[Answered] NPN transistors: load on emitter

Hi all,

I've always understood that the load to be switched by an NPN should be between the collector and +ve power, the common emitter configuration.

Yet I also know that an NPN can be used in common collector mode, where the load is below the transistor between the emitter and ground.

The latter goes against my understanding that the load should be on the collector.

When used simply to provide a digital signal, such as one would do with the NPN output from a typical opto-isolator, the two configurations provide active low and active high logic so the user may prefer a pulldown on the emitter to get active high.

So question: if I'm right that the load on an NPN ought to be on the collector, how does that square with the common collector, load on emitter config? Is it ok to have the load on the emitter for signalling purposes but not for "real" loads like motors and so on, mayb

Do some reading here
[url=http://en.wikipedia.org/wiki/Common_collector[http://en.wikipedia.org/wiki/Common_collector

and here
http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic
especially the part about
“TTL with a “totem-pole” output stage”]

and here
](http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic]http://en.wikipedia.org/wiki/Common_collector<a%20target="_blank"%20href="%3Cbr%3E%3Cbr%3Eand%20here%3Cbr%3E%3Ca%20target=%22_blank%22%20href=%22http://en.wikipedia.org/wiki/Transistor%2525E2%252580%252593transistor_logic%22%3Ehttp://en.wikipedia.org/wiki/Transistor%25E2%2580%2593transistor_logic%3C/a%3E%3Cbr%3Eespecially%20the%20part%20about%3Cbr%3E%22TTL%20with%20a%20%22totem-pole%22%20output%20stage%22"><br><br>and%20here<br><a%20target="_blank"%20href=“http://en.wikipedia.org/wiki/Transistor%25E2%2580%2593transistor_logic”>http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic</a><br>especially%20the%20part%20about<br>“TTL%20with%20a%20"totem-pole”%20output%20stage"</a>)[http://en.wikipedia.org/wiki/Common_collector

and here
http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic
especially the part about
“TTL with a “totem-pole” output stage”]http://en.wikipedia.org/wiki/Transistor–transistor_logic]http://en.wikipedia.org/wiki/Common_collector](http://en.wikipedia.org/wiki/Transistor–transistor_logic)[

and here

especially the part about
“TTL with a “totem-pole” output stage”](http://en.wikipedia.org/wiki/Transistor%E2%80%93transistor_logic)

Wow, Modify sure made a mess of that!

I had read the Wiki on the common collector config: that's how I got to know about that in the first place.

But I'll read up on the totem pole thing....

The common collector mode is also known as an emitter follower. It is used as an impedance buffer, that is where you have a high impedance input and want a low impedance output. It is more often used in analogue work because the voltage on the emitter is exactly the voltage on the base minus the 0.7V Vbe drop.

Hence it is not sutiable for switching loads because the emitter never reaches the base voltage.

Now go and look up the common base configuration of a transistor.

Grumpy_Mike:
Now go and look up the common base configuration of a transistor.

Heh, heh ...


An Emitter Follower has a gain slightly less than unity, at least in terms of voltage. As such, it has the useful characteristic that the output is a quite faithful (very low distortion in general - it effectively has built-in negative feedback) reproduction of the input.

It provides current gain or - given that there is no voltage gain, "impedance gain" which is to say that the impedance at the output is reduced in proportion to the beta of the transistor.

As long as you can tolerate the loss of the emitter-base voltage (or worse, a gate-source voltage) it is quite useful for driving heavy loads such as LEDs or relays needing substantially more current than the 20mA pin rating of an Arduino.

It is particularly useful for buffered Charlieplexing.

Have a read of this:-

JimboZA:
Hi all,
the load to be switched by an NPN should be between the collector and +ve power, the common emitter configuration.

There should not be always in "should be between".
There are 3 ways to connect a transistor: common emitter, common collector and common base. They are all correct and they all can be used.

Here, Wikipedia is not at it's best. So I'll write here things which have helped me.

NPN transistors.

Common collector
Emitter is grounded
Base is at 0.7V or 0V
Collector can be anything. It is connected to the load, and the load is connected to supply voltage.
With CPUs, collector is usually either 0V or supply voltage.

Common emitter
Collector is at some posive voltage, like supply.
Base. With 5V CPUs, it is at either 5V or 0V
Emitter is connected to the load. It is around 4.3V(5V-0.7V) or 0V.
other end of the load is grounded.
If collector is at 5V and base too, emitter voltage may be a little less than 4.3V, because there is not enough base current. Use higher supply voltage at collector.

Common base is not so useful with single supply or in DC amplifiers. I'll leave it out here.

Hope this helps

Common base is not so useful with single supply or in DC amplifiers.

Wrong on both counts.
I have only ever used it on a single supply, and I have used in amplifiers that are of DC.

It is used a lot in high frequency amplifiers, however the main use in digital electronics is in logic level conversion ( voltage shift ) from 3V3 to 5V without any signal inversion.

An NPN transistor is just a component. It's doesn't respond to philosophical debate If it's properties make it suitable for a circuit just use it and enjoy.

LMI:
Hope this helps

Well.... apart from the fact that you have the descriptions of common collector / common emitter under the wrong headings?

So I have. Common collector and common emitter names are swapped in my text

LMI:
So I have. Common collector and common emitter names are swapped in my text

Better go edit it then.