Discrete bandgap reference? - Page 1
Forum section: electronics / projects

author: baoshi
author_rank: Regular Contributor
timestamp: May 27, 2014, 01:24:30 am
content: Hi,In a design I need a very crude reference voltage, about 300-500ppm is pretty acceptable.Since I have one opamp left unused and BC857BS dual-pnp transistor somewhere else in the BOM, I'm thinking of making a discrete bandgap reference out of it. The design is below.Can someone tell me if it makes any sense?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2014, 04:31:18 am
content: Seems about right. But what about thermal gradients?300-500ppm out of what? That's considerably precise if you mean first-time accuracy and drift. That's at least as good as a TL431. If you mean just drift, under unspecified ambient conditions (hopefully minimal convection and gradients), maybe it'll be good enough?Tim

author: Vgkid
author_rank: Super Contributor
timestamp: May 27, 2014, 04:53:29 am
content: ^^^ Or you could glue them together.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 27, 2014, 06:55:59 am
content: Even the best discrete pair (including two dies in a single package!) is little better than the transistors alone; RthJA for each die might be 100 C/W, and >70 C/W between them.Whereas proper monolithic might be... Idunno, single digit C/W between -- but more importantly, they can be fabricated in a crossed interdigitated pattern that nulls the effect of external gradients to at least the first order.Unless specifically stated, and supported with necessary data, do not assume any "dual transistor" product is monolithic -- those things have almost entirely disappeared!But yeah, if you insist on doing it discrete... do everything you can to match them. A big glob of "hot snot" to "thermally lag" them might come in handy.Tim

author: baoshi
author_rank: Regular Contributor
timestamp: May 27, 2014, 08:18:10 am
content: The BC857BS I'm using is dual pnp transistors in the same package.http://www.nxp.com/products/bipolar_transistors/general_purpose_bipolar_transistors/general_purpose_transistors/double_transistors/double_pnp/BC857BS.htmlI assume they are very well thermally coupled.I'm looking at 5% drifting within temperature span of 0-70C, and initial accuracy within 2%. I guess that is not too much for the crude reference.

author: Andreas
author_rank: Super Contributor
timestamp: May 27, 2014, 07:34:09 pm
content: Hello,sorry but are you shure that this is a "bandgap" circuit.I have never seen a bandgap with 2 diode connected transistors.The Widlar circuit has 2 npns where one emitter is connected by a resistor to ground.http://www.ami.ac.uk/courses/ami4409_amsicd/u03/images/amsicd_vrd_img05.gifThe cirquit which is used together with OP-Amps I have built some times ago:It worked ok for my purposes. Although I had 2 discrete transistors.Over a 10 to 60 deg C temperature range the 3.3V changed by about 20mV.By the way: If you have a capacitive load you will need some phase compensation.With best regardsAndreas

author: dannyf
author_rank: Super Contributor
timestamp: May 27, 2014, 10:17:52 pm
content: QuoteIt will work. The principle is the same.

author: tszaboo
author_rank: Super Contributor
timestamp: May 28, 2014, 07:57:49 am
content: I dont think it makes sense. A TL431 is cheaper than the LMV321, and better than your circuit.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 28, 2014, 09:33:45 am
content: Hi,To the original poster.Don't forget to consider the Input Offset voltage of the op-amp and the temperature coefficient of Vos.You will get Vos x (1+R2/R3) added to the bandgap voltage.With the LMV321 the maximum offset is 7mVso you can get +/- 77mV variation in output voltage from the Vos.The LMV321 Vos has an average tempco of 5uV / CThis gets multiplied by the gainto 55uV per degree C.55E-6 /1.2 = 45ppm / degree CYou also need to look at the temperature coefficient of the resistors.A voltage reference IC is beginning to look pretty goodRegards,Jay_Diddy_B

author: baoshi
author_rank: Regular Contributor
timestamp: May 28, 2014, 09:38:01 am
content: Hi J_D_B, very thoughtful indeed. Thanks for remindingBaoshiSent from my iPhone using Tapatalk