Lecture 18

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Lecture 18 - PowerPoint PPT Presentation

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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Presentation Transcript
Lecture 18

ANNOUNCEMENTS

• HW#10 will be posted tonight

OUTLINE

• Basic MOSFET amplifier
• MOSFET biasing
• MOSFET current sources
• Common-source 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

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.
• 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.

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.