Bias current sources - Page 1
Forum section: electronics / beginners

author: aheid
author_rank: Regular Contributor
timestamp: January 02, 2019, 10:13:23 pm
content: The attached schematic was pulled from an opamp datasheet. It shows a few bias current sources and sinks.If I were to implement the circuit as shown, how could I implement those current sources and sinks? Not that I am going to implement the circuit, I'm just curious how this is done in practice.I've read about theWidlar current sourceandWilson current source, would either be a viable option? If so, for a sink, would I switch the NPNs for PNPs and swap the rails or?

author: aheid
author_rank: Regular Contributor
timestamp: January 03, 2019, 01:18:33 am
content: Aaand I forgot to add the image of course

author: Wimberleytech
author_rank: Super Contributor
timestamp: January 03, 2019, 02:31:53 am
content: Quote from: aheid on January 02, 2019, 10:13:23 pmShort answer: Yes, either could be used. Which,...depends on a number of things...many outlined in the wiki article.For a current sourced from Vcc, use PNP. For current sunk to ground, use NPN.

author: aheid
author_rank: Regular Contributor
timestamp: January 06, 2019, 02:59:43 am
content: Excellent, thanks!

author: exe
author_rank: Unknown
timestamp: January 06, 2019, 11:40:18 am
content: I like jfet current sources due to their simplicity. Just a jfet and a resistor.The catch is for each jfet the resistor has to be selected individually.

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2019, 01:23:39 pm
content: It is not shown in the simplified schematic but those are current mirrors with multiple outputs and there is a separate series circuit between the top mirror and the bottom mirror which generates a temperature compensated constant current so only one current reference is required.

author: Zero999
author_rank: Super Contributor
timestamp: January 06, 2019, 07:25:49 pm
content: Quote from: David Hess on January 06, 2019, 01:23:39 pmYes, a J-FET has been used in the past but no doubt modern ICs use something a bit more sophisticated.

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2019, 11:51:30 pm
content: Quote from: Zero999 on January 06, 2019, 07:25:49 pmThe 741 just used a resistor but the 301 used a bipolar constant current generator and the 301A used a temperature compensated constant current generator with a JFET which is actually a pinch resistor. Modern parts including the 324/358 are very much like the 301A so not much has changed. Linear Technology was good about publishing full schematics for their early parts which are modern in every way so check them out for examples.If your application has fixed supply voltages, then just use a resistor.

author: aheid
author_rank: Regular Contributor
timestamp: January 07, 2019, 11:30:00 pm
content: Thanks a lot for the additional input. I'll go study some LT datasheets then!

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2019, 02:59:52 am
content: I checked a few more old schematics and every JFET in the bias control circuit is actually a three lead space saving pinch resistor. Pinch resistors have a structure sort of like a JFET which contributes to very high resistance per square. I guess they might be considered a very leaky JFET with an excessively long channel.