Exercise 1: Examination of Op amp Data Sheets and Their Parameters

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1 Villanova University ECE 053 Fundamentals of Electrical Engineering I Lab Fall 00 8 Op Amp Circuits Introduction The op amp, which stands for operational amplifier, is a general-purpose feedback amplifier that used vacuum tubes in the early 940 s. It has evolved to its present form as an integrated circuit (IC). The op amp is used in applications where amplification, integration, summation, or wave shaping are required. The op amp has two input terminals and one output terminal as shown in this schematic symbol. Inverting input Noninverting input - + OPAMP OUT U A signal connected to the upper input terminal (marked with a ) is increased (amplified), and leaves the output terminal, inverted. The input terminal is designated the inverting input. On the other hand, a signal connected to the lower input terminal (marked with a +) is amplified but is not inverted. Thus the + input terminal is designated the noninverting input. The essential characteristics of an op amp are: - very high voltage gain, of the order, 0 5 to 0 7 volts/volt, - high input resistance, e.g., 00 k, - low output resistance, typically 0. In linear applications, the op amp output signal (or portion of it) is fed back to the inverting input. This results in what is called negative feedback. Learning Objectives At the end of this experiment, you should be able to:. Wire a positive and a negative dc supply to the op amp.. Wire an inverting amplifier, a noninverting amplifier, an integrator, and a summing amplifier and measure their basic characteristics. Exercise : Examination of Op amp Data Sheets and Their Parameters We will consider the LM74 op amp. LM stands for linear and monolithic. This means that the LM74 is a linear circuit (as opposed to a digital logic circuit) and it is made on one chip, i.e., monolithic. You will examine the manufacturer s data sheet and determine some important parameters. Note: All production processes have variability. Integrated circuits are less expensive to produce if their parameters are not required to be precise. For example, the data sheet may show minimum values, typical values, and maximum values; these represent a range within which a good batch of parts will lie. For example, consider the input offset voltage parameter of an op amp. It is specified to lie between.0 mv and 5.0 mv, with a typical value of.0 mv. Any part whose offset voltage parameter is outside this range is considered a bad part and would be discarded. 8 Op Amp Circuits nov 0.doc

2 Note that not all of the parameters on the data sheet have minimum, maximum, or typical values.. Examine the data sheets for the LM74op amp. Go to the web site of the National Semiconductor Corporation at its URL Look for the search box; type LM74 and press Search. Click on LM74 details and download the pdf file.. Record the minimum value, the typical value, and the maximum value for the following items (not all parameters have these three values): (a) The open loop gain (identified as "Large Signal Voltage Gain"). Record the gain in volts/millivolts (V/mV) and in volts/volt (V/V). (b) The input resistance. (c) The maximum output current (identified as "Output Short Circuit Current"). (d) The input bias current. Exercise : Wiring the +/ DC Power Supplies to the Op amp. Refer to Figure A- in the Appendix. Notice how the +/ 5-V power supply (PS) is wired to the breadboard, and then to the op amp.. Place your proto board (PB) in landscape orientation as shown. Connect a green wire from the COM terminal of PS to the PB at the bottom row indicated by a +; this is your ground bus or reference point. 3. Connect a red wire from the + terminal of the PS to the top (indicated as row +). This is your positive bus or positive rail. 4. Now connect a black wire from the terminal of the PS to the bottom (indicated as row. This is your negative bus or negative rail. 5. Set the two sources to volts and the current limits to 5 ma. 6. Obtain a 74 op amp and examine the top of the IC with the pins pointing away from you. Look for a small dot or indentation (refer to Figure A-). This identifies the location of pin. The other pin numbers go in sequence counter clockwise around the IC. 7. Place the IC over the gutter (slot) as shown in Figure A- and push in, gently. 8. The last bit of wiring is to connect the dc power on the PB to the LM74. Connect a short black wire from the negative rail to pin 4 of the IC, and connect a short red wire from the positive rail to pin 7 of the IC. 9. Turn on the power supply. Connect the black clip lead of your digital voltmeter (DVM) to the ground bus and leave it there. Verify that the voltages at pins 4 and 7 are correct. Then turn off the power.

