Electronics > Beginners

Bias current sources

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David Hess:
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.

Zero999:

--- Quote from: David Hess on January 06, 2019, 01:23:39 pm ---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.

--- End quote ---
Yes, a J-FET has been used in the past but no doubt modern ICs use something a bit more sophisticated.

David Hess:

--- Quote from: Zero999 on January 06, 2019, 07:25:49 pm ---
--- Quote from: David Hess on January 06, 2019, 01:23:39 pm ---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.
--- End quote ---

Yes, a J-FET has been used in the past but no doubt modern ICs use something a bit more sophisticated.
--- End quote ---

The 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.

aheid:
Thanks a lot for the additional input. I'll go study some LT datasheets then! :)

David Hess:
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.

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