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Lecture 18

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  1. Lecture 18 ANNOUNCEMENTS • HW#10 will be posted tonight OUTLINE • Basic MOSFET amplifier • MOSFET biasing • MOSFET current sources • Common-source amplifier Reading: Chapter 7.1-7.2

  2. Basic MOSFET Amplifier • For large small-signal gain, the MOSFET should be operated in the saturation region. Vout should not fall below Vin by more than VTH.

  3. MOSFET Biasing The voltage at node X is determined by VDD, R1, and R2: Also,

  4. Self-Biased MOSFET Stage • Note that there is no voltage dropped across RG  M1 is operating in the saturation region.

  5. MOSFETs as Current Sources • A MOSFET behaves as a current source when it is operating in the saturation region. • An NMOSFET draws current from a point to ground (“sinks current”), whereas a PMOSFET draws current from VDD to a point (“sources current”).

  6. Common-Source Stage:  = 0 Amplifier circuit Small-signal analysis circuit for determining voltage gain, Av Small-signal analysis circuit for determining output resistance, Rout

  7. Common-Source Stage:   0 • Channel-length modulation results in reduced small-signal voltage gain and amplifier output resistance. Small-signal analysis circuit for determining voltage gain, Av Small-signal analysis circuit for determining output resistance, Rout

  8. CS Gain Variation with L • An ideal current source has infinite small-signal resistance. The largest Av is achieved with a current source as the load. • Since  is inversely proportional to L, Av increases with L.

  9. CS Stage with Current-Source Load • Recall that a PMOSFET can be used as a current source from VDD. • Use a PMOSFET as a load of an NMOSFET CS amplifier.

  10. PMOS CS Stage with NMOS Load • An NMOSFET can be used as the load for a PMOSFET CS amplifier.

  11. CS Stage with Diode-Connected Load Av is lower, but it is less dependent on process parameters (mn and Cox and drain current (ID). Amplifier circuit Small-signal analysis circuit including MOSFET output resistances

  12. CS Stage with Diode-Connected PMOS Load

  13. CS Stage with Degeneration Amplifier circuit Small-signal analysis circuit for determining voltage gain, Av

  14. Example • A diode-connected device degenerates a CS stage.

  15. Rout of CS Stage with Degeneration • Degeneration boosts the output impedance: Small-signal analysis circuit for determining output resistance, Rout Current flowing down through ro is

  16. Output Impedance Examples

  17. CS Stage with Gate Resistance • For low signal frequencies, the gate conducts no current.  Gate resistance does not affect the gain or I/O impedances.

  18. CS Core with Biasing