BJT differential amplifer

You're describing the input stage to an opamp. Search around until you find a data sheet for something simple like a 741 or an LM353 and look at the input stage.

If you want to do it in real life you need matched transistors, though.

You have severely under specified the problem. What range of differential voltage must the circuit respond to? What output current per volt of input do you require? What voltage range must the current source comply with? What load resistance can the differential amplifier apply to the input voltage? What frequency range is involved? Etc.

You are not ready to even start thinking about circuit designs till you have the problem nailed down.

"derek" ...

Study "Howland current source", i.e. in the AoE.

Regards, Arie de Muynck

"derek" ha scritto nel messaggio news: snipped-for-privacy@c13g2000cwb.googlegroups.com...

you need a differntial Gm cell.... it's possible to create a programmable Gm cell where Iout= Gm_code*Vin_diff it's very important f you want to realize it using discrete component to use a feedback to cancel the mismatch of the compnent...reed Pelgrom's paper on this topic anyway you should give some spec on your circuit...

You're referring to this paper? M. Pelgrom, A. Duinmajer and A. Welbers, "Matching properties of MOS transistors", IEEE J. Solid-State Circuits, vol. 24., No. 5,pp. 1433-1439, 1989.

Bipolar transistors are more easily matched than MOFETs, and therefore arguably are better suited for this task. An introductory discussion of transconductance (voltage-to-current) circuits should include the famous old RCA CA3080 chip, and mention of its relations, the CA3094, LM13600 and LM13700. There are modern matched-transistor arrays suited for the basis of transconductance circuits, such as Intersil's hfa3134 Gilbert cell, their hfa3102, and matched pairs, hfa3134 and hfa3135.

If the requirements aren't too stringent, the dual 3904's are dirt cheap in high volume today (under 5¢ per pair. They don't claim to be matched, but I've used them and had good luck. I think dual 2n2222's are available too.