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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.

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lecture 18
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
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
MOSFET Biasing

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

Also,

self biased mosfet stage
Self-Biased MOSFET Stage
  • Note that there is no voltage dropped across RG

 M1 is operating in the saturation region.

mosfets as current sources
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
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 07
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
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
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
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
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
CS Stage with Degeneration

Amplifier circuit

Small-signal analysis circuit

for determining voltage gain, Av

example
Example
  • A diode-connected device degenerates a CS stage.
r out of cs stage with degeneration
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
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.