at you, just offering friendly advice

Dave