Help with Unity Gain Amplifier

Discussion in 'Homework Help' started by sailmike, Apr 7, 2014.

  1. sailmike

    Thread Starter Member

    Nov 11, 2013
    143
    3
    I need to design a unity gain amplifier for a sample and hold circuit. I've decided to use the circuit shown in the attachment. Now I know the gain = 0.5(Gm2*R1)[(Gm3*R2)/(1 + Gm3*R2)]. The 2N7000 has a Gm of 0.1, so that gets me the values of R1 and R2. I don't know if these are good values to use. Components Q1, M1, R3, and R4 comprise the bias current source. Obviously, I'm not understanding the theory behind the circuits. I hope someone here can help me understand this better.

    Thanks a lot,
    Mike
     
  2. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    Many things wrong. Mosfets aren't anywhere near their operating point with microvolts gate to source. The 2N7000 will pass 1 microamp at zero Vgs, so the labels couldn't possibly be right when they show a 2N7000 passing 15 picoamps at Vgs = 250 uV. Picoamps are less than normal leakage currents. For instance, the 2N2222 is not guaranteed to shut off to less than 10 nanoamps. You need to jack up the current levels to the milliamp range to get this to work at all.
     
    Last edited: Apr 7, 2014
  3. sailmike

    Thread Starter Member

    Nov 11, 2013
    143
    3
    The 2N7000 in this circuit is a custom part I made for PSpice and I'm not sure I connected the pin numbers to the right label. The data sheet doesn't say if pin 20 is the drain, gate, or source so I had to guess. Do you know?

    Thanks,
    Mike
     
  4. #12

    Expert

    Nov 30, 2010
    16,257
    6,763
    Not a clue..
     
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