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 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
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
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.