Hello,

First off, I love this website. I'm just learning about electronics (I'm 37) and AAC has been an info packed source for me.

I did have a question regarding wiring up an NPN transistor. Most other info sources show a base-emitter circuit and a collector-emitter circuit, but on on here it shows a base-emitter and base-collector circuit. Is there a functional difference?

On a little breadboard I have 5V pos. going to the collector and out the emitter, then to a resistor and finally an LED to ground. (collector-emitter circuit?) Then I have a 1.5v battery pos to 10K resistor to base lead, then out emitter to battery ground. (base-emitter circuit?) My circuit works as a switch. No led with 5V on collector-emitter circuit, and led on when 1.5v battery is applied to base. But isn't this configuration fundamentally different than what is shown on AAC?


Thanks for any and all help!
Chris

 

Perhaps our Ebook can give some help - http://www.allaboutcircuits.com/vol_3/chpt_4/2.html

Schematics always help insure all parties know what's going on.

 

Hi,
Thanks for the link. That diagram made sense.
I should have mentioned where I was looking before, however. It was in Vol III-Semiconductors, Solid State Device Theory, under Bipolar junction transistors. In those pictures, the wiring is different. (reverse bias base-collector circuit, forward bias base-emitter circuit.)

 

This will be my last post on this thread, promise.
On this page, http://www.allaboutcircuits.com/vol_3/chpt_2/8.html the npn transistor has a bc circuit and a be circuit. In the link that beenthere posted above, the two circuits are ce and be.

What is the reason for the different wiring setups? It seems they both work the same when a pos V is applied to the base, but the bc circuit is a little confusing to me. Thanks!

 

Do recall that a transistor is an entity. It does not act as a loosely coupled collection of internal circuits. The C-E, C-B, and E-B circuits are to illustrate internal workings.

In the case of your NPN switch, you put the emitter to ground, the load between the positive supply and the collector, and arrange a controlled current into the base, on the order of 1/10th the collector current. You do not need to do an analysis of the internal operation, just use the transistor.

 

Yes, that is usually my problem. I can follow instructions, but I tend to want to know how the instructions came to be.

 

What you are referring to could be described as common-emitter and common-base circuit configurations, where "common" refers to the transistor electrode shared by the input and output circuits. The purpose of the circuit might dictate the use of either of these connections, and in other circumstances a third (common collector) configuration might be preferred.

The common-emitter circuit is probably the most commonly used, as it normally gives the most amplification at low to moderate frequencies. The other configurations have their own advantages, particularly related to input and output impedances and frequency response.

In the case of the AAC description, the focus on the individual junctions may help the reader to think about the junction bias voltages.

 

This will be my last post on this thread, promise.

Please don't think you have to limit the number of questions in a post
As long as it's the same topic keep asking until you get it. This is the most helpful electronics forum on the web.

 

Your transistor is not a switch and is not even turned on.
You forgot that the base to emitter voltage of a transistor is about 0.7V.

 

Your transistor is not a switch and is not even turned on.

Your diagram of my circuit was just as I had it, but my LED did turn on, enough to see it in a well lit room, anyway. I do realize that I had it wired incorrectly, though. My main interest in transistors is to make logic gates and eventually an adder, and I'm seeing why loads and/or outputs come off the collector. I may start with relays instead, they seem to follow a more predictable path (at least at my current level of understanding.)

Thanks to all for your info.

 

Your diagram of my circuit was just as I had it, but my LED did turn on, enough to see it in a well lit room, anyway.

Impossible.
The base of the transistor was +1.5V and the emitter was only 0.8V.
A 2V red LED might light very dimly with 1.5V but not only 0.8V.

Maybe you have a high power radio or TV station near you that lighted the LED.