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Opamp class A bias using FET - how to calculate resistor value?

Posted by Saurav (A) on May 3, 2002 at 11:08:54

Hi,

I'm sure this is a very basic question. I've been looking up threads on biasing opamps into class A, especially in Thorsten's El Cheapo phono stage. I found a schematic that shows me how to do this - is this correct? The emitter connects to the opamp output, the base connects to V-, and the collector connects to V- through a resistor. I might have the emitter/collector wrong, because (a) I'm really rusty with all this, and (b) the datasheet doesn't show any arrows on the FET.

Anyway, on this diagram, V- is -18V, the opamp is an LT1115, the FET is a 2N4304, and the resistor says "~ 250 ohms, select for 2mA".

Questions:

* Is that the way to hook this up?
* How does one calculate the required resistor value? 18V / 2mA = 9K, which isn't anywhere close to 250, so the calculation obviously isn't that straightforward.

If it makes any difference, I'm using an OPA2134, and the supply voltage is 12V. I found a post in the archives where someone used a 2N5485 for this, so I'll probably do the same thing.

Thanks in advance for all help.

Saurav

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