# Lect9EEE 2021 Op Amp Circuits Dr. Holbert February 13, 2008.

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Lect9EEE 2021 Op Amp Circuits Dr. Holbert February 13, 2008

Lect9EEE 2022 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

Lect9EEE 2023 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

Lect9EEE 2024 A Generic Digital Meter Input Switching and Ranging Amplifier A/D Converter ProcessorDisplay

Lect9EEE 2025 Voltage Measurements HiCom 10V 1V 100V

Lect9EEE 2026 Model for Meter The ideal meter measures the voltage across its inputs. No current flows into the ideal meter; it has infinite input resistance 10M  Ideal Meter Hi Com

Lect9EEE 2027 10M  Ideal Meter Hi Com R Meter Loading The 10M  meter resistance in parallel with R may change the voltage that you measure

Lect9EEE 2028 Loading When measuring the voltage across R, we need to make sure that R is much less than 10 M  If R is close to 10 M , significant current flows through the meter, changing the voltage across R

Lect9EEE 2029 Loading Example Without Meter: voltage is 100 V With Meter: measured voltage is 83.3 V 10M  Ideal Meter Hi Com 2M  50  A

Lect9EEE 20210 Current Measurements AmpCom 10V 1V 100V

Lect9EEE 20211 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

Lect9EEE 20212 RsRs Ideal Meter Amp Com R Meter Loading The R s shunt resistance in series with R may change the current that you measure

Lect9EEE 20213 The Voltage Follower v in + – v out + – +–+–

Lect9EEE 20214 Without a Voltage Follower v A/D is not equal to v s vsvs RsRs R A/D + – v A/D Sensor A/D Converter +–+–

Lect9EEE 20215 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

Lect9EEE 20216 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

Lect9EEE 20217 With a Voltage Follower v A/D is equal to v s vsvs RsRs Sensor R A/D + – v A/D A/D Converter + – +–+–

Lect9EEE 20218 An Integrator – + V in + – V out R C +–+–

Lect9EEE 20219 KCL at the Inverting Input – + v in (t) + – R C v out (t) iR(t)iR(t) iC(t)iC(t) i–i– +–+–

Lect9EEE 20220 Solve for v out (t) From the KCL: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

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