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Electrical & Computer Engineering
Lab#1: Operational Amplifier Performance
Preparations
Read chapter 2: ”operational Amplifiers” of text book. Refer to this material for background and theory for the various measurements to be performed in this lab. You will be using a 741-type Operational Amplifier IC, made by one of various companies including National Semiconductor, Fairchild and Texas Instruments. Familiarize yourself with the electrical characteristics and the performance curves. You will be using the eight pin version, the so called ”Minidip”
1. Inverting Amplifiers
Design an amplifier stage with:
- Gain = −50 Input Impedance = 2k
- Gain = −5 Input Impedance = 100k
Using the circuit configuration shown below:
2. Non-Inverting Amplifiers
Design an amplifier stage with:
- Gain = 2 smallestresistor = 10kΩ
- Gain = 200 smallestresistor = 1kΩ
Using the circuit configuration shown below:
3. Frequency Response
- Design an inverting operational amplifier with:
Gain = −100 Input impedance = 1KΩ
- From the theory in the text book and from the ”Open Loop Voltage Amplification vs. Frequency” performance curve in the data sheets, DRAW the expected frequency response curve from 10Hz to 1MHz.
- Carry out PSPICE simulation of your design.
4. Common Mode Rejection Ratio (CMRR)
Based on the test circuit given below, consider the procedure which you will use in order to determine CMRR, where in decibels CMRR = 20log(CMRR). Note that:
6. Input Bias Current - IB
Explain briefly how the test circuit shown below can measure the input bias current to the non-inverting input of the operational amplifier.
An equation that expresses the output voltage of the Op-Amp with respect to the total effects of the offset voltage and bias current is given below:
where IB1 and IB2 are the bias currents into the − and + terminal, respectively. As you can see, the test circuit is designed to determine IB2, because RF ≪ R2*(1 +RF/R1). Write the expression for IB2, using this simplifying condition. What is the expected output voltage from the test circuit based on the manufacturer’s data for Input Bias Current when Vos = 0? The input offset current, Ios is defined as:
Show that for R2 = R1 || RF,
7. Input Offset Current - Ios
Modify the test circuit so that Rf = 1MΩ but the gain is unchanged (Av = 11). Choose R2 = Rf || R1. Assuming Vos = 0, what is the expected output voltage from the test circuit based upon the manufacturer’s data?
Experiments
Build the operational amplifier stages on the laboratory breadboards. Use +15V and −15V supply voltages. Check that the actual supply voltages are within 1% of the nominal value.
1. Inverting Amplifiers
Perform the gain measurements at 500Hz. Measure the resistance of the input and the feedback resistor and calculate the gain based on their values.
2. Non-inverting Amplifiers
Perform the gain measurements at 500Hz. Also here measure the resistance of the input and the feedback resistor and calculate gain.
3. Frequency Response
With constant input signal, measure Vo as a function of frequency over a frequency range from 100Hz to 1MHz for gain = −100. Plot the results.
4. Measurements of Common Mode Rejection Ratio-CMRR
Match the two 10kΩ and the two 470kΩ resistors if possible. At 500Hz measure ACM = Vo/Vs and determine CMRR. The waveform averaging function on the scope may be useful for improving the signal to noise ratio when measuring the very low level Vo signal. Replace the 470kΩ resistor to ground with a 1MΩ potentiometer. Adjust the potentiometer for minimum Vo (maximum CMRR), measure this Vo value and calculate the new CMRR value. Compare with the typical and minimum value given in the data sheets.
5. Measurement of the Offset Voltage – Vos
