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ECE 2006 University of Minnesota Duluth Lab 6 ECE Department Page 1 March 5/7, 2007 Operational Amplifiers: Part II 1. Introduction The name "operational amplifier" comes from this amplifier's ability to perform mathematical operations. Three good examples of this are the summing amplifier, the differentiator, and the integrator. In this experiment, the student will learn how to build a summing amplifier, a differentiator and an integrator using operational amplifiers and then design a final circuit that uses a combination of them. The Op Amp used for the experiment is the LM741. The diagram for the LM741 is shown in Figure 1. Figure 1: LM741 Pinout Diagram 2. Procedure 2.1. Equipment • Tektronix TDS 3012B Digital Phosphor Oscilloscope • Agilent E3631A DC Power Supply • Agilent 33120A Waveform Generator • Fluke 8050A Digital MultiMeter (DMM) • Resistors, Capacitors, and LM741 Op Amps as Needed 2.2. Summing Amplifier The summing amplifier can be used as an audio mixer. For that application, it allows the circuit to add waveforms (sounds) from different channels (vocals, instruments) together before sending the combined signal to a recorder. Figure 2 shows the circuit of a summing amplifier. In this circuit the resistor R3 is used to limit the current through the op amp (what would the current be through R3 in theory for an ideal Op Amp?). 2.2.1. PRELAB Theoretical Procedure Using the summing amplifier equations given in Figure 2, compute the output voltage for the circuit. Assume RA = RB = Rc= 1 kΩ, R1 = R2 = R =10 kΩ, R3 = 1 kΩ, +VCC = 10 V, -VCC = -10 V, and Vs = +5 V DC. Record your result in Table 1.ECE 2006 University of Minnesota Duluth Lab 6 ECE Department Page 2 March 5/7, 2007 Figure 2: Summing Amplifier 2.2.2. Experimental Procedure Connect the Op Amp circuit shown in Figure 2. Use the same voltages and resistances as listed above for the theoretical procedure. Turn ON the output of the power supply. Measure Vin1, Vin2 and Vo with the DMM. Record these values in Table 1. Turn OFF the output of the power supply. Set the function generator to output a 5 Volts Peak-to-Peak (VPP), sinusoidal wave with a frequency of 1 kHz (Vs = 5 VPP, f = 1KHz). Turn ON the output of the power supply. Connect the function generator as the voltage source Vs to your summing amplifier. Using the oscilloscope measure, the input signals Vin1, Vin2, and the output signal Vo. Measure the voltage and frequency of the three waveforms. Record these values in Table 2 and save snapshots of Vo with each of the other two signals on the display (one at a time). Obtain your instructor’s signature for the oscilloscope display. IMPORTANT: • NEVER APPLY AN INPUT SIGNAL WHEN THE POWER SUPPLY IS SWIITCHED OFF. • THE INPUT VOLTAGE SHOULD NOT EXCEED THE SUPPLY VOLTAGE. 2.3. The Differentiator The differentiation is useful for obtaining velocity measurements from a signal representing a position or determining a signal's frequency. Figure 2a shows an ideal Op Amp differentiator with an input-output relationship that is theoretically correct, but has practical implementation issues. Analyzing the circuit in Fig. 2a, we see that since the input circuit element is a capacitor, this circuit will only allow AC signal components and will block DC signal components. The faster and larger the change in input voltage is, the greater the input current,ECE 2006 University of Minnesota Duluth Lab 6 ECE Department Page 3 March 5/7, 2007 therefore the greater the output voltage in response. Since the output voltage will reflect the rate of change of the input, this circuit will indeed perform differentiation. The general equation for the output voltage is shown in equation (1). (1) The ideal Op Amp differentiator is not used in real applications. The basic reason for this is that high-frequency noise signals will not be suppressed by this circuit; instead, they will be amplified far beyond the amplification of the desired signal. In some applications, it may be possible to add a series input resistor, as shown in Fig. 2b. This limits the high frequency gain of the circuit to the ratio R /Rin. The low frequency gain is still set by R and C, as before. The cutoff frequency, where these two effects meet, is determined by Rin and C, according to the expression: fco = (1/2 ) RinC. 2.3.1. PRELAB Theoretical Procedure On a sheet of paper, sketch the derivative of the following waveforms: a) sinusoidal waveform b) triangular waveform c) rectangular waveform Include these sketches in your write-up (you may want to reformat them). 2.3.2. Experimental Procedure Connect the differentiator shown in Fig. 2b. Use the following values for the components and voltages: R = 10 kΩ, Rin = 470 Ω, C = 0.022 µF, +VCC = 10 V, -VCC = -10 VECE 2006 University of Minnesota Duluth Lab 6 ECE Department Page 4 March 5/7, 2007 Set the function generator to provide the following input signals: a. Vin = 2.5 VPP, 2 KHz, sine wave b. Vin = 2.5 VPP, 2 KHz, square wave c. Vin = 2.5 VPP, 2 KHz, triangular wave Turn ON the output of the power supply to provide the +10V and -10V to the Op Amp. Connect the signal generator to the input terminal Vin of your differentiator. Measure the voltage and frequency of the output waveforms. Repeat this for each of the inputs and record these values in Table 3 and save snapshots of Vo with the corresponding input for each of the three cases. Obtain your instructor’s signature for the oscilloscope display. IMPORTANT: • NEVER APPLY AN INPUT SIGNAL WHEN THE POWER SUPPLY IS SWIITCHED OFF. • THE INPUT VOLTAGE SHOULD NOT EXCEED THE SUPPLY VOLTAGE. 2.4. The Integrator In Fig. 3a, the feedback element is a capacitor. This circuit is an ideal op-amp integrator with input-output relationship that is theoretically correct, but again has practical implementation issues. Observe that any feedback current must be based on a change in output voltage. As feedback current flows, the capacitor will gain an electric charge, which will change according to the cumulative effects of the output signal. If the input voltage is zero, no input current will flow. Therefore no feedback current


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