Simple Logarithmic AmplifierDEPARTMENT OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE MASSACHUSETTS INSTITUTE OF TECHNOLOGY CAMBRIDGE, MASSACHUSETTS 02139 Spring Term 2007 6.101 Introductory Analog Electronics Laboratory Laboratory No. 2 READING ASSIGNMENT You should have read the diode reading assignments in the course outline before doing this lab. For this laboratory, you will need to refer to the instruction manual for the Tektronix 577-177 or 575 Curve Tracers [manuals may be signed out at the stockroom window]. At a minimum, you should refer to the section of the manuals entitled Applications that describes how to set up the curve tracer to make measurements on specific devices such as diodes, bipolar transistors, etc. Since the 577-177 is a severely ergonometrically-challenged design, it is strongly suggested that you use the model 575. Simplified instructions for the 575 curve tracers are chained to the instruments in the lab and are also available at the stockroom window. DANGER: HIGH VOLTAGE is available on the collector terminals of any curve tracer depending on the setting of the collector voltage switch and the variable collector voltage pot position. Be sure to turn the transistor selector switch to the center “off” position before inserting or removing transistors, and to keep your hands free while applying voltage. [This voltage is pulsed and is current limited, but may still “surprise” you if you touch the collector terminals!] SPECIAL CURVE TRACER SETUP: One of the curve tracers, located in building 38, on the6th floor, has been modified to allow the curves to be displayed on the new Tektronix samplingscopes. While the curves are not as clean as those displayed on an analog display, the new scopesdo allow you to print your displays to a floppy disc. I suggest that you use .TIF file types forthese curves. Instructions for using this hybrid curve tracer are posted on the equipment. This willsave lots of sketching of device curves. There are 3 more curve tracers on the 5th floor. OBJECTIVE Diodes! Zener diodes! Bipolar Transistors! OpAmps! Power Supplies! These are some of the fundamental devices and circuits in analog electronics. You will learn more about our test equipment, and you’ll study some of the properties of the devices above. You will also build three [count ‘em!] [3] different linear [non-switching] power supplies and compare them. You will learn how to display the input-output characteristics of some of these devices on our antique [but still very useful] curve tracers. Lab. No. 2 1 2/14/07 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].Experiment 1: Diode Fundamentals: Building a Simple Log Amplifier. In this experiment, you will learn more about the diode by studying a simple log amplifier. Simple Logarithmic Amplifier [You need to know that the voltage gain of this amplifier is Ω−==kRVVAdiodeinoutV5.1; you also need to know that the inverting input, pin 2, is a virtual ground, in this case, for both AC and DC signals.] [We suggest that you express Rdiode in more useful terms!] 1. Build the logarithmic amplifier shown in figure 1. Q 1.1 What is the ideal input/output relationship VOUT versus VIN for this amplifier when the diode is biased with forward current? Q1.2 For what range of input voltages is there a logarithmic VOUT versus VIN relationship? 2. When the diode is forward biased, the diode voltage/current relationship is given by: IDISeqVDkTISeqVDkT=−≈⎡⎣⎢⎤⎦⎥⎡⎣⎢⎤⎦⎥()1 where kT/q is the thermal voltage [≈ 26 mV at room temperature] and IS is the diode reverse saturation current which depends on the particular device, temperature, etc. Q 1.3 When the diode forward current is increased by a factor of 10, how much does the diode voltage change? -++15LF35623476vinvout+_-151.5kΩ1N9140.1μF0.1μF Figure 1: Circuit for Experiment 1 Lab. No. 2 2 2/14/07 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].3. Plot the amplifier input/output relationship for 10 mV < VIN < 1000 mV. Use the DC offset adjustment control on your Function Generator to provide the DC voltage source. Make sure that the AC signal from the generator is turned off. To do this, press and hold the button labeled OFFSET for two seconds or until you hear a click. This will produce a pure DC output without any AC riding on top of it. Put one of your scope probes at the output of the amplifier to observe the output and to make sure that the only signal present is a DC signal. You should use your DMM to read first the input voltage [set to a convenient value], and then move the DMM to the output terminal to read the output DC voltage. Measure for several points per decade of input voltage, and use semi-log axes; this should result in a linear plot on semi-log axes. 4. Q 1.4 Find the diode coefficient IS from your data. You will need to know that the current entering pin 2 of the operational amplifier is negligible compared to the current through the diode, at least at the higher DC input voltages. If you find that the logarithmic relationship between input and output disappears at low DC input voltages, it may be due to the fact that the pin 2 input current is the same order of magnitude as the current through the diode at low input voltages. This can still happen even though the LM356 has a JFET input stage, and thus very very low input bias current. 5. Q 1.5 Measure an approximate temperature coefficient for the diode voltage, in mV/oC. [Hint: this can be done with the diode operating in your circuit; we want you to make a very rough estimate of the diode temperature coefficient. One possible method to change the temperature of the diode is to hold the diode between your thumb and forefinger with your fingers. Hold the diode until the output voltage stops changing.] Q 1.6 Comment on the practicality of using this amplifier in a piece of equipment where the temperature may vary widely [for instance, in a piece of commercial electronics where the ambient temperature may vary from 40oF to 150oF]. 6. Replace the 1N914 in your circuit with