# Homework Help: Circuit symbol?

1. Dec 29, 2013

### 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?

2. Dec 29, 2013

### KiNGGeexD

3. Dec 29, 2013

### Staff: Mentor

4. Dec 29, 2013

### KiNGGeexD

So what is gm then?

5. Dec 29, 2013

### haruspex

The page you posted tells you that it is defined as ic/vbe, 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)

6. Dec 30, 2013

### Staff: Mentor

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".