# Transistor Question

• posted

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???

• posted

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

• posted

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

• posted

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

• posted

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!

"Dave D" wrote in news:4274bb06\$0\$287\$ snipped-for-privacy@news-text.dial.pipex.com:

• posted

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: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337
• posted

Hello, all, and I should have added in my previous post that one can certainly be an electronics hobbyist or electronics technician (with appropriate training) without a university-conferred EE degree (BS, MS. PhD). Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337
• posted

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

• posted

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

• posted

I went over the message last night and it was full of information. My problem is I'm terrible at looking at this and creating 2 loop equations with 2 unknowns.

I knew all the formulas, but it was unclear to me as to how to come up with the numbers. Even when I first looked at the two equations nothing came to me. then I sat down and said ohhh 2 equations and 2 unknowns.

Even though I came up with Vb=14.72 and Ve=14.07, Multisim for some reason was using around 0.8something volts from B to E. It's fine, you can't get exact numbers and I'm sure if I played around with the parameters I can get these numbers. It may be due to the higher Ie creating a higher voltage drop.

Thanks again!

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