Download - Lecture 4 Differential Amplifiers
Dr. Cuong HuynhTelecommunications DepartmentHCMUT
CMOS ANALOG IC DESIGN Spring 2013
1
Dr. Cuong Huynh [email protected]
Department of Telecommunications
Faculty of Electrical and Electronics Engineering
Ho Chi Minh city University of Technology
Dr. Cuong HuynhTelecommunications DepartmentHCMUT
CMOS ANALOG IC DESIGN Spring 2013
2
Dr. Cuong Huynh [email protected]
Department of Telecommunications
Faculty of Electrical and Electronics Engineering
Ho Chi Minh city University of Technology
Lecture 4: Differential Amplifiers
• A single-ended signal is measured with respect to a fixed potential (ground)
• A differential signal is measured between two equal and opposite signals which swing around a fixed potential (common-mode level)
• You can decompose differential signals into a differential mode (difference) and a common-mode (average
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• Differential signaling advantages
• Common-mode noise rejection
• Higher (ideally double) potential output swing
• Simpler biasing
• Improved linearity
• Main disadvantage is area, which is roughly double
• Although, to get the same performance in single-ended designs, we often have to increase the area dramatically
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Common-Mode Level Sensitivity
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• A design which uses two single-ended amplifiers to realize a differential amplifier is very sensitive to the common-mode input level
• The transistors’ bias current and transconductance can vary dramatically with the common-mode input
• Impacts small-signal gain
• Changes the output common-mode, which impacts the maximum output swing
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• An improved differential amplifier topology utilizes a “tail” current source to keep the transistor bias current ideally constant over the common-mode input range
• Allows for a constant small-signal gain and output common-mode level
• Note, you still have to have keep the input pair and tail current source transistors in saturation
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• For large-signal differential inputs, the maximum output levels are well defined and ideally independent of the input common-mode
• For small-signal differential inputs, the small-signal gain is maximum at low-input signal levels
• As the differential input level increases, the circuit becomes more nonlinear and the gain decreases
Differential Pair I-V Characteristics
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• The differential current is an odd function of the differential input voltage which increases linearly for small inputs
• For large differential input voltages, the output differential current compresses due to the sqrt term
• The differential output current maxes out when all the current flows through one transistor at Vin1
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• The differential output current will saturate if the differential input voltage exceeds sqrt(2) times the equilibrium input overdrive voltage
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• The differential pair transconductance and gain is maximum near zero input differential voltage
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• The X output from Vin1 is modeled as a degenerated CS amplifier
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• The Y output from Vin1 is modeled as a Thevenin equivalent driving a CG amplifier
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• The symmetric differential pair can be modeled as a Thevenin equivalent to observe how the tail node P changes with the differential input signal
• If RT1=RT2 and the input is a truly differential signal, node P remains constant
• This allows the tail node to be treated as a “virtual ground”
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Differential Pair Small-Signal AnalysisMethod 2 – Half Circuit
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Differential Pair Common-Mode Response
• Ideally, a differential amplifier completely rejects common-mode signals, i.e. Av,CM=0
• In reality, the finite tail current source impedance results in a finite common-mode gain
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Differential Pair with Diode Loads
• While the gain of this amplifier is relatively small, it is somewhat predictable, as it is defined by the ratio of the transistor sizes and the n/p mobility
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• While the gain of this amplifier is higher, it is somewhat unpredictable, as it is defined by the transistor output resistance, which changes dramatically with process variations
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Differential Pair w/ Diode & Parallel Current-Source Loads
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• Using a cascode differential pair and cascode current-source loads allows for aconsiderable increase in gain
• However, a relatively large power supply may be required to supply the necessary voltage “headroom” to keep all the transistors in saturation