Gain-V Swing- trade-off Common Source CMOS amplifier

Discussion in 'General Electronics Chat' started by aman92ullah, Jun 29, 2012.

  1. aman92ullah

    Thread Starter New Member

    Jan 19, 2011
    12
    0
    In a common source stage with resistive load, the approximate gain is given by
    Av=-gm*Rd. [Figure attached]
    Now, the book I'm referring to, Analog CMOS Design by Razavi, says that if we increase Rd, we increase the gain(that's fine), but we also DECREASE THE VOLTAGE SWING. I didn't get this; isn't the output still moving from Vdd (at Vin<Vth) to some value Vout=Vin-Vth (so that our MOS is in saturation).

    In fact, if we increase the gain using Rd, then the circuit will achieve this Vout at smaller Vin, thus increasing the voltage swing?

    Where am I going wrong?
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,000
    3,229
    The voltage swing would only significantly decrease with an increase in the drain resistor value if there were also an un-bipassed source resistor. What is the source connected to?
     
  3. aman92ullah

    Thread Starter New Member

    Jan 19, 2011
    12
    0
    The source is grounded, sorry I didn't mark that.
     
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