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Common Drain Amplifier

Discussion in 'General Electronics Discussion' started by oreee, Oct 1, 2014.

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  1. oreee

    oreee

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    Sep 22, 2014
    Untitled.png

    Hi,
    The above image is of two common drain amplifiers. From what I know, the PMOS common drain amplifier shifts the input voltage up by ~Vth(threshold voltage) and the NMOS common drain amplifier shifts the input voltage down by ~Vth. Since the PMOS common drain is not subjected to bulk effect it has a gain close to 1 (~0.99, ~1.01). The NMOS common drain on the other hand will have bulk effect causing its gain to drop lower than 1 (~0.9 maybe).

    Q1: Is there a way to increase the gain of the NMOS common drain amplifier efficiently to ~1?
    Q2: I'm not 100% sure how to vary the threshold voltage, could changing the biasing of the transistor vary the threshold voltage?

    Thanks.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    A1: I don't know of any. This is a property of the circuit as Vout follows Vin closely once the transistor is on (Vgs > Vth).
    A2: Vth is a property of the transistor. You cannot change it by biasing the transistor in-circuit. You need to modifiy the parameters of the production process to change Vth. (Vth is also temperature dependent, but that's not how you'd control it in a reliable way).


    Addendum: I don't see how a PMOS CD amplifier could have a voltage gain >1. A gain >1 implies that the source voltage increases more than the gate voltage, thereby reducing Vgs and consequently reducing Vout.
     
  3. oreee

    oreee

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    Sep 22, 2014
    Yeah I'm having a hard time understanding this, its what i gathered from lecture notes and probably made some stupid assumptions too.
    The minus sign in front of the values are meant to be "~" for "about", not sure why it was converted to minus signs.
    In the lecture notes I am currently reading, it says the NMOS CD amplifier suffers from bulk effect and its gain is reduced to about 0.9 (depending on the property of the circuit like you said) and would require external circuitry to fix this problem. Any ideas here?

    thanks!
     
  4. oreee

    oreee

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    Sep 22, 2014
    lol, the sign got converted to a negative sign again.
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    I see tildes...

    And no ideas as to your requested external circuitry, sorry.
     
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