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
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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”
I-V characteristics:
ID =
W
W
2
2
µ n Cox (VGS − VTn ) =
µ n Cox (VDS − VTn )
2L
2L
Beyond the threshold voltage, the MOSFET looks like a
“diode” with quadratic I-V 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
I-V 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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