Lecture 40 Last time: Bias and output swing for BiCMOS voltage - - PowerPoint PPT Presentation

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Lecture 40

• Last time:

– Bias and output swing for BiCMOS voltage amp – Start open-circuit time constant analysis (back to Chapter 10)

• Today :

– Applications of open-circuit time constant analysis: CE amplifier and cascode amplifier

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Common-Emitter Voltage Amplifier

Time constant for base-emitter capacitance Cπ:

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Base-Collector Time Constant

Must apply a test source (can’t see RTµ by inspection):

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Solving for RTµ

( )

in t S t

R i r R i v ′ − = − =

π π

|| Find vπ:

( ) ( )

• ut

in m t

• ut

m t

• ut
• R

R g i R v g i R i v ′ ′ + = ′ − = ′ − = 1

π

Find vo:

( )

) ( 1

in t

• ut

in m t

• t

R i R R g i v v v − − ′ ′ + = − =

π

Find vt: Solve for Thèvenin resistance:

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Dominant Pole of CE Amplifier

Estimate dominant pole as inverse of sum of OCTCs:

( )

1 1

] [ 1

−

′ ′ + ′ + ′ + ′ = + ≈

µ π

µ π

τ τ ω C R R g R R C R

• ut

in m

• ut

in in C C

Identical to the “exact” analysis in Chapter 10 Why bother with the OCTC technique … add effect of Ccs

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Multistage Amplifier Frequency Response

CS*-CB cascode

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Systematic Approach

• 1. Construct two-port small-signal models for each stage
• 2. Add the capacitors for each device across the appropriate

nodes in the two-port models. Make sure how the gate, drain, and source (or base, collector, and emitter) terminals

• f each device fit onto the two-port models!
• 3. (Optional) Use Miller’s Theorem to transform capacitors

across amplifiers into effective capacitances to ground (note that we ignore the “output Miller” in this course)

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Cascode Two-Port Model

Add capacitors: Cgs1, Cgd1, Cπ2, Cµ2 … what about Cdb1, Ccs2?

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Finding the Thèvenin Resistances

Cgs1:

S TC

R R

gs = 1

Cgd1: = ′ ′ + ′ + ′ =

• ut

in m

• ut

in TC

R R g R R R

gd

1

1

=

2 π

TC

R Cπ2: =

2 µ

TC

R Cµ2:

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Dominant Pole

Applying the theorem:

( )

2 2 2 1 2 1 1 1 1

) / 1 ( / 1

µ π

ω C R C g C g g R C R

L m gd m m S gs S

+ + + + ≈

−

Find approximate voltage transfer function:

( )

L

• c
• m
• m

R R s

• ut

vo

R r r g r r g v v A

L S

|| || / 1

2 2 2 1 1 1 ,

β         + − = =

• R. T. Howe

EECS 105 Spring 2002 Lecture 40

• Dept. of EECS

University of California at Berkeley

Gain-Bandwidth Product

Metric for amplifier performance: note that

( )

1

* =

ω j Av

when

1 *

ω ω

vo

A =

( )

2 2 2 1 2 1 1 1 1

/ / 1

µ π

ω C R g C C g g R C R R g A

L m gd m m S gs S L m vo

+ + + + =

Special case: small RS

2 2 2 1 1

/

µ π

ω C R g C R g A

L m L m vo

+ ≈