High side current source with NPN transistors, possible? - Page 1
Forum section: electronics / beginners

author: Marco
author_rank: Super Contributor
timestamp: July 19, 2013, 11:56:24 pm
content: As the subject says, how do you do a high side current source with NPN transistors? I'm usually good with google, but I can't seem to enter the right query at the moment. With PNP it's trivial to build a good high side current source with transistors and zeners (or LEDs) but NPN is giving me headaches.

author: daveatol
author_rank: Regular Contributor
timestamp: July 20, 2013, 01:50:16 am
content: You can't make a current mirror if that's what you're referring to. I'm not sure what your restrictions are, so I'll just post a basic current source. The image on the left is the normal form and the one on the right should work for high-side current sourcing. R2 is probably not useful.[Original image from wikipedia.org]

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 02:13:10 am
content: Quote from: daveatol on July 20, 2013, 01:50:16 amIt doesn't work well, the problem is that the voltage across R1 is Vo-2*Vbe ... R1 essentially becomes the upper limit of the output resistance of the current source. You could make R1 large ... but the limited beta of Q1 restricts just how large you can make it for a given output current. You can make Q1 a darlington, but then it's response will be sluggish.Guess there is no real alternative to PNP, thought there might be some smart trick.

author: c4757p
author_rank: Super Contributor
timestamp: July 20, 2013, 02:14:38 am
content: What's wrong with PNP?

author: Paul Price
author_rank: Super Contributor
timestamp: July 20, 2013, 12:25:15 pm
content: Yes it is possible and easy, see attached.

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 12:30:20 pm
content: Same problem as the first circuit, the top resistor sees the output voltage and contributes to the current ... limited beta restricts how large you can make it, giving a pretty lousy dynamic resistance to the current source. At least for something like setting bias currents in a large swing amplifier.

author: Paul Price
author_rank: Super Contributor
timestamp: July 20, 2013, 01:18:40 pm
content: what??? The top resistor? It can be a relatively small <1% but necessary bias contribution to the constant current.

author: Paul Price
author_rank: Super Contributor
timestamp: July 20, 2013, 01:22:24 pm
content: Just how large a current are you trying to constant?

author: digsys
author_rank: Unknown
timestamp: July 20, 2013, 01:28:52 pm
content: To create a high power high-side NPN with ultra-low Von, you need a supply about 2V above the collector.In past cases I generated that with an isolated DC-DC converter or a capacitor voltage doubler type circuit. There are many highside charge pump ICs for doing just that as well, usually for NFETs, but they can be used on BJTs

author: Marco
author_rank: Super Contributor
timestamp: July 20, 2013, 01:40:22 pm
content: Quote from: Paul Price on July 20, 2013, 01:22:24 pmless than 100 mA, it's not really important though.It wasn't ever a necessity to do it with NPN ... they're just more easily available for any given range of parameters and I thought there might be some smart way to do it with NPN transistors which I was unaware of ... since I'm unaware of a lot of things.Two PNP's, a zener and two resistors will do just fine.

author: Paul Price
author_rank: Super Contributor
timestamp: July 20, 2013, 01:45:07 pm
content: If you use a RED LED and a high-gain medium power NPN in my circuit it would do the job well. The LED is the "zener."