1Copyright Baylor University 2006 1Operational Amplifiers and Electrical SensorsDuration: Approximately 30 minutesIntroduction to Engineering 1301Copyright Baylor University 20062Dr. Steven Eisenbarth, Professor of Electrical and Computer EngineeringCopyright Baylor University 20063Overview The operational amplifier, otherwise known as the op-amp, is an circuit traditionally used for signal amplification and analog signal processing. Although op-amps can operate in an open loop circuit, it is generally used in a feedback network creating a closed loop whereby part of the output is fed back into the input.http://www.eetimes.com/anniversary/designclassics/opamp.htmlhomepages.nildram.co.ukFairchild uA709op-amp2Copyright Baylor University 20064Schematic Symbol The schematic symbol for the op-amp is shown below. It has both positive and negative inputs (V+ and V-), an output (Vout), and two leads for powering the op-amp (Vs- and Vs+). National Semiconductor LM7301Copyright Baylor University 20065The Ideal Op Amp An ideal op amp consists of the following characteristics: Infinite input impedance Infinite open loop gain for the differential signal Zero gain for the common mode signal Zero output resistance Infinite bandwidth.dB = 20 log10(Vx/Vy)Copyright Baylor University 20066Negative Feedback (Inverting) Amp1. Op-amp open-loop mode: Vout = A*(V+ minus V-)2. In closed-loop (feedback) mode V+ = V- implies that V- is 0 Volts.3. In closed-loop mode currents flowing into V- must be equal to those flowing out (since no current actually flow in/from V-.Calculating currents at V-node:V-/R2 = -Vout/R1Vout = -(R1/R2) VinZero VoltsCurrent = 03Copyright Baylor University 20067Negative Feedback (Inverting) Amp The output voltage will be inverted to that of the input voltageby a gain (A) of the ratio of R1 over R2.Output signal compared to the input signal.Inverted OutputInputCopyright Baylor University 20068Non-Inverting Amplifier Negative feedback is still present, but the input voltage is connected to the positive terminal of the Op Amp. Voltage difference between V- and V+ must be zero.1. V- = Vout * R2/(R1+R2)2. Vout = Vin * (R1+R2)/R2or3. Vout = Vin * (1 +R1/R2)Copyright Baylor University 20069Non-inverting Unity Gain Op-ampOutput signal compared to the input signal with unity gain. As the name implies, the output voltage is in phase with the input voltage, but offset is present in the output signal.VoutVin4Copyright Baylor University 200610Voltage Follower The voltage follower sometimes referred to as a buffer amplifier is an op-amp with a zero resistance negative feedback to provide unity gain (output voltage equals input voltage). The typical voltage follower is used to transfer a voltage from a circuit at a high impedance level to another circuit at a low impedance level.Voltage difference between V- and V+ must be zero. This implies: Vout = Vin.Copyright Baylor University 200611Summing Amplifier For a Summing Amplifier the input to the op-amp is composed of several voltage sources. The currents flowing into V- must be equal to those flowing out and voltage at V- must be equal to V+ which is grounded (zero volts).Summing current at V- input:V1/R1 + V2/R2 = -Vout/R31. Vout = -R3(V1/R1 + V2/R2)2. Vout = -R3/R1(V1 + V2) {special case: R1=R2}3. Vout = -(V1 + V2) {special case: R1=R2=R3}Copyright Baylor University 200612Differential Amplifier1. V+ is set by the voltage divider composed of R3 and R4.2. V- must be equal to V+ (zero voltage difference).3. Sum of currents at V- must be zero.1. V+ = V2 * R4/(R3+R4)2. V- = V+3. (V1-V+)/R1 = (V+ -Vout)/R2{if R4 = R2 and R3 = R1}4. Vout = R2/R1*(V2-V1)5Copyright Baylor University 200613Integrating Amplifier with Reset1. Zc = 1/(jωC)2. Vin/R1 = -Vout/Zc3. Vout = -(Zc/R1)*Vin4. Vout = (1/C1)∫i(t)dt + c5. Vout = (1/R1*C1)∫Vin dt1. V- must be equal to V+ (zero voltage difference).2. Sum of currents at V- must be zero.3. Voltage across C1 changes with time: V(t) = (1/C1)∫i(t)dt + V(t0).Copyright Baylor University 200614Power Supply Rails The power inputs (Vs+and Vs-) are connected to a power supply. Vs+and Vs-set the maximum value to which a signal can be amplified. An amplified signal exceeding the power supply specification is cut-off at the Vs+and Vs-values.Clipped OutputSine InputCopyright Baylor University 200615Designing Amplifiers1. Determine the specific application configuration.2. Select a suitable op-amp3. Select feedback circuit values to set the correct function and gain.4. Keep in mind specific op-amps may require additional circuitry to minimized output errors such as zero offset.  When designing an amplifier circuit using op-amps:6Copyright Baylor University 200616Example Suppose a PZT sensor produces a maximum voltage of 1.0 volt output but the output must be amplified to 10 volts to be accurately measured by a data acquisition (ADC) interface on a computer. Design an inverting amplifier to amplify the signal to 10V to make it readable to the ADC card.Figure from: http://www.williamson-labs.com/480_opam.htmCopyright Baylor University 200617Typical ApplicationsSensorAmpActuatorAmplifier/signal-conditioner/driverCopyright Baylor University 200618Electrical Sensor Types Electrical Voltage Current Resistance Impedance Conductance Frequency Electric field Magnetic field Polarization MagnetizationPicture from: http://upload.wikimedia.org/wikipedia/commons/a/a6/Digital_Multimeter_Aka.jpg7Copyright Baylor University 200619Optical Sensor Types Optical Intensity Frequency Reflectance Transmittance Scattering Interference Polarization Refractive Index ImagePicture from: http://www.subzeropromotions.co.uk/admin/images/Printed-Computer-Mouse-Home.jpgCopyright Baylor University 200620Mechanical Sensor Types Mechanical Position Displacement (linear or rotational) Velocity (linear or rotational) Acceleration (linear or rotational) Range or distance Vibration Tilt Proximity Torque Pressure Strain (linear or rotational)Art from: http://mechatron.me.wisc.edu/Courses/me601/terms/sensorterm.html#exampleCopyright Baylor University 200621Fluid Sensor Types Mass Surface Morphology Thermal Temperature Image Chemical Concentration pH Enzymes Ions Gases Fluids Density Flow Viscosity Thickness8Copyright Baylor University 200622Acoustic