Download DC Voltage and Current Sources

DC Voltage and Current Sources
*
Output characteristics of a BJT or MOSFET look like a family of current sources
... how do we pick one?
specify the gate-source voltage VGS in order to select the desired current level
for a MOSFET ( specifiy VBE exactly for a BJT)
how do we generate a precise voltage? ... we use a current source to set the
current in a “diode-connected” MOSFET
note that this is a
“one-port” ... NOT
an amplifier!
(wait a minute ... where do we find IREF? Assume that one is available!)
2
W
i D = I REF + i OUT ≅  ------ µ C ox ( v OUT – V Tn )
 2L n
EE 105 Spring 2000
Page 1
Week 11, Lecture 26
DC Voltage Sources
*
Solving for the output voltage
I REF + i OUT
v OUT = V Tn + -----------------------------W
 ------ µ C
 2L n ox
If ID = 100 µA, µn = 50 µAV-2, (W / L) = 20, VTn = 1 V, then
VOUT = 1.45 V for IOUT = 0 A.
*
Stack up two diode-connected MOSFETs
EE 105 Spring 2000
Page 2
Week 11, Lecture 26
Totem Pole Voltage Sources
*
Define a series of bias voltages between the positive and the negative supply
voltages.
*
In practice, output currents are small (or zero), so that the DC bias voltages are
set by IREF
EE 105 Spring 2000
Page 3
Week 11, Lecture 26
MOSFET Current Sources
*
Bias the n-channel MOSFET with a MOSFET DC voltage source!
*
Intuitively, VREF is set by IREF and determines the output current of M2
I REF
V REF = V Tn + ------------------------------ = V GS1 = V GS2
W
 ------ µ C
 2L 1 n ox
Substituting into the drain current of M2 (and neglecting (1 + λnVDS2) term)
2
W
i OUT = i D2 =  ------ µ n C ox ( V GS2 – V Tn )
 2L 2
EE 105 Spring 2000
Page 4
Week 11, Lecture 26
MOSFET Current Sources (cont.)
*
Output current is scaled from IREF by a geometrical ratio:

2
I REF


W
i OUT = i D2 =  ------ µ n C ox  V Tn + -----------------------------V Tn
–
 2L 2
W


 ----- µ n C ox


 2L 1
 ( W ⁄ L ) 2
I OUT =  -------------------- I REF
 ( W ⁄ L ) 1
EE 105 Spring 2000
Page 5
Week 11, Lecture 26
MOSFET Current Source Equivalent Circuit
*
Small-signal model: source resistance is ro2 by inspection
*
Combine output resistance with DC output current for approximate equivalent
circuit ... actual iOUT vs. vOUT characteristics are those of M2 with VGS2 = VREF
The model is only valid for vDS = vOUT > vDS(SAT) = VGS - VTn
EE 105 Spring 2000
Page 6
Week 11, Lecture 26
The Cascode Current Source
*
In order to boost the source resistance, we can study our single-stage building
blocks and recognize that a common-gate is attractive, due to its high output
resistance
*
Adapting the output resistance for a common gate amplifier, the cascode current
source has a source resistance of
r oc = ( 1 + g m4 r o2 )r o4 ≈ g m4 r o4 r o2
*
Penalty for cascode:
needs larger VOUT to function
EE 105 Spring 2000
Page 7
Week 11, Lecture 26
MOSFET Current “Mirrors”
*
n-channel current source sinks current to ground ... how do we source current
from the positive supply? Answer: p-channel current sources...?
*
By mixing n-channel and p-channel diode-connected devices, we can produce
current sinks and sources from a reference current connected to VDD or ground.
EE 105 Spring 2000
Page 8
Week 11, Lecture 26
EE 105 Spring 2000
Page 9
Week 11, Lecture 26