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Original question:

JFE2140: JFE2140

JFE2140: JFE2140 differential amp gain computation

J Scott Merritt
Prodigy 180 points

Part Number: JFE2140

Tool/software:

Referring to Figure 9-10, and using the JFET Common Source voltage gain equation (gm * Rd) / (1 + gm Rs), I compute a gain of 30 (= ~30db) where as Figure 9-11 show a gain of around 13 db.  What am I missing ?  Thanks !

  • 0
    TI__Genius 15445 points

    Hello,

    What are you using for the "Rd" value?  The gain isn't set by the actual resistors "Rd1" - it is the parallel combination of Rd1,2 and RI1,2.  So the actual Rd value for the calculation should give about 857 Ohms.

    When I use Rd = 857, gm = 0.01, and Rs = 50, I get a gain of 5.7, which is around ~15 dB.  I would attribute the 2 dB difference to the value of gm.

    Let me know if you have any questions on this.

    Regards,
    Mike

  • 0 in reply to Michael Hartshorne
    Prodigy 180 points

    Mike - many thanks for the response, very helpful and much appreciated !

    I was using the wrong Rd, and also the wrong gm, all good now.  Thanks !