• Simplified transistor model : MOSFET and BJT

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(1)

Microelectronics Circuits 1 Review

Prof. Y.Kwon

(2)

Electronic Circuits

• Static phenomenon only

• Simplified transistor model : MOSFET and BJT

회로이론

반도체 전자장 소자

(3)

Contents to Cover in this Course and Evaluation

• Chap 8 : Feedback Amplifier

• Chap 9 : Operational Amplifier

• Chap 12 : Filters and Tuned Amplifier (12.1 ~ 12.7)

• Chap 13 : Signal Generators and Waveform-Shaping Circuits (13.1~13.5)

• Chap 14 : Output Stage and Power Amplifiers (14.1 ~ 14.5)

• Chap 10 & 11 : Basics of Digital Circuits

• Evaluation

– Two mid terms and One final exam – Attendance

– Homework

(4)

Physical understanding of STC

Ærequires large I to reduce t Ærequires small R

Time Constant = RC

takes more charge to raise voltage level

s s

o

o o

V C t

Q V

t V

t V t

Q

C V Q

* )

( )

(

0 ) 0 (

0 ) 0 (

=

=

=

=

=

=

=

=

=

=

R V I V

d I t

Q

s t

I

=

= ∫

0

( ) )

( τ τ

V s

(5)

Comparison of the MOSFET & the BJT

TABLE 6.3 Comparison of the MOSFET and the BJT

Circuit Symbol

NMOS npn

To Operate in the Active Mode, Two Conditions Have To Be Satisfied

( ) ( )

1 Induce a channel; (1)Forward-bias EBJ;

0 5 0 7 V 0 5

Let

2 Pinch-off channel at drain;

GS t t BE BE on BE on

GS t ov

V V ,V . . v V , V . V

V V v

≥ = − ≥ ≅

= +

(2)Reverse-bias CBJ;

0 4V or equivalently, or equ

GD t BC BCon BCon

V < V v < V ,V.

ivalently

vV ,V = v

(6)

Comparison of the MOSFET & the BJT

Current-Voltage Characteristics in the

Active Region

( )

2

2

1 1 1

2

1 1 2

0

BE T A

DS v /V CE

D n ox GS t C S

A

DS

n ox ov

A G

v v

i C W v V i I e

L V V

W v

C v

L V

i

⎛ ⎞ ⎛ ⎞

= μ − ⎜ ⎜ ⎝ + ⎟ ⎟ ⎠ = ⎜ ⎝ + ⎟ ⎠

⎛ ⎞

= μ ⎜ + ⎟

⎝ ⎠

= i

B

= i /β

C

Low-Frequency Hybrid-π Model

(7)

Comparison of the MOSFET & the BJT

Transconductance gm

Output resistance

r0 0 0

0

is inversely proportional to the bias current.

A

A D A C

D

r V /I V' L r V /I I

r

= = =

D ox

n m

ov ox

n m

T C m

ov D

m

L I C W

g

L V C W

g

V I g

V I

g

⎟ ⎠

⎜ ⎞

= ⎛

⎟ ⎠

⎜ ⎞

= ⎛

=

=

) (

2 ) (

/ )

2 / /(

μ μ

Input Resistance with Source (Emitter)

Grounded

r

π

= β/g

m

(8)

Comparison of the MOSFET & the BJT

Intrinsic Gain A0=gmro

D ox n A

OV A

T A C

A T C o

m OV

A

I

WL C

A V

V L A V

V I V

V V r I

g A

V V A

μ 2 2

/ )

2 / /(

' 0

' 0

0 0

=

=

=

=

=

=

V

V

A

0

= 1000 ~ 5000 /

(9)

( )

1

2 1

1

Transistor : Drain is shorted to its gate. Saturation mode 1

2

Since the gate currents are zero,

D n ox GS tn

Q

i C W V V

L

⎛ ⎞

= μ ⎜ ⎝ ⎟ ⎠ −

1

2

: In most cases, outside the IC chip Assuming in the satuaration mode,

DD GS

D REF

V -V I I

R R

Q

= =

( )

( ) ( )

2 2

2

2

1

= 1

2

=

/ is sole

O D n GS tn

O REF

O REF

I I k ' W V V

L

I W L

W

I L

I I

⎛ ⎞

= ⎜ ⎝ ⎟ ⎠ −

y determined by the geometries of the transistors.

Current Mirror Circuit

(10)

2

02 2

To ensure that is saturated, or equivalentely

At , .

As increases above , will increase according to

the incremental output resistance of .

O GS t O OV

O GS O REF

O GS

O

Q

V V V V V .

V V I I

V V

I

r Q

≥ − ≥

= =

Current Mirror Circuit

(11)

( )

( )

02 2

2 2

2

As is increases above , will increase according to the incremental output resistance of .

1

O GS O

O A

o o

O O

O GS

O REF

V V I

r Q

ΔV V

R r

ΔI I

W/L V V

I I

W/L V

≡ = =

⎛ − ⎞

= ⎜ + ⎟

⎝ ⎠

Current Mirror Circuit

(12)

Once a constant current is generated, it can be replicated to provide dc bias currents for various amplifier stages in an IC.

