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Download Op Amp Circuits - Keith E. Holbert
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Op Amp Circuits Dr. Holbert February 13, 2008 Lect9 EEE 202 1 Digital Meters and Oscilloscopes • Most multimeters and oscilloscopes are now digital • A digital multimeter or a digital oscilloscope has an analog-to-digital (A/D) converter • Most digital meters and all digital oscilloscopes have one or more processors Lect9 EEE 202 2 Data Acquisition Systems • In many applications, digital meters and scopes are being replaced by data acquisition cards that fit into a computer • The data acquisition cards have A/D converters • The computer provides processing and storage for the data Lect9 EEE 202 3 A Generic Digital Meter Input Switching and Ranging A/D Converter Amplifier Display Lect9 Processor EEE 202 4 Voltage Measurements 100V 10V 1V Hi Lect9 EEE 202 Com 5 Model for Meter Hi 10MW Ideal Meter Com The ideal meter measures the voltage across its inputs. No current flows into the ideal meter; it has infinite input resistance Lect9 EEE 202 6 Meter Loading Hi R 10MW Ideal Meter Com The 10MW meter resistance in parallel with R may change the voltage that you measure Lect9 EEE 202 7 Loading • When measuring the voltage across R, we need to make sure that R is much less than 10 MW • If R is close to 10 MW, significant current flows through the meter, changing the voltage across R Lect9 EEE 202 8 Loading Example Hi 50mA 2MW 10MW Ideal Meter Com • Without Meter: voltage is 100 V • With Meter: measured voltage is 83.3 V Lect9 EEE 202 9 Current Measurements 100V 10V 1V Com Amp Lect9 EEE 202 10 Measuring Large Currents (> 100 mA) • The current to be measured is passed through a small resistor (called a shunt resistor) and the resulting voltage across the shunt resistor is measured • From the voltage, the current can be computed Lect9 EEE 202 11 Meter Loading Amp R Rs Ideal Meter Com The Rs shunt resistance in series with R may change the current that you measure Lect9 EEE 202 12 The Voltage Follower + – vin + – + vout – Lect9 EEE 202 13 Without a Voltage Follower Rs Sensor vs + – + vA/D RA/D A/D Converter – vA/D is not equal to vs Lect9 EEE 202 14 Op-Amp Review • The ideal op-amp model leads to the following conditions: i+ = i– = 0 v+ = v– • The op amp will set the output voltage to whatever value results in the same voltages at the inputs Lect9 EEE 202 15 Op-Amp Review • To solve an op-amp circuit, we usually apply KCL (nodal analysis) at one or both of the inputs • We then invoke the consequences of the ideal model • We solve for the op-amp output voltage Lect9 EEE 202 16 With a Voltage Follower + vs + – Rs – + vA/D RA/D – Sensor vA/D is equal to vs Lect9 EEE 202 A/D Converter 17 An Integrator C R – Vin Lect9 + – + EEE 202 + Vout – 18 KCL at the Inverting Input vin (t ) v vin (t ) iR (t ) R R C iC(t) R i 0 iR(t) i– vin(t) + – – + + vout(t) – d vout (t ) v dvout (t ) iC (t ) C C dt dt Lect9 EEE 202 19 Solve for vout(t) • From the KCL: iR (t ) iC (t ) i 0 vin (t ) dvout (t ) C 0 R dt dvout (t ) vin (t ) dt RC t vin ( x) vout (t ) dx RC Lect9 • Hence, the output voltage is equal to the time integration of the input voltage—an electronic method of integrating • Now, if we could only make a differentiator EEE 202 20