Lecture 18 ANNOUNCEMENTS HW10 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 EE 105 Fall 2007 Lecture 18, Slide 1 Prof. Liu, UC Berkeley

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. EE 105 Fall 2007 Lecture 18, Slide 2 Prof. Liu, UC Berkeley

MOSFET Biasing The voltage at node X is determined by VDD, R 1, and R 2: Also, EE 105 Fall 2007 Lecture 18, Slide 3 Prof. Liu, UC Berkeley

Self-Biased MOSFET Stage • Note that there is no voltage dropped across RG M 1 is operating in the saturation region. EE 105 Fall 2007 Lecture 18, Slide 4 Prof. Liu, UC Berkeley

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”). EE 105 Fall 2007 Lecture 18, Slide 5 Prof. Liu, UC Berkeley

Common-Source Stage: = 0 Amplifier circuit Small-signal analysis circuit for determining voltage gain, Av Small-signal analysis circuit for determining output resistance, Rout EE 105 Fall 2007 Lecture 18, Slide 6 Prof. Liu, UC Berkeley

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 EE 105 Fall 2007 Small-signal analysis circuit for determining output resistance, Rout Lecture 18, Slide 7 Prof. Liu, UC Berkeley

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. EE 105 Fall 2007 Lecture 18, Slide 8 Prof. Liu, UC Berkeley

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. EE 105 Fall 2007 Lecture 18, Slide 9 Prof. Liu, UC Berkeley

PMOS CS Stage with NMOS Load • An NMOSFET can be used as the load for a PMOSFET CS amplifier. EE 105 Fall 2007 Lecture 18, Slide 10 Prof. Liu, UC Berkeley

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). EE 105 Fall 2007 Lecture 18, Slide 11 Prof. Liu, UC Berkeley

CS Stage with Diode-Connected PMOS Load EE 105 Fall 2007 Lecture 18, Slide 12 Prof. Liu, UC Berkeley

CS Stage with Degeneration Amplifier circuit EE 105 Fall 2007 Small-signal analysis circuit for determining voltage gain, Av Lecture 18, Slide 13 Prof. Liu, UC Berkeley

Example • A diode-connected device degenerates a CS stage. EE 105 Fall 2007 Lecture 18, Slide 14 Prof. Liu, UC Berkeley

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 EE 105 Fall 2007 Lecture 18, Slide 15 Prof. Liu, UC Berkeley

Output Impedance Examples EE 105 Fall 2007 Lecture 18, Slide 16 Prof. Liu, UC Berkeley

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. EE 105 Fall 2007 Lecture 18, Slide 17 Prof. Liu, UC Berkeley

CS Core with Biasing EE 105 Fall 2007 Lecture 18, Slide 18 Prof. Liu, UC Berkeley