constant current source

[steelerfan11]

I am working on designing a constant current source. I was told to use a transconductance amplifier and a pot to achieve this. I have never used or learned about a transconductance amplifier so I'm not exactly sure how they work. I am wondering if it will work and how it works. I have read that they work like a transistor. Is that true? Any help would be greatly appreciated, thanks.

 

[RadioRon]

A forum like this is not the best way to learn how something like this works. I suggest that you use a search engine to find technical articles about the transconductance amplifier. Or you might go to a library and read a book about basic electronic circuits to learn more about such amplifiers. I recommend the "book" approach.

 

[HarveyH42]

What is a transconductance amplifier? Is it a LM317 regulator type circuit? Pretty simple, works great for LEDs...

 

[RadioRon]

A transconductance amplifier is one which provides transconductance. In other words, its gain is defined as amps/volts. The unit of transconductance is the Siemen, which is amps/volt. The output current is proportional to the input voltage of such an amplifier.

 

[pc88]

Transconductance is also just 1/resistance, and another unit that's used is the mho ("ohm" spelled backwards). 1 mho is 1/(1 ohm).

Here's a good intro to OTA's (operational transconductance amplifiers):

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[audioguru]

Deja-Vu.
I read the same articles by Ray Marston about the very old CA3080 (remember when RCA made ICs?) in Popular Electronics magazine in the '70s.

 

[Bob Scott]

The below link is the site of Apex Microtech. They are a very professional outfit that makes very high power op-amps.

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Click on App note #13: Voltage to current conversion. In that app note pay attention to the circuit called a "Howland Current Pump".

Note: If you build the grounded-load circuit in Figure 7, be aware that the circuit is very sensitive to resistor tolerance. I suggest 0.1% or better metal film for all Ri and Rf. This does not concern the very low resistance of Rs. Rs is (1ohm or so). Any resistance tolerance error of both Ri's and Rf's should be small when compared to the value of Rs.

As a matter of fact, all of Apex's app notes are interesting and well worth reading for any engineer or technologist.

Bob