ESE 372 / Spring 2013 / Lecture 22
MOSFET differential stage
Ideal differential amplifier
Common mode gain
Differential mode gain
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ESE 372 / Spring 2013 / Lecture 22
Differential amplifier
Output can be either differential or single‐ended.
Differential:
Single‐ended:
Ideally
Then only differential signal gets amplified:
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ESE 372 / Spring 2013 / Lecture 22
Differential amplifier
EXAMPLE
Single‐ended output
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Assume that M1 and M2 are identical
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Common mode input range
Value of Vc is limited
since both M1 and M2 are to be in saturation.
Overdrive for I0/2
Hence
Min. voltage drop across current source I0 – it is not ideal.
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Hence, can expect gain for this case!
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Balanced (complementary) input signals
In this case each branch acts as a common source amplifier with source on ground
source amplifier with source on ground for signal.
For ideal current source bias: ACM = 0 and 7
ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Bias by real current source
For AC signal
Symmetric out of phase branches – no change from ideal current source case
Symmetric, hence, ACM = 1 for differential output,
but not for single‐ended since now VGS can change
Half of
circuit
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Effect of the parameter mismatch on CMRR
1. Mismatch of drain resistors
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Effect of the parameter mismatch on CMRR
2. Mismatch of transconductance
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Single ended input and output
Single-ended
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Single ended input and output
Single-ended
What is the max value of Vin?
When Vin makes
When V
then M2 closed, hence
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Single ended output needs only one RD
Single-ended
With active load one can get higher gain
‐ RD is replaced with r0 >> RD;
‐ Active load has current mirror action
A i l dh
i
i
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Frequency response
Frequency response of differential mode gain AD is the same as in CS amplifier
gain A
is the same as in CS amplifier
What about ACM ?
Recall low freq.
Recall low freq. cutoff of CS amp
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ESE 372 / Spring 2013 / Lecture 22
MOSFET differential pair
Frequency response
Common mode gain ACM
Differential mode gain AD
Often BW of Dif. Pair is limited by ACM
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