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FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS EXPERIMENT 9 OPERATIONAL AMPLIFIERS 1.0 INTRODUCTION 1.1 OP AMP FUNCTION The operational amplifier, or op amp, is a high performance linear integrated circuit. It offers a high gain, high impedance and low output impedance, all important characteristics for an amplifier. Modern op amps are typically housed in 8 or 14 pin dual-in-line packs and are extremely versatile components. Many different op amps are available form a variety of manufacturers all offering different specifications and operating characteristics. All however confirm to the same basic format of two inputs, one labelled “+” and the other “-”, and one output. The symbol for an op amp is shown in Figure 1.1. Figure 1.1 The two inputs are known as inverting (-) and non-inverting (+) and under normal operating conditions, any signal applied to the inverting will undergo a 180 phase shift (i.e. it will be inverted). Figure 1.2 shows the resulting outputs of various inputs, and the effect of signals being sent to either the noninverting or inverting input. Figure 1.2 Dr. Daniel Nankoo 1 of 5 FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS The gain, or amount of amplification, is set through the use of external components, such as resistors. 1.2 NON-INVERTING AMPLIFIER A typical non-inverting amplifier is shown in Figure 1.3. RF +VS R1 + Vout -VS Vin Figure 1.3 Note how the signal is applied to the non-inverting (+) input, and that the inverting (-) input is grounded. Also note the feedback resistor RF, and the power supply connections +VS and –VS. The gain (AV), which is the ratio of the output voltage to the input voltage, for the non-inverting amplifier can be shown to be: AV 1 RF R1 or R Vout Vin 1 F R1 1.3 INVERTING AMPLIFIER A typical inverting amplifier is shown in Figure 1.4. Note how for the amplifier configured in inverting mode, the input signal is applied to the inverting (-) input via R1, and that the non-inverting input (+) is set to ground. Dr. Daniel Nankoo 2 of 5 FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS RF +VS R1 Vin + Vout -VS Figure 1.4 The gain for this amplifier is AV RF R1 or R Vout Vin F R1 1.4 OP AMP CHIP The op amp chip used for the purpose of this experiment is the UA741CN, and is shown in Figure 1.5. Its advantages are numerous, including having a large input voltage range and a high gain capability. It is a high performance operational amplifier constructed on a single silicon chip. It is intended for a wide range of analogue applications, such as a summing amplifier, voltage follower, integrator, active filter and function generator. Figure 1.5 Its pin connections, as viewed from the top of the chip are shown in Figure 1.6. Note that there is a notch or black spot on the top surface of the chip. Held with this notch or black spot pointing away from you, pin 1 can be identified as the leg which is in the top left position. The bottom left leg is thus pin 4, the bottom right is pin 5, and the top right is pin 8. Dr. Daniel Nankoo 3 of 5 FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS Figure 1.6 Pins 4 and 7 are the power terminals for the chip. These pins are not to be confused with the input pins (2 and 3). Pins 4 and 7 need to be connected to a power supply in order for the device to function, just like any other electronic device. Pin 4 needs to be connected to a negative power source, whereas pin 7 needs to be connected to a positive power source. 2.0 PROCEDURE 2.1 OPERATION AS A NON-INVERTING AMPLIFIER Connect up the op amp to function as a non-inverting amplifier, as shown in Figure 1.3. Use RF = R1 = 8.2k. Use the pin layout shown in Figure 1.6 to make the correct connections. Make sure your chip has been connected in a way so that the left and right sides occupy different halves of the breadboard, i.e. make use of the board’s central groove. Pins 4 and 7 will need to be connected to the DC power supply, which needs to be set up in order to supply a dual voltage of +5V and –5V. This is done by implemented the wiring configuration shown in Figure 2.1. Figure 2.1 Draw your circuit in your lab book, showing components values, pin connections and the configuration of the dual voltage supply. Calculate the expected value of non-inverting gain and record your results in your lab book. Dr. Daniel Nankoo 4 of 5 FOUNDATION EXP 9 – OPERATIONAL AMPLIFIERS Set the output of the function generator to produce a bipolar sine wave output of 300mV peak-to-peak, at 1 kHz. Connect the signal generator to the input of your circuit and display your input and output waveforms on the oscilloscope (using Channels 1 and 2). Measure the peak to peak voltages of VIN and VOUT from the oscilloscope display and record your results in your lab book. Use these values to calculate the actual gain of your circuits. Sketch VIN and VOUT in your lab book, and note down their respective dimensions. Turn off your circuit and replace the 8.2k resistor used for RF with a 150 k resistor. Repeat the above procedure, recording your results in your lab book. Increase the value of VIN to 700 mV. Observe the output on the oscilloscope and sketch the waveform in your lab book. Explain what has happened to the output and why. You may want to consider the DC power supply voltages when considering an explanation for what you have observed. 2.2 OPERATION AS AN INVERITNG AMPLIFIER Turn off your circuit and reconstruct it as an inverting amplifier, as shown in Figure 1.4. To begin with, use RF = R1 = 8.2k. Reset your signal generator to produce a 300 mV peak-to peak sinusoidal output and carry out the same procedures as laid out in Section 2.1. Record the relevant circuit diagrams, measurements, oscilloscope displays and calculations in your lab book. 3.0 COMMENTS AND CONCLUSIONS Comment on the discrepancies (if any) between theory and practice. Also identify where your waveforms illustrate the difference between inverting and non-inverting configurations. Finally, discuss the saturation phenomenon, and explain where your waveforms show this for positive and negative signals. Dr. Daniel Nankoo 5 of 5