L07 lecture - Operational Transconductance Amplifiers OTA...

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EECS 240 Lecture 7: OTA © 2003 B. Boser 1 Operational Transconductance Amplifiers OTA versus OpAmp, Applications Differential versus Single-Ended Output Characteristics Frequency response, settling time, stability Open-loop gain Noise, dynamic range PSRR, CMRR Common-mode feedback circuit Topologies Single-stage (telescopic, folded cascode, …) Multi-stage, Miller compensation Class A, A/B Design EECS 240 Lecture 7: OTA © 2003 B. Boser 2 OpAmp versus OTA OpAmp Voltage source output (low impedance) Essential to drive resistive loads Essentially OTA + buffer Buffer increases power dissipation, noise OTA Current source output (high impedance) Cannot drive resistive loads Use capacitive (SC) feedback Transistors are transconductors
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EECS 240 Lecture 7: OTA © 2003 B. Boser 3 Resistive Feedback OpAmp Gain independent of feedback network Feedback network adds noise OTA Resistive feedback network lowers loop gain Large feedback resistors? Large area Parasitic poles Æ stability? Solution: capacitive feedback Needs initialization Needs clock Æ Linear, time-variant circuit kT/C noise EECS 240 Lecture 7: OTA © 2003 B. Boser 4 OpAmp versus OTA Noise Opamp and switch noise add OTA contributes no excess noise (actual designs can increase noise) + = switch noise 2 R 1 R C kT v oT C kT v oT = 2 Rswitch C 4kT / Gm 4kT Rswitch Rswitch C Gm Rswitch C 4kT (1/Gm + Rswitch) 1/Gm + Rswitch C 1 Gm
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EECS 240 Lecture 7: OTA © 2003 B. Boser 5 SC Gain Stage
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