Current Steering Circuit

(13)

F H Estimation : Open-circuit time constant Method

• High Frequency Gain Function :

• Practical Bandwidth is determined by 3dB roll-off point ( )

• When dominant pole exists, &

• Gray and Searle Æ

• IF there is dominant pole,

Pn P

P

n n

n n Pn

P P

Zn Z

Z H

b

s b s

b s b

s a s

a s a s

s s

s s

s s F

ω ω

ω

ω ω

ω

ω ω

ω

1 1

1

1 1 ) / 1 ( ) / 1 )(

/ 1 (

) / 1 ( ) / 1 )(

/ 1 ) ( (

2 1

1

2 2 1

2 2 1

2 1

2 1

+

⋅⋅

⋅ + +

=

+

⋅⋅

⋅ + +

+

+

⋅⋅

⋅ + +

= + +

⋅⋅

⋅ +

+

+

⋅⋅

⋅ +

= +

ω

H

/

1

1 ) 1 (

P

H

s s

F ≅ + ω ω

H

≅ ω

P1

=

i

io i

R C b

1

=

i

io i P

H

P

b C R

b

1 1 1

1 1 1

1

ω ω

ω

) ( )

( s A F s

A =

M H

A

M

: Midband Gain

(14)

Miller’s Theorem

• When the circuit conditions remain to make

• Proof : derive the same I/V relationship at both ports

• Æ Miller multiplication (“Miller Effect”) for conductance or susceptance

) 1 ( K 2

1

KV

V =

) 1

( − K

(15)

The MOSFET internal capacitances and high- frequency model

gs gd gb sb db

Five capacitance

C , C , C , C and C

(16)

• The gate capacitive effect – Triode region

Cgs= Cgd = 1/2WLCox

– Saturation region

Cgs= 2/3WLCox

Cgd = 0

– Cutoff

Cgs= Cgd = 0

Cgb = WLCox

• The junction capacitances

,

1 1

sbo dbo

sb db

SB DB

o o

C C

C C

V V

V V

= =

+ +

The MOSFET internal capacitances and high-

frequency model

(17)

• The high-frequency MOSFET model

(a) High-frequency equivalent circuit model for the MOSFET.

(b) The equivalent circuit for the case in which the source is connected to the substrate (body).

(c) The equivalent circuit model of (b) with Cdbneglected (to simplify analysis).

The MOSFET internal capacitances and high-

frequency model

(18)

The MOSFET unity-gain frequency (f

T

)

Determining the short-circuit current gain Io /Ii.

The frequency at which the short-circuit curent-gain of the common-source configuration becomes unity.

/ ( )

o m gs gd gs

o m gs

gs i gs gd

I g V sC V I g V

V I s C C

= −

= +

( )

For physical frequencies

/( )

2 ( )

o m

i gs gd

T m gs gd

m T

gs gd

I g

I s C C

s j

g C C

f g

C C

ω ω

π

= +

=

= +

= +

The MOSFET internal capacitances and high-

frequency model

(19)

(1) The current source can be

implemented using a PMOS transistor.

Î Active load

(2) Obviously, Q

1

is biased at I

D

=I.

But what are V

DS

and V

GS

?

Î We just assume that the MOSFET is biased to operate in the saturation region throughout this chapter.

CS Amplifier

(20)

i i

i RL

o

vo m o

i RL

R v i

A v g r

v

=∞

=∞

= = ∞

= = −

CS Amplifier

(21)

( )

2 3

2 2

The value of is set by passing the reference bias current through

When exceeds operates in satuaration.

When operates in satuaration, behaves as a current source.

Wh

SG R EF

SD SG tp

V I Q .

v v V - V , Q

Q Q

=

2

2 2

en is in satuaration, it exhibits a finite incremental resistance :

o

V

A

Q r

= I

CS Amplifier

(22)

1

When the amplifier is biased at a point in region III,

small signal voltage gain can be determined

by replacing Q with its

CS Amplifier

(23)

0

vo m o

x

o o

x vi

A g r

R v r

i

=

= −

= =

CS Amplifier

(24)

( )

( ) ( )

( ) ( )

1 1 1 2

2

1 1 1 1 2

2 1

1 1 2

, and in this case , , and

Or from the other circuit,

VO VO

o L

v m o O o L o

i L O

o

v m o m o o

o o

o m i o o

v R

A A A g r R r R r .

v R R

A g r r g r r

r r

v g v r r

≡ = = − = =

+

= − = −

+

= −

CS Amplifier

(25)

( ) ( ) ( ) ( )

CMOS common-source amplifier 1 Voltage gain of 15 to 100

2 Very high input resistance 3 Output resistance is also high.

Two final comments

1 The circuit is not affected by the body effect since the source

( )

1 2

terminals of both and are at signal ground.

2 The circuit operates in region III, which is ensured by the negative feedback: the circuit is usually part of a larger amplifier (Chapters 7 and 9)

Q Q

.

CS Amplifier

(26)

Common-Gate Amplifier

(27)

Common-Gate Amplifier

• Impedance transformation

• Circuits to calculate R

out

(28)

Source Follower

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