Linear operation of NPN transistor as a current amplifier trouble - Page 1
Forum section: electronics / projects

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 01:33:56 am
content: I'm trying to power a LED and vary the intensity of the led from a DC voltage source that varies from 0 to 5 volts from a 5 volt supply. Normally, this wouldn't be a problem but I also need it to operate below the Vbe biasing point to fully reflect the value of the DC voltage source in the LED intensity. Dual channel rail to rail Op-Amps are too expensive or maybe not? I was thinking about using a p-FET but I don't have any in stock right now and I have a ton of NPN transistors.I can't remember if there is a way to do this with a single supply and low component count. Can anybody think of anything or do I need to get some p-FET'S?

author: c4757p
author_rank: Super Contributor
timestamp: February 16, 2013, 01:38:58 am
content: I would use an op amp made into a voltage-controlled current source for that. No need for a good op amp at all - LM358 should do just fine.Like this:

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 01:52:13 am
content: Gosh, that's one hell of a fancy reply! I think that I'm going to go with a three transistor method.. One to turn it on and off, one to limit the led current, and one to pull up the base. I know it's not totally linear, but whatever. Thanks for the reply!

author: c4757p
author_rank: Super Contributor
timestamp: February 16, 2013, 01:53:54 am
content: I made one with just a single PN2222 using the same idea (voltage-controlled current source) and I challenge you to figure it out on your own. I'll give you a hint for dealing with VBE: voltage dividers don't have to go to ground. Remember that two 1k resistors, one to V+ and one to ground, is like a 500 ohm resistor to a point halfway between.Here is the graph from that one: slightly less linear, but very good for a one-transistor solution.

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 02:00:46 am
content: *I meant three resistor and one transistor!That graph looks about right. Thanks again.

author: c4757p
author_rank: Super Contributor
timestamp: February 16, 2013, 02:05:07 am
content: Ah, OK.Three transistors would be a bit too much.FWIW, here's what I had:

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 02:10:01 am
content: That's great! I eliminated R4, Replaced R3 with a Diode ( Solved the linearity Prob) and put R 2 in series with VCC. Voila!

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 02:13:36 am
content: Sorry, I didn't put r2 in series, I put another resistor in series... I can't see your picture when I'm posting.

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 08:02:26 pm
content: Aagh.. The polarity is reversed and I can't reverse it elsewhere in the circuit. Is there a dual channel version of the LM386 or something cheap that is equivalent?

author: DaveHardy
author_rank: Regular Contributor
timestamp: February 16, 2013, 08:06:26 pm
content: Never mind, I found it... LM358

author: mrflibble
author_rank: Super Contributor
timestamp: February 16, 2013, 08:35:42 pm
content: Not sure if it is "equivalent enough" for your purposes, but the LM324 is a nice cheap quad.