b 20080305 single-stage ics part 1

1ELECTRONICS II BE(EE) 3 AB


Single-Stage Integrated Circuit Amplifier(Part 1)

Introduction!


Introduction : We have studied discretecircuit amplifier

configurations. The next domain is integratedcircuit amplifiers.

There is a difference in IC design philosophy.

Circuits combine MOS and bipolar transistors in a technology known as BiMOS or BiCMOS.

Chip-area considerations dictate that while resistors are to be avoided, constant current sources are readily available.

Single-Stage Integrated Circuit Amplifier

contd!


Large capacitors are components external to the IC chip but should be used cautiously to keep number of chip terminals small to avoid high cost.

As of 2003, CMOS process technologies are capable of producing devices with a 0.1 m channel length. Advantages? These require overdrive voltages of only 0.2 volts or so. However, bipolar circuits can still provide much higher output currents. Due to reliability under severe environmental conditions, the bipolar

circuits are preferred for applications in the automotive industry.

CMOS is the most popular technology for the implementation of digital systems due to :

Small size Ease of fabrication and Low power dissipation

Integrated Circuit Design Philosophy

Typical values of MOSFET parameters!


To pack more transistors on a chip, the trend is to reduce the minimum allowable channel length.

Magnitude of threshold voltage Vt has been decreasing with decreasing length. Additionally VDD has been reduced from 5 volts to 1.8 volts for newer technologies to keep power dissipation as low as possible.

With submicron technologies, channel length modulation effect isvery pronounced. As a result VA has been steadily decreasing which causes Early voltage VA = VAL to become very small so short-channel MOSFETs exhibit low output resistance. Because r0 = VA/ID

Two major MOSFET capacitances are Cgs and Cgd. We see that: Shorter devices exhibit much higher operating speeds and wider

amplifier bandwidths. EX: fT for 0.25 m NMOS transistor can be as high as 10 GHz.

Typical Values of BJT/MOSFET Parameters

contd!


Till date (2003), the major drawback of standard bipolar integrated circuit fabrication process has been the lack of pnp transistors of a quality equal to that of npn devices.

is lower for pnp transistors And pnp transistors have much larger forward transit time F . And transit time F determines the emitter-base diffusion

capacitance Cde and hence the transistor speed of operation.

Comparison of important characteristics!

Typical Values of BJT/MOSFET Parameters


Comparison of Important Characteristics

MOSFET Induce a channel vGS > vt where

vt = 0.5 to 0.7 V vDS > vGS vt i v characteristics iG = 0 Input resistance (CS) is infinite Transconductance

BJT Forward-bias EBJ vBE >vBEon

where vBEon = 0.5 V Reverse-bias CBJ i v characteristics iB = iC/ Input resistance (CE) r = /gm Transconductance gm=Ic/VT

( )tGS Dm VVIg =

2gs

dm v

ig = ( )tGSn VVLWk = or

( )

+=

A

DStGSnD V

VVVLWkI 1

21 2

where 1/VA = process-technology parameter

Low/High Frequency Models!


Low/High Frequency Models

The IC Biasing & MOSFET Current Mirror/Source!

Vgs

Cgd

gmVgs roCgs+

_

vo+

_gmv

+

_rov r

V

C

gmVro

C+

_

rx

r

The frequency at which magnitude of hfe drops to unity is called unity-gain bandwidth wT. fT is called unity-gain frequency or transition frequency. fT is 10 to 20 GHz for npn and 5 to 15 GHz for NMOS. The high-frequency response of IC amplifiers is limited by the transistor internal

capacitances, mainly Cgs and Cgd in the MOSFET and C and C in the BJT.

rovgs+

_gmvgs

id

is

ig = 0


Biasing Using a Current Source

Consider the following circuit:

Vcc

Rc

vO

- 10 V

= 100

1 mA

8 k

RB 100 k

+

_RL 5 k

The BJT can be biased using a constant current source I.

It has the advantage that the emitter current is independent of and RB.

Thus RB can be made large, enabling an increase in the input resistance at the base without adversely affecting bias stability.

Implementation of constant current source?

10

ELECTRONICS II BE(EE) 3 AB

Biasing Using a Current Source : The Circuit

Q1 and Q2 are matched transistors. Assuming high , and small value of

base current, the current through Irefwill be:

Since VBE is same so collector currents of Q1 and Q2 will be the same.

V

R

+_

- VEE

Vcc

I

VBE

Q1 Q2

IrefIref = {VCC (- VEE) VBE} / R

So I = Iref = {VCC + VEE VBE} / R

This is known as a current mirror.

Implementation!

