HW#10 will be posted tonight 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

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.

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

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

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”).

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

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

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.

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.

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

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

CS Stage with Diode-Connected PMOS Load

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

Example A diode-connected device degenerates a CS stage.

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

Output Impedance Examples

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.

CS Core with Biasing