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Analog CMOS Integrated Circuit Design
Differential Amplifiers
Jawdat Abu-Taha Department of Electrical and Computer Engineering
Islamic University of Gaza
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Overview
• The “differential amplifier” is one of the most important circuit inventions.
• Their invention dates back to vacuum tube era (1930s.)
• Alan Dower Blumlein (a British Electronics Engineer, 1903- 1942) is regarded as the inventor
differential pair. of the vacuum-tube version of
• Differential operation offers many useful properties and is widely used in analog and mixed-signal integrated circuits
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Single-ended and Differential Signals
•A “single-ended” signal is a signal that is measured with respect to a fixed potential (typically ground.)
“ •Differential signal” is generally referred to a signal that
is measured as a difference between two nodes that have equal but opposite-phase signal excursions around a
fixed potential (the fixed potential is called common-mode
(CM) level.)
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Why Differential?
• Better immunity to environmental noise
• Improved linearity
• Higher signal swing compared to single-ended
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Higher Immunity to Noise Coupling
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Supply Noise Reduction
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Basic Differential Pair
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Basic Differential Pair
• Problem: Sensitive to input common-mode (CM) level
– Bias current of the transistors input CM level changes
M1 and M2 changes as the
– gm of the devices as well as output CM level change
• Can we think of a solution?
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Differential Pair
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Common-Mode Response
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Common-Mode Input versus Output Swing
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Example
• Using the half-circuit concept, calculate the small-signal
differential λ≠ 0.)
gain of the following circuit (for two cases of λ=0 and
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Example
• Using the half-circuit concept, calculate the small-signal
differential λ≠ 0.)
gain of the following circuit (for two cases of λ=0 and
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Example
• Sketch the small-signal gain of a differential pair as a function of
its input common-mode level.
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Analysis of Differential Amplifier
2 I D − V = V − V = + V , V VTH in1 in 2 GS1 GS 2 GS W
µnCox L
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Analysis of Differential Amplifier
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Analysis of Differential Amplifier
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Analysis of Differential Amplifier
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Analysis of Differential Amplifier
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Differential Gain
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Differential Pair as a CS and CD-CG Amplifier
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Common-Mode Response
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Example
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Common-Mode Response
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Common-Mode Response
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Common-Mode Response
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Differential Pair with MOS Loads
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MOS Loads
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MOS Loads
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Gilbert Cell
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Gilbert Cell
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Gilbert Cell
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