2N2222 for analog switching?

[things]

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?

 

[Speakerguy]

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.

 

[pc88]

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.

 

[Willbe]

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.

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