Lab. No. 4 1 3/1/07 Cite as: Ron Roscoe, course materials for 6.101 Introductory Analog Electronics Laboratory, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. DEPARTMENT OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS 02139 Spring Term 2007 6.101 Introductory Analog Electronics Laboratory Laboratory No. 4 READING ASSIGNMENT In this laboratory, you will investigate the performance of operational amplifiers in simple circuit configurations. We will discuss various aspects of operational amplifier behavior in class. In addition, you should have read at least the Operational Amplifiers class reading assignments in the class outline under section 7. Objective: At last! The Bigtime!! Op-amps!!! You will be using the LM741, which is one of the earliest of popular operational amplifiers, the LF356 which is a precision JFET-input op-amp with very low input bias current, wider open-loop bandwidth, higher slew rate, etc., the LT 1632C, a dual rail-to-rail op-amp, and the LT1011A comparator. Acknowledgement: I would like to thank National Semiconductor Corporation for their generous donation of all of the integrated circuits used in this laboratory, with the exception of the LT1011 and the LT1632C devices, which were generously donated by Linear Technology, Inc. Experiment 1: The Inverting Configuration. In this experiment, you will be connecting a LM741 in the inverting configuration of Figure 1. You will learn to adjust the offset of the amplifier, measure its bandwidth and see how its performance is limited by its slew rate. 1. Construct the circuit of Figure 1. [Refer to the LM741 data sheet to make sure that you connect to the proper pins of your LM741]. Choose resistor values R1 = R2 = 15 kΩ and R3 = 7.5 kΩ and do not install the 10 kΩ potentiometer at this point. Ground the input vin and measure the output voltage [it will probably differ by some number of millivolts from zero]. This voltage is caused by the input offset voltage that can be modeled as a dc voltage source in series with the non-inverting input to the amplifier and the gain of the amplifier [in this case the gain is two for a voltage applied to the non-inverting input]. Calculate the corresponding input offset voltage and compare this value with that found on the LM741 data sheet.-++15-15237410 k ΩLM741156R1R2R3vinvout0.1μF0.1μF Figure 1: Circuit for experiment 1. 2. Now install the 10 kΩ offset-null potentiometer. Depending on the style of the potentiometer, you may have to solder some leads onto it so that you can plug it into your protoboard. [Soldering irons are installed near the 6.071 lab area, and solder and other tools are available from the instrument room window.] Adjust the potentiometer to zero the output voltage. Q 1.1 What range of output-offset voltage can be achieved by adjusting the potentiometer over its entire range? 3. Now connect the signal generator to the input and adjust it to produce a 0.2 Vp-p, 1 kHz sinusoid. [Remember that your function generator source impedance is default calibrated for a 50 Ω load. Be sure to switch it to High Z.] Measure the magnitude of the voltage gain of this connection. Q 1.2 Do you need an input coupling capacitor between the function generator and R1? Why or why not? 4. Increase the frequency of the signal generator until the amplitude of the output voltage begins to decrease. Find the frequency at which the amplifier gain is 12 of its low-frequency [1 kHz] value. This frequency can be considered to be the bandwidth [-3dB point] of this particular configuration. Measure the phase-shift between the input and output voltages at this frequency. [See the attached instructions for measuring phase-shift with the Tektronix 2445 oscilloscope, the Tektronix 34XX series have a phase measurement available under the “measurements” menu.] 5. Now change the feedback resistor R2 to 150 kΩ, calculate a new value for R3, and repeat parts 2, 3 and 4. If you disconnect the offset potentiometer, you will notice that the output offset is approximately 5.5 times larger than that found when the amplifier was configured for a gain of 1 from the inverting input. Q 1.3 Why? Q 1.4 Why did we change R3.? Q 1.5 What is the ideal value of R3 relative to the values of R1 and R2? Lab. No. 4 2 3/1/07 Cite as: Ron Roscoe, course materials for 6.101 Introductory Analog Electronics Laboratory, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY].Lab. No. 4 3 3/1/07 Cite as: Ron Roscoe, course materials for 6.101 Introductory Analog Electronics Laboratory, Spring 2007. MIT OpenCourseWare (http://ocw.mit.edu/), Massachusetts Institute of Technology. Downloaded on [DD Month YYYY]. • Notice that while the inverting amplifier gain is a factor of 10 larger than that of the first configuration, the bandwidth is approximately a factor of 5.5 lower. If you were to examine this configuration for other values of gain, you would find out that the higher the gain, the lower the bandwidth; specifically, that the product of (one plus the gain) times the bandwidth is a constant. 6. With the signal generator set to the bandwidth frequency [-3dB point] for the gain of -10, which you found in part 5, increase the amplitude of the input voltage until the output voltage begins to distort [i.e. it will no longer look sinusoidal, but more like a triangle wave]. At this point, the amplifier has reached its slew-rate limit. The slew-rate limit of an operational amplifier is caused by a current source within the amplifier [biasing the first stage, the input differential stage, of the amplifier] that limits the amount of current that can be supplied by the first stage of the amplifier. When the amplifier is pushed to the point that this limit is reached, it can no longer function properly. The slew-rate limit manifests itself as a maximum value of dvout/dt for the amplifier because there is an internal amplifier capacitance that must be charged by the first-stage output current and a first-stage current limit thus corresponds to a maximum dv/dt for this capacitor. With the input amplitude set to the value at which the output voltage just starts to distort, calculate the maximum value of dvout/dt on the output voltage. Q 1.6 How does