Lecture 25
Multistage Amplifiers (II)
DC VOLTAGE AND CURRENT SOURCES
Outline
1.
2.
DC Voltage Sources
DC Current Sources and Sinks
Reading Assignment:
Howe and Sodini, Chapter 9, Sections 9-3-9.4
Final: Monday, May 19, 2009, 9am, Johnson Ice
Rink
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1. DC Voltage Sources
Characteristics of DC Voltage Sources :
• A well controlled output voltage
• Output voltage does not depend on current
drawn from source ⇒ Low Thevenin
Resistance
Consider a MOSFET connected in “diode configuration”
I­V characteristics:
ID =
W
W
2
2
µ n Cox (VGS − VTn ) =
µ nCox (VDS − VTn )
2L
2L
Beyond the threshold voltage, the MOSFET looks
like a “diode” with quadratic I­V characteristics
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How does one synthesize a voltage source with
this?
Assume a current source is available
VGS = VDS takes a value needed to sink current
W
2
i D = I REF + iOUT =
µnC ox (vOUT − VTn )
2L
Then:
W
2
iOUT =
µn Cox (v OUT − VTn ) − I REF
2L
Solving for vOUT:
I REF + iOUT
vOUT = VTn +
W
µn Cox
2L
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Synthesizing Voltage Sources (contd.)
vOUT is a function of IREF and W/L of MOSFET:
• IREF ↑ ⇒ vOUT ↑
• W/L ↑ ⇒ vOUT ↓
Small Signal Equivalent Circuit Model:
1
1
Rout =
|| ro ≈
gm
gm
Rout is small (good!)
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PMOS voltage source
Same operation and characteristics as NMOS
voltage source. PMOS needs to be larger to attain
the same Rout.
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3. DC Current Sources and Sinks
Characteristics of Current Sources
• A well controlled output current
• Supplied current does not depend on output
voltage ⇒ High Norton Resistance
Connect a voltage source to the gate of another MOSFET
:
1  W
2
IOUT ≈   µnCox (VREF −VTn )
2 L 2
1 W
2
I REF ≈   µn Cox (VREF − VTn )
2  L 1
Then:
 W
 
 L 2
I OUT = I REF
 W
 
 L 1
IOUT scales with IREF by W/L ratios of two MOSFETs
⇒ Current Mirror Circuit
Well “matched” transistors important.
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DC Current Sources and Sinks (contd.)
Small Signal Equivalent Circuit Model:
Rout 2 = ro2
I­V characteristics of NMOS current source:
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PMOS Current Source
• NMOS current source sinks current to ground
• PMOS current source sources current from
positive supply
PMOS Current Mirror:
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3. Multiple Current Sources
Since there is no DC gate current in MOSFET, we
can tie up multiple current mirrors to single current
source:
 W
 
 L n
I OUTn = I REF
 W
 
 L R
Similar idea with NMOS current sinks:
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Multiple Current Sources and
Sinks
Often, in a given circuit, we need current sources
and sinks. We can build them all out of a single
current source.
W 
 
 L 1
I OUT1 = I REF
W
 
 LR
W
 
 L2
I OUT2 = I REF
W
 
 LR
W
 W  W
 
   
 L4
 L  4  L 1
I OUT4 = I OUT1
= I REF
 W
W W
 
   
 L 3
 L 3 L  R
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Generating IREF
Simple circuit:
VDD − VOUT
I REF =
R
VOUT = VTn +
For large W/L:
I REF
W
µ nCox
2L
VDD − VTn
I REF ≈
R
• Advantages
– IREF set by value of resistor
• Disadvantages
– VDD also affects IREF.
– VTn and R are functions of temperature ⇒ IREF(T).
In the real world, more sophisticated circuits are
used to generate IREF that are VDD and T
independent.
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What did we learn today?
Summary of Key Concepts
• Voltage source easily synthesized from
reference current source using MOSFET in
diode configuration
• Current source easily synthesized from current
source using current mirror circuit.
• Multiple current sources and sinks with different
magnitudes can be synthesized from a single
current source.
• Voltage and current sources rely on the
availability of well “matched” transistors in IC
technology.
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