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ECE 342 – Jose Schutt-Aine 1
ECE 242Solid-State Devices & Circuits
15. Current Sources
Jose E. Schutt-AineElectrical & Computer Engineering
University of [email protected]
ECE 342 – Jose Schutt-Aine 2
Amplifier with Diode-Connected Load
Amplifier is single stage M1 driving a load impedance which is that looking into the source of M2
ECE 342 – Jose Schutt-Aine 3
Output Impedance
22 2 2 2 2
2
ss m s mb s
ds
vi g v g v
r
To calculate output impedance, connect voltage supply at source of M2 and measure current
ECE 342 – Jose Schutt-Aine 4
Output Impedance
22
2 2 2 2
1ss
s m mb ds
vR
i g g g
1 22 2 2 1
1||out ds S
m mb ds ds
R r Rg g g g
M1 sees this load in parallel with its own output impedance,rds1
ECE 342 – Jose Schutt-Aine 5
Incremental model
The source to-body voltage changes in M2 use transconductance term gmb2
11
2 2 2 1
mMB m out
m mb ds ds
gA g R
g g g g
ECE 342 – Jose Schutt-Aine 6
Gain – Diode-Connected Amp
1
2
mMB
m
gA
g
2 ( / )m n ox Dg C W L I
1/ 2
1 1 1 1
2 22 2
2 ( / ) ( / )
( / )2 ( / )n ox D
MB
n ox D
C W L I W LA
W LC W L I
The magnitude of the voltage gain varies as the square root of the aspect ratio
ECE 342 – Jose Schutt-Aine 7
pMOS Diode-Connected Amp
• Analog Design Requirements – Load device can be
replaced by pMOS transistor– Eliminates body effect of the
load device– Increases the resistance
1/ 2 1/ 2
1 1 1 1
2 2 2 2
/ 3 /
/ /n oxn
MBp oxp
C W L W LA
C W L W L
ECE 342 – Jose Schutt-Aine 8
Integrated Circuits
• IC Requirements – Biasing of ICs is based on the use of constant
current sources– Use current mirrors– Source circuits are used as loads
ECE 342 – Jose Schutt-Aine 9
• Analog Design Requirements – Analog ICs may need resistors and capacitors for
the design of amplifiers– Resistors and capacitors occupy the space of
tens or hundreds of MOS devices– It is important to minimize their use
Integrated Circuits
ECE 342 – Jose Schutt-Aine 10
Transistor Biasing
ECE 342 – Jose Schutt-Aine 11
Transistor Biasing
ECE 342 – Jose Schutt-Aine 12
Current Mirrors
A current mirror will reproduce a reference current to the output while allowing the output voltage to assume any value within a specified range. Io=KIin where K is a factor that can be less than or equal or greater than 1
ECE 342 – Jose Schutt-Aine 13
MOS Current Mirror
2'1
1
1
2D n GS Tn
WI k V V
L
1DD GS
D REF
V VI I
R
2'2
2
1
2o D n GS Tn
WI I k V V
L
R is usually external to IC
ECE 342 – Jose Schutt-Aine 14
MOS Current Mirror
2 12
1 21
/
/o
REF
W LI W L
I W L W L
Assuming that the transistors are using the same process
• Can be limited by – Channel length modulation (l)– Threshold voltage mismatch– Imperfect geometrical matching
ECE 342 – Jose Schutt-Aine 15
MOS Current Mirror
2 22 1
1 2 1 1
1
1DS DSPo
REF DS DSP
V VI W L
I W L V V
• Some Properties1. MOS current mirrors draw zero control
currentbetter than BJT’s2. Matching of threshold voltages harder than
in BJT’s
ECE 342 – Jose Schutt-Aine 16
ExampleA matched pair of MOSFETs are used in a current mirror witl l = 0.032 V-1, mCox=70 mA/V2, W/2L =10, and VT = 0.9 V. Find the value of R to create an input current of 100 mA. Calculate the output current when Vo = 3 V.
2
1 112ox
D GS T DS
C WI V V V
L
Use drain current equation in active region to calculate
2
1 100 700 0.9 1 0.032*D GS GSI V V
We can now solve for the value of VGS
ECE 342 – Jose Schutt-Aine 17
Example
MOS Current Mirror
ECE 342 – Jose Schutt-Aine 18
Example
1
1
5 5 1.27237.2
0.1DS
D
VR k
I
The resistance needed is:
The output current is calculated from:
2
2 700 1.272 0.9 1 0.032 3 106DI A
VGS = 1.272 V
2 106DI A
2
2 112ox
D GS T DS
C WI V V V
L
ECE 342 – Jose Schutt-Aine 19
BJT Current Mirror
2 2
1 1
o s
REF s
I I Area of EBJ of Q
I I Area of EBJ of Q
• Characteristics – Base current is not zero– Depends on relative areas
of emitter-base junction– Want Io=IREF
ECE 342 – Jose Schutt-Aine 20
Multiple Output Current Mirror