3 7 4 V+ V- Exercise 3: Inverting Amplifier Figure is an inverting amplifier circuit. It consists of an op amp, an input resistor R and a feedback resistor R f. Pin is the inverting input, pin 3 is the noninverting input, and pin 6 is the output. The FG supplies a signal, V i, to the amplifier. The output voltage V o is an amplified and inverted version of the input voltage V i. An analysis shows that the output voltage equals the input voltage multiplied by the voltage gain, G v. In equation form, V o = G v V i Rf k V Vdc Vfg 500mVac-max 0Vdc Vi R k U3 - OS OUT 3 + OS ua Vo V Vdc Figure Inverting amplifier. where G v = V o / V i = R f / R () The equivalent input resistance, R in, of the amplifier is given by R in = V i / I i It can be shown that R in equals the value of resistor R, that is R in = R (). Design an inverting amplifier that has an input resistance of at least 4 kω, with a voltage gain of G v = 0 V/V. Select resistors in the range of 4.7 kω to 8 kω. Measure and record their resistance.. Look at the layout in Figure A-, and wire your circuit accordingly (do not use the resistor values shown). Note that Pin 3 goes to the ground bus. 3. Set the FG for a 500 mv peak sine wave signal at 00 Hz with zero offset. Connect the scope probes to display the input and output signals. Don t forget the scope ground clip. Hint: Set the FG to display the correct p-p voltage readout refer to Exp. 6 (Wheatstone Bridge) for details. 4. The input and output waveforms should be sinusoidal and 80 degrees out of phase. If everything is ok, record the peak values of the signals. Determine the voltage gain G v from the measurements. Compare this measured value with the value predicted by Equation (). 3

4 5. Slowly increase the FG amplitude to volts peak as seen on the scope (press Autoscale often). Observe how the output waveform starts to flatten out on top and bottom the output has reached saturation. Record the input and output waveforms, noting the +/ saturation voltages. 6. Change R to 680 Ω and R f to 0 kω. Readjust the amplitude of the input signal, so the output is not saturated. Record the peak values of the signals and determine the voltage gain. 7. Reset the input signal to a triangular wave and then to a square wave. Exercise 4: Integrator If the feedback resistor in the inverting amplifier is replaced with a capacitor, C, the resulting circuit becomes an integrator, i.e., the output v o (t) is the integral of v i (t) multiplied by the gain G v = /RC.. Select a 0. μf capacitor (it is very small, rectangular, and yellow and is labeled 4). Put this in place of the 0 kω feedback resistor. Put the 0 kω resistor in place of the 680 Ω resistor.. Set the FG for a 00 Hz, zero offset, square wave of.5 volts peak (as seen on the scope). Display the input and output waveforms. The output should be a triangle waveform. Notice that the output has a dc value. Record the peak-topeak (p-p) value and dc value of both input and output signals. If the output waveform drifts up or down, place a 00 kω across the capacitor. 3. Set the input to a sine wave. The output will be a sine wave but with phase shift. Record the p-p value and the dc value of the output. 4. Measure and record the phase shift of the output relative to the input. 5. Set the input to a triangle wave. The output appears to be sinusoidal. Is it? Finally try a ramp wave. 6. Remove the capacitor and turn off the dc power. Exercise 5: Noninverting Amplifier Figure is a noninverting amplifier. The dc supplies are omitted for convenience. The output voltage is an amplified version of the input voltage but without inversion. Analysis shows that the output voltage, V o, equals the input voltage, V i, multiplied by the voltage gain, G v, i.e., V o = G v V i where, G v = V o / V i = + (R f / R ) (3) 4

5 4 V- V+ 7. Use the same resistor values of R = 680 Ω and R f = 0 k Ω (do not use the resistor values shown on the schematic) for this amplifier.. In order to wire this circuit you will only make minor changes to the wiring of your existing inverting amplifier. Follow the instructions below. Vfg 500mVac-max 0Vdc Vi OS OUT OS U ua Vo Rf k R k Figure Noninverting amplifier. 3. Remove the wire, which comes out of the red terminal of the FG, from R. Remove the ground wire at pin 3. Connect the left side of R to ground. And connect the wire of the FG to pin 3. This completes the wiring. 4. Set the FG for a 500 mv peak sine wave signal at 00 Hz with zero offset. Turn on the dc power and connect the scope probes to display the input and output signals. The input and output waveforms should be sinusoidal and in phase. If everything is ok, record the peak values of the signals. Determine the voltage gain G v from the measurements, and compare this measured value with the value in Equation (3). 5. Connect a resistor, R s, equal to 6.8 k between pin 3 and the FG output wire. To see the effect of R s, get a piece of wire and jumper the resistor intermittently. Is there any difference in the output waveform? What is the explanation for this? Exercise 6: Summing Amplifier Figure 3 shows the amplifier. The output voltage is an inverted sum of the two inputs. Analysis shows that the output voltage, V o, is given by V o = G v V i + G v V i (4a) where G v = R f / R G v = R f / R (4b). Use the following resistor vales: R = 680 Ω, R = kω and R f = 0 k Ω (do not use the resistor values shown on the schematic). 5

6 7 4 V+ V- R k Rf k V Vdc Vsync Vac 0Vdc Vfg 500mVac-max 0Vdc R k U3-3 + ua74 OS OUT OS 6 5 Vo V Vdc Figure 3 Summing amplifier.. Set the FG output to a 500 mv peak sine wave at 00 Hz with zero offset. The FG sync output signal is a square wave with minimum value of 0 V and maximum value of about 3 V. 3. Record the two input waveforms (noting their phase relations) and the output waveform. 6

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