11


Biasing using a current source : implementation

V

R

+_

- VEE

Vcc

I

VBE

Q1 Q2

Iref

Vcc

Rc

vO

- 10 V

= 100

1 mA

8 k

RB 100 k

+

_

RL 5 k

The IC Biasing & MOSFET Current Mirror/Source!

12


Biasing in IC design is based on the use of constantcurrent sources.

The IC Biasing & MOSFET Current Mirror/Source

Now: ( )21

1 21

tGSnD VVLWkI

=

Also: ( )RVVII GSDDrefD

==1

G

DS

GD

S

R

VGS

IRef Io

+

-

Q1 Q2

ID1

VDD

And: ( )22

2 21

tGSnDo VVLWkII

==

Then:

1

2

=

LWLW

II

ref

o

For identical transistors: refo II =

MOS Current Steering Circuits!

A constant dc current (called reference current) is generated at one location and then replicated at various other locations through a process called current steering.

Usually a precision resistor external to the chip is used for generating the reference current.

Because the circuit replicates or mirrors the reference current, it is given the name of current mirror.

13


The circuit:

MOS Current Steering Circuits

Q1 and R determine ref current Iref.

Q1, Q2 & Q3 form a two-port current- mirror where:

1

2ref2 II

=

LWLW

and

1

3ref3 II

=

LWLW

For saturation region operation, the voltages at the drains of Q2 and Q3 are constrained as follows:

vD2, vD3 = - vSS + vGS1 - vtn The CS Circuit With Active Load!

Current SourceCurrent Sink

I5Q5

R

VGS1

IRef I2

+

-- VSS

VDD

I3 I4

Q1 Q2Q3

Q4

+

-VSG5

VDD

14


The most basic IC MOS amplifier is:-

The CS Circuit with Active Load

The drain resistance RD has been replaced by a constant current source I. Because this current source load can be implemented using a PMOS

transistor, it is called an active-load and the amplifier is said to be active-loaded.

The small signal analysis of the amplifier is:-

For the CS amplifier, Ri = , Avo = - gmro and Ro = r0

The magnitude of Avo is the maximum gain available from a CS amplifier and is called the intrinsic gain,

Ao = gmro

The active-loaded CE Circuit!

I

vivo

vDD

vgs+

_gmvgs

+

_ro

15


The circuit is :-

The Active-Loaded CE Circuit

To keep analysis simple, the bias network is not shown. The small signal analysis of the amplifier is:-

For the CE amplifier, Ri = r Avo = - gmro and Ro = r0

The magnitude of Avo is the maximum gain available from a CE amplifier and is called the intrinsic gain,

Ao = gmro

High-Frequency Response of the CS & CE Amplifier!

I

vivo

vCC

voror+

_vvi

gmv

16


The MOSFET internal capacitances

There will be five capacitances in total namely: Cgs , Cgd ,Cgb , Csb, and Cdb It can be shown that :

Cgs = Cgd =1/2 WLCox (triode region)Cgs = 2/3 WLCox and Cgd = 0 (saturation region)Cgs = Cgd = 0 and Cgb = WLCox (cut-off region)

Another small capacitance that should be added to Cgs and Cgd is the capacitance that results from the fact that the source and drain diffusions extend slightly under the gate oxide.

If overlap length is denoted by Lov, the overlap capacitance Cov = WLovCox (typically Lov = 0.05 to 0.1 L) The junction capacitances are given by:

Where Csbo = value of Csb at zero-body source bias, VSB is equal to magnitude of reverse bias voltage. Vo is equal to junction built in voltage of 0.6 to 0.8 volts.

contd.

o

SB

sbosb

VV

CC+

=1

and

o

DB

dbodb

VV

CC+

=1

17


The MOSFET internal capacitances contd

So the high frequency response of the MOSFET amplifier can be predicted by the model:

G D

S B

Vgs

Cgd

gmVgsro Cdb

Csb

gmbVbs

Cgs+

_ vbs

+

_

Due to its complexity, such models are limited to computer simulation.

contd.

18


The MOSFET internal capacitances contd(Source and body connected)

G D

S

Vgs

Cgd

gmVgsro

Cgs+

_

Cdb

G D

S B

Vgs

Cgd

gmVgsro Cdb

Csb

gmbVbsCgs

+

_

19


The MOSFET internal capacitances contd(Source and body connected: with Cdb neglected)

G D

S

Vgs

Cgd

gmVgsro

Cgs+

_

This is the commonly used high frequency model of the MOSFET.

20


The MOSFET internal capacitances contd

Mid bandLF band HF band

fL fH

Gain falls due toCC!, CC2,CS

Gain falls due toCgs, Cgd

Vo/vin (dB)

f (Hz)

b 20080305 single-stage ics part 1

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