Operational Amplifiers

Transcription

1 Objective Operational Amplifiers Understand the basics and general concepts of operational amplifier (op amp) function. Build and observe output of a comparator and an amplifier (inverting amplifier). Equipment Introduction Oscilloscope Signal generator Power supplies Digital Multimeter Use a op amp esistors An op amp can be used to amplify signals and build circuits that perform mathematical operations such as addition or integration. Op amps are typically manufactured in an integrated circuit (IC), sometimes called a chip or microchip. Each op amp chip has several metal conductors, called pins, that enable connections to the op amp terminals. Op Amp Basics The circuit symbol for an op amp is shown in Figure 1. The two terminals on the left are the inverting () and non-inverting () input terminals. The terminal on the right is the output terminal. The terminals on the top and bottom are the power supply terminals. Many op amps, including the 741 op amp used in this lab, require both a positive and negative power supply. Although op amps must be powered to function properly, the power supplies are not always shown explicitly in circuit diagrams. s s Figure 1: Op amp circuit symbol. Note that the voltages,, out, s, and s are voltages with respect to the common ground (called as Node oltage). The ground reference symbol should always appear in op amp circuit diagrams to indicate which terminals are to be grounded.

2 There are many different op amps made by many different manufacturers. In this lab, we will be using the 741 op amp (which itself has several different manufacturers). It is packaged in what is called a chip or microchip. Inside the chip is a thin layer called an integrated circuit (IC) that contains the op amp circuitry. Several pins (metal conductors) stick out of the chip and allow you to make electrical connections to the IC. The pin configuration for the 741 is shown in Figure 2. (Note that we will not use pins 1, 5, or 8). According to the 741 data sheet, the power supply voltages should be between 22 and 22. Do not violate these specifications because the op amp may be damaged! 1 8 Inverting Input ( ) 2 7 Positive power supply ( s ) Non-inverting input ( ) 3 6 Output ( ) Negative power supply ( s ) 4 5 Figure 2: 741 op amp chip (left) and pin configuration (right). A semicircular or circular mark on one end of the chip indicates the location of pin 1. Op Amp Modes of Operation The op amp differential input voltage is defined to be d. The output of the op amp is related to the differential input voltage as shown in Figure 3, which gives rise to the three regions of operations shown. = A d sat d Negative saturation sat Linear region Positive saturation Figure 3: Op amp regions of operation. Generally, when there is no feedback from the output to the input (open-loop), the op amp operates in one of the saturation region and works as a Comparator (Measurement-1). In the presence of a feedback (closed-loop) from the output to the inverting input (negative feedback) an op am can be designed to operate in the linear region and work as an Amplifier (Measurment-2).

3 Measurement 1: The Op Amp as a Comparator 1. Carefully place the 741 op amp on the breadboard. Make sure you place it in such a way that no pins are shorted together. If you need to remove the chip from the breadboard, ask for help. Do not remove the chip from the breadboard with your fingers, because you are likely to bend and possibly break the pins! 2. Set up the positive/negative power supplies to generate ±5 and the remaining connection to 1, then turn it off. 3. Build the open-loop op amp comparator circuit shown in Figure 4 below. Consult Figure 2 for pin assignment. 1 = 20 k and 2 = 15 k. Consult with the professor if you do not have these values. Do not turn on the power supply until both lab group members independently verify the circuit connections. Ask your professor if you are unsure. Make sure you connect the positive and negative power supplies to the appropriate pins on the op amp (S = 5 and = -5. Do not get the power supplies backwards or you will destroy the chip! Connect the 0.5 to in. Also, do not forget to connect the common ground terminal of the power supplies to the appropriate point(s) in the circuit. S S S- Figure 4: Comparator circuit. 4. Set the multimeter to measure voltage and connect to out (negative lead goes to the common ground). 5. Measure voltage across resistor 2 (-). 6. Increase in with an incremental step size of 1 up to 3.5. ecord the reading on the Lab 8 esults report as well as your lab notebook. You will need these results to prepare your lab report. Now decrease the in from 3.5 to 0.5 with the same step size. ecord your results.

4 Amplifiers Inverting Amplifier The circuit diagram for an inverting amplifier is shown in Figure Figure 5: Inverting amplifier. The ideal op amp assumptions can be used to show that the relationship between the input voltage,, and output voltage,, is given by. (4) out 2 in 1 Therefore, the output voltage has the opposite polarity of the input voltage (hence the name inverting amplifier). The ratio, out /, is called the closed-loop voltage gain. Non-inverting Amplifier The circuit diagram for a non-inverting amplifier is shown in figure 6. in 1 2 in Figure 6: Non-inverting amplifier. The ideal op amp assumptions can be used to show that the relationship between the input voltage, in, and output voltage, out, is given by 1. (5) out 2 in 1 Therefore, the output voltage has the same polarity of the input voltage (hence the name non-inverting amplifier).

5 Measurement 2: Inverting Amplifier 1. Put the signal generator in high Z mode, and set it to generate a 1 khz sine wave of peak-to-peak amplitude of 1. Use this as the input signal ( ). 2. Set up the positive/negative power supplies to generate ±5, then turn them off. 3. Use a 10 k resistor and a 20 k resistor to construct an inverting amplifier with a voltage gain of about 2 (Figure 5). Consult with the professor if you do not have these values. Do not turn on the power supply until both lab group members independently verify the circuit connections. Ask your professor if you are unsure. Make sure you connect the positive and negative power supplies to the appropriate pins on the op amp (S = 5 and = -5. Do not get the power supplies backwards or you will destroy the chip! Also, do not forget to connect the common ground terminal of the power supplies to the appropriate point(s) in the circuit. 4. Using the two oscilloscope channels, simultaneously display the input signal ( on channel 1 and the output signal ( ) on channel 2. Use the scope cursors to measure the peak-to-peak amplitudes of the input and output signals. The peak-topeak amplitude of the input should be smaller than the output, the output should be an inverted and amplified version of the input. 5. ecord the results in your lab notebook and Lab 8 esults report. )