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Common collector vs Common drain amplifier

  1. Nov 19, 2009 #1
    I was wondering whats the main advantage of choosing one over the other, besides common drain having to suffer body effect. How do we determine if a CC is better or a CD is better to use as a buffer?
     
  2. jcsd
  3. Nov 19, 2009 #2

    vk6kro

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    You would use a common drain amplifier if you were using a FET. If you had a BJT transistor, you would use a common collector circuit.

    Both have slightly less than unity gain (gain < 1 ) and low output impedance, so they are used as buffer amplifiers.
    Common collector:
    130px-NPN_emitter_follower.svg.png
    Common drain:
    180px-N-channel_JFET_source_follower.svg.png
     
  4. Nov 20, 2009 #3
    The main operational difference (at least in my mind) is that FET's are Voltage controlled devices, whereas BJT's are current controlled devices.
     
  5. Nov 20, 2009 #4
    With Vcc=5 volts, BJTs (common collector) work much better than NMOSs (common drain).
    Bob S
     
  6. Nov 20, 2009 #5
    The threshold of a FET is several volts, but only 0.65V for a bjt. If the input is 10V, the FET will output around 7V, where as the bjt will output 9.35V. For low voltages, the FET threshold is a problem. As Bob. S, stated, when the voltage is LOW, the bjt is a better choice since it drops much less voltage.

    Claude
     
    Last edited: Nov 20, 2009
  7. Nov 20, 2009 #6

    vk6kro

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    The threshold of a FET is several volts,
    You wouldn't use an enhancement mode FET as a common drain amplifier. They do have a large threshold voltage.
    Something like an MPF102 actually requires a negative voltage on the gate (relative to the source) to bias it.
    This makes the biassing a little more complex as a large source resistor may be needed for the load but a smaller one may be needed for the biassing. So, such circuits often have both.
    They have a small bypassed source resistor for bias purposes and a larger unbypassed one for the load. A gate resistor takes the voltage between the two to the gate.

    However FETs perform badly at low voltages as Bob S has pointed out.
    The high input impedance of an emitter follower is surprising if you haven't seen it before and it certainly becomes a voltage driven device.

    If the supply voltage is 10 volts or so, which one you use doesn't make much difference to the performance.
     
  8. Nov 20, 2009 #7
    Finally getting back to the "good ol' days" of the familiar cathode bias circuits.

    Bob S
     
  9. Nov 21, 2009 #8
    Thanks everyone for the input!!
     
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