ECEN326: Electronic Circuits Fall 2017 - Texas A&M University
Transcript of ECEN326: Electronic Circuits Fall 2017 - Texas A&M University
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Sam PalermoAnalog & Mixed-Signal Center
Texas A&M University
ECEN326: Electronic CircuitsFall 2017
Lecture 6: Operational Transconductance Amplifiers (OTAs)
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Announcements
• HW4 due today• HW5 due 11/1
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OpAmps and OTAs
• High voltage gain• High input impedance• Voltage source output
(low impedance)3
OpAmp OTA
• High “voltage” gain• High input impedance• Current source output
(high impedance)
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Simple OTA
• Important Parameters• Transconductance• Output Resistance• Differential Gain• Common-Mode Input Range
4
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Simple OTA Transconductance: Rigorous Analysis
5
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Simple OTA Transconductance: Informal Virtual Ground Approach
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• As the differential circuit is not purely symmetric, we cannot formally assume a virtual ground at node P
• However, if the differential pair transistors M1 and M2 have high output resistance, a virtual ground can be approximated at node P to simplify the analysis
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Simple OTA Output Resistance
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Simple OTA Differential Gain
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Simple OTA Common-Mode Input Range
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• Common-mode input range set by transistor saturation conditions• Low-end set by tail current source
saturation
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Bipolar Simple OTA
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• Following a similar procedure
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3 Current Mirror OTA
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• While Gm has increased by the current mirror factor B, the voltage gain remains the same due to the output resistance being reduced by B-1
• Common-mode input range expression remains the same as the previous simple OTA
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Bipolar 3 Current Mirror OTA w/ Degenerated Gm (Lab 7)
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• In order to improve the distortion performance, emitter resistors have been added in the input differential pair
• In order to improve the output resistance, emitter resistance has been added to all the current mirror/source transistors
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Bipolar 3 Current Mirror OTA w/ Degenerated Gm (Lab 7)
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Bipolar 3 Current Mirror OTA w/ Degenerated Gm (Lab 7)
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1112 Eeid RrR
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• Low-end Vicm set by keeping tail current source in active mode
• High-end Vicm set by keeping differential pair in active mode
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• Maximum differential input amp. for good distortion
1max, ETid RIv
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Simulating the 3 Current Mirror OTA
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• To simulate differential amplifiers, use voltage-controlled voltage sources (VCVS or “E” elements) to generate the differential input signal and monitor the current through the load resistor
Single-ended input source
2 VCVS to generate differential inputs
DC voltage source to set input DC level
Monitor I(RL)Note: This example is designed for 2X the Lab 7 Gm
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Simulating Transconductance
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Single-ended input sourceDC=0AC=1
Gain=0.5DC voltage source to set input DC level
• Set Inputs E1 Gain=0.5 and E2 Gain=-0.5• With input source AC=1, simply plot the load resistor current I(RL)
to get the transconductance
Gain=-0.5
Monitor I(RL)
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Simulating Transconductance
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• Gm = 2.01mA/V
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Simulating Differential Rind
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• The differential input resistance is equivalent to the differential input (Vi) divided by the input current, where I use the base current of Q1 or IB(Q1)
• Rind = 159k
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Simulating THD
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• Set input source to differential input amplitude spec • For Lab 7, that is 2V
• Check the THD at 3 different common-mode points• 0V, VCM,min, VCM,max
VCM=0V
VCM=-2V
VCM=2V
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Simulating Output Resistance
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Ground input source
• Ground input source and apply an AC-coupled voltage stimulus at output• With output source AC=1, plot the ratio of V(Vout) over the current
through the coupling capacitorOutput stimulus
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Simulating Ro
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• The output resistance is equivalent to the output stimulus V(Vout) divided by the output current, which is equal to the current through the output capacitor I(C5)
• Ro = 25.4k