constant current source - Electronic Circuits Projects Diagrams Free

steelerfan11 · 4th July 2007, 06:27 PM

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 · 4th July 2007, 06:33 PM

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 · 4th July 2007, 08:22 PM

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

RadioRon · 4th July 2007, 08:28 PM

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 · 4th July 2007, 08:46 PM

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):

http://www.nutsvolts.com/~downloads/...%20Op-Amps.pdf

http://nutsvolts.com/~downloads/May%...20Part%202.pdf

audioguru · 4th July 2007, 09:42 PM

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 · 4th July 2007, 10:45 PM

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

http://eportal.apexmicrotech.com/mai.../app_notes.asp

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

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4th July 2007, 06:27 PM	(permalink)
steelerfan11	constant current source 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.

4th July 2007, 06:33 PM	(permalink)
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. __________________ RadioRon

4th July 2007, 08:22 PM	(permalink)
HarveyH42	What is a transconductance amplifier? Is it a LM317 regulator type circuit? Pretty simple, works great for LEDs...

4th July 2007, 08:28 PM	(permalink)
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. __________________ RadioRon

4th July 2007, 08:46 PM	(permalink)
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): http://www.nutsvolts.com/~downloads/...%20Op-Amps.pdf http://nutsvolts.com/~downloads/May%...20Part%202.pdf

4th July 2007, 09:42 PM	(permalink)
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. __________________ Uncle $crooge

4th July 2007, 10:45 PM	(permalink)
Bob Scott	The below link is the site of Apex Microtech. They are a very professional outfit that makes very high power op-amps. http://eportal.apexmicrotech.com/mai.../app_notes.asp 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 Last edited by Bob Scott; 4th July 2007 at 10:54 PM.