2.996/6.971 Biomedical Devices Design Laboratory Lecture 7: OpAmpsFundamental Circuit: Source and LoadIdeal AmplifierOperational AmplifierNon-inverting AmplifierInverting AmplifierOp Amp CircuitsFeedback T-NetworkDifferential AmplifierAccurate Peak DetectorOp Amp PackagesVoltage OffsetChopper Stablized Op AmpInput Bias CurrentUltra Low Input Bias Current: OPA129Op Amp NoiseOPA374 Voltage Noise SpectrumLT1792 Voltage Noise SpectrumSlew RateOp Amp in FeedbackReview of Feedback SystemsOp Amp Frequency ResponseReview of 2nd Order SystemsDriving Capacitive LoadsCommon Mode Rejection Ratio Power Supply Rejection RatioMore on the Power Supply: The Op Amp is a 5 Terminal Device!Rail-to-Rail (R-R) Op AmpsCross-over DistortionCheck List for Selecting Op AmpsOp Amps Types (Industry Jargon)2.996/6.971 Biomedical Devices Design LaboratoryLecture 7: OpAmpsInstructor: Dr. Hong MaOct. 3, 2007Fundamental Circuit: Source and Load• Optimize for Voltage: ZLOAD>> ZSOURCE• Optimize for Current: ZLOAD<< ZSOURCE• Optimize for Power: ZLOAD= ZSOURCE• Amplifier / active circuit – impedance transformSourcesPower supplySignal GeneratorSensorAmplifier outputLoadsActuatorMeasurement deviceAmplifier inputIdeal Amplifier• High input impedance• Low output impedance• Accurate and stable gainOperational Amplifier• High input impedance• Low output impedance• Very high gain Æ Trade gain for accuracyNon-inverting Amplifier• High input impedanceInverting Amplifier• Input impedance defined by R1• Can be used for current input• Can be used as an adderOp Amp Circuits• Level shifters• Simple filtersFeedback T-Network• Measuring small currents• Use Thevenin-Norton to analyze more complex feedback networksDifferential Amplifier• Differential Amplifier• Instrumentation Amp (Buffered differential amp)• 2-op amp instrumentation ampAccurate Peak Detector• Also: Op Amp power driverOp Amp PackagesExamples:• OPA374• LT1792Voltage Offset• Range: ≈ 1mV or less• Magnified by the gainChopper Stablized Op Amp• Periodically calibrate the offset using switches• More noisy than standard op amps by ~5x• Some sacrifice in speed, performance, and costImage and text removed due to copyright restrictions.Information sheet for Linear Technology LTC1051/LTC1053 dual/quad zero-drift op amps.Input Bias Current• Mismatched input Æ input offset current• Bipolar input op amps– Can be quite large Æ >1nA– Match input impedance to reduce error• CMOS or J-FET input op amps– 100fA to 1nA– Match input impedance is not necessary• Increases with increasing temperature• How to address: keep bias impedances low• Input bias current important for measuring small current levelsUltra Low Input Bias Current: OPA129• ±100 fA maximum• Intended Purpose:– Capacitive sensing– Photodiode preampConventional Pin-OutOPA129 Pin-OutImage and text removed due to copyright restrictions.Information sheet for Burr-Brown OPA129 op amp.Op Amp Noise• Frequency dependent– 1/f characteristic at low frequencies– Devices specify 1/f corner• Specified in µV over some frequency range• Or as spectral density (nV / Hz0.5)• Include both noise voltage and noise current• Analysis is the same as the offsetsOPA374 Voltage Noise SpectrumLT1792 Voltage Noise Spectrum• Much lower 1/f corner!• Trade-off:– OPA374 input capacitance ~ 3pF– LT1792 input capacitance ~ 27pFSlew Rate• Example 1: Multiplexed input• Example 2: RectifierOp Amp in Feedback• Rules:– Inputs draw no current– Output will do whatever is necessary to make the voltage difference between the inputs zeroReview of Feedback SystemsOp Amp Frequency Response• Open-loop frequency response• Gain-Bandwidth (GBW) productReview of 2ndOrder Systems• If loop gain has 2 poles or more:– 180° phase shift turns negative FB into positive FB– potential for instability• Phase margin (180° - loop phase shift when gain=1)– Settling time– Overshoot– Possibility for oscillation• Rule of thumb for phase margin– aim for 60°– minimum 45°• Check datasheets for PM at various gainsDriving Capacitive Loads• Possible instability when driving purely capacitive loads• Cause: op amp output resistance ~ 20ΩCommon Mode Rejection RatioPower Supply Rejection Ratio• CMRR = ADIFF/ ACM• PSRR = VCC/ VOSMore on the Power Supply:The Op Amp is a 5 Terminal Device!• Output cannot exceed the power supply!• Traditional op amps need 1-2V head room• Dual supply vs. Single supply• ‘Single-supply op amp’ Æ accept input down to Vs-Rail-to-Rail (R-R) Op Amps• Overused industry buzzword• Can be R-R input or R-R output or both• R-R output = low output impedance near supplies• ‘Single-supply op amp’ = half of a R-R input op amp• R-R input op amps Æ cross-over distortionCross-over Distortion• Bias current has a similar error• Remedy: Avoid cross-over distortion• OPA386 R-R op amp with no cross-over distortionCheck List for Selecting Op Amps1. Power supply: range, rail-to-rail2. Gain-Bandwidth3. Cost4. Voltage offset5. Stability at the intended gain, settling time6. Output current7. Noise8. Special functions: e.g. shut-down pinOp Amps Types (Industry Jargon)• Precision Amplifier - Could be any of the following:– Low offset– Low input bias current– Low noise• Zero-drift amplifier Æ chopper stablized, poor noise•Low power Æ Low bandwidth• Video Amps Æ High speed, poor DC characteristics•Audio Amps Æ Low distortion, poor DC characteristics• Current Feedback Amps Æ High speed, poor DC, cannot be used as a conventional voltage FB amplifier• Differential / Instrumentation amplifiers• High voltage / high