Lecture 20
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Lecture 20. ANNOUNCEMENTS HW#11 is due in 2 weeks, on 11/20. Review session: Fri. 11/9, 3-5PM in 306 Soda (HP Auditorium) Midterm #2 (Thursday 11/15 in Sibley Auditorium): Material of Lectures 11-18 (HW# 7-10; Chapters 6,9,11) 4 pgs of notes (double-sided, 8.5”×11”), calculator allowed.

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Lecture 20

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Lecture 20

Lecture 20

ANNOUNCEMENTS

  • HW#11 is due in 2 weeks, on 11/20.

  • Review session: Fri. 11/9, 3-5PM in 306 Soda (HP Auditorium)

  • Midterm #2 (Thursday 11/15 in Sibley Auditorium):

    • Material of Lectures 11-18 (HW# 7-10; Chapters 6,9,11)

    • 4 pgs of notes (double-sided, 8.5”×11”), calculator allowed

OUTLINE

  • Review of MOSFET Amplifiers

  • MOSFET Cascode Stage

  • MOSFET Current Mirror

    Reading: Chapter 9


Review mosfet amplifier design

Review: MOSFET Amplifier Design

  • A MOSFET amplifier circuit should be designed to

    • ensure that the MOSFET operates in the saturation region,

    • allow the desired level of DC current to flow, and

    • couple to a small-signal input source and to an output “load”.

  • Proper “DC biasing” is required!

    (DC analysis using large-signal MOSFET model)

  • Key amplifier parameters:

    (AC analysis using small-signal MOSFET model)

    • Voltage gain Av  vout/vin

    • Input resistance Rin resistance seen between the input node and ground (with output terminal floating)

    • Output resistance Rout resistance seen between the output node and ground (with input terminal grounded)


Mosfet models

MOSFET Models

  • The large-signal model is used to determine the DC operating point (VGS, VDS, ID) of the MOSFET.

  • The small-signal model is used to determine how the output responds to an input signal.


Comparison of amplifier topologies

Comparison of Amplifier Topologies

Common Source

  • Large Av < 0

    - degraded by RS

  • Large Rin

    • determined by biasing circuitry

  • Rout RD

  • ro decreases Av & Rout

    but impedance seen

    looking into the drain

    can be “boosted” by

    source degeneration

  • Common Gate

    • Large Av > 0

      -degraded by RS

    • Small Rin

      • - decreased by RS

    • Rout RD

    • ro decreases Av & Rout

      • but impedance seen

      • looking into the drain

      • can be “boosted” by

      • source degeneration

    Source Follower

    • 0 < Av ≤ 1

    • Large Rin

      • determined by biasing circuitry

    • Small Rout

      • - decreased by RS

    • ro decreases Av & Rout


    Common source stage

    Common Source Stage


    Common gate stage

    Common Gate Stage


    Source follower

    Source Follower


    Cs stage example 1

    CS Stage Example 1

    • M1 is the amplifying device; M2 and M3 serve as the load.

    Equivalent circuit for small-signal analysis,

    showing resistances connected to the drain


    Cs stage example 2

    CS Stage Example 2

    • M1 is the amplifying device; M3 serves as a source (degeneration) resistance; M2 serves as the load.

    Equivalent circuit for small-signal analysis


    Cs stage vs cg stage

    CS Stage vs. CG Stage

    • With the input signal applied at different locations, these circuits behave differently, although they are identical in other aspects.

    Common gate amplifier

    Common source amplifier


    Composite stage example 1

    Composite Stage Example 1

    • By replacing M1 and the current source with a Thevenin equivalent circuit, and recognizing the right side as a CG stage, the voltage gain can be easily obtained.


    Composite stage example 2

    Composite Stage Example 2

    • This example shows that by probing different nodes in a circuit, different output signals can be obtained.

    • Vout1 is a result of M1 acting as a source follower, whereas Vout2 is a result of M1 acting as a CS stage with degeneration.


    Nmos cascode stage

    NMOS Cascode Stage

    • Unlike a BJT cascode, the output impedance is not limited by .


    Pmos cascode stage

    PMOS Cascode Stage


    Short circuit transconductance

    Short-Circuit Transconductance

    • The short-circuit transconductance is a measure of the strength of a circuit in converting an input voltage signal into an output current signal:

    • The voltage gain of a linear circuit is

      (Rout is the output resistance of the circuit)


    Transconductance example

    Transconductance Example


    Mos cascode amplifier

    MOS Cascode Amplifier


    Pmos cascode current source as load

    PMOS Cascode Current Source as Load

    • A large load impedance can be achieved by using a PMOS cascode current source.


    Mos current mirror

    MOS Current Mirror

    • The motivation behind a current mirror is to duplicate a (scaled version of the) “golden current” to other locations.

    Current Mirror Circuitry

    Current mirror concept

    Generation of required VGS


    Mos current mirror not

    MOS Current Mirror – NOT!

    • This is not a current mirror, because the relationship between VX and IREF is not clearly defined.

    • The only way to clearly define VX with IREF is to use a diode-connected MOS since it provides square-law I-V relationship.


    Example current scaling

    Example: Current Scaling

    • MOS current mirrors can be used to scale IREF up or down

      • I1 = 0.2mA; I2 = 0.5mA


    Impact of channel length modulation

    Impact of Channel-Length Modulation


    Cmos current mirror

    CMOS Current Mirror


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