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1ELECTRONICS II BE(EE) 3 AB
2ELECTRONICS II BE(EE) 3 AB
Single-Stage Integrated Circuit Amplifier(Part 1)
Introduction!
3ELECTRONICS II BE(EE) 3 AB
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!
4ELECTRONICS II BE(EE) 3 AB
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!
5ELECTRONICS II BE(EE) 3 AB
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!
6ELECTRONICS II BE(EE) 3 AB
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
7ELECTRONICS II BE(EE) 3 AB
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!
8ELECTRONICS II BE(EE) 3 AB
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
9ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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
ELECTRONICS II BE(EE) 3 AB
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)