Understanding Circuits: Investigating an Unlabeled Circuit

[KiNGGeexD]

I have a circuit and in the circuit there is two over lapping circles and I'm not sure what it is because there is no description or anything. I think it is a current or something but next to it is the symbol gm *Vbe and I know Vbe is the voltage between the base and emitter but what is gm?

 

[KiNGGeexD]

 

[berkeman]

It's one way to draw a current source:

http://commons.wikimedia.org/wiki/File:International_symbols_of_current_source.svg

In the case of your circuit, it is a voltage-controlled current source (VCCS)

 

[KiNGGeexD]

So what is gm then?

 

[haruspex]

So what is gm then?

The page you posted tells you that it is defined as i_c/v_be, that it is the same as 1/resistance, that it is described as a conductivity, and that its units can be variously termed Siemens, Ohm^-1, or Mho (Ohm backwards)

 

[NascentOxygen]

So what is gm then?

It should be written $g_m$and is usually pronounced "transconductance". Its origin is the older term "mutual conductance", the 'g' coming from the fact that it has units of amperes per volt which, as haruspex elaborated, corresponds to units of conductance (which in physics we represent by the letter "G").

You can say that $g_m$is the gain of this voltage-controlled current source. Personally, I prefer to write its units as amps/volt, finding that more descriptive of its function than plain "Siemens".

see http://transconductance.askdefine.com/

 

[rude man]

Symbols for voltage and current sources, controlled or not, seem to change year by year and/or are not universal. Be aware of this as you grow older ...