2N2222 for analog switching?

hey guys. Basically, I am trying to come up with a way, I could do some analog "switching". Basically, I have a 0 to 5V signal. I'm trying to make up a circuit, that would take that signal, and switch a transistor to vary power. For example, if I gave the circuit a 2.5V signal, the transistor would be half open. If I gave it 5V, it would be fully on etc etc. The transistor would only have to pass a max voltage of like 4V, at no more than 1.3A.


Any ideas?

If you can add about 0.6V to the signal, you can just use an emitter follower for current gain. I wouldn't use a 2n2222, I would use something with much more current capability.

For an emmitter follower, output = input minus Vbe (diode) drop. So 3.1V in ~= 2.5V out (if you didn't know).

If you want a pure transconductance amplifier, well that's something else.

Sounds like you're looking for a VCA (voltage controlled amplifier). Just google the term for some schematics such as (**broken link removed**)

The Wikipedia entry for "Variable-gain amplifier" suggests using a VCR (voltage-controlled resistor) in the feedback loop of an op-amp amplifier. A VCR can be implemented using a JFET. Of course, the devil's in the details.

things said:

if I gave the circuit a 2.5V signal, the transistor would be half open.
If I gave it 5V, it would be fully on etc etc.

Click to expand...

Half open?
A linear voltage controlled resistor, like a FET?