ECE Electronics 315 Lab 2 In Person 2021 (1)

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Binghamton University *

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315

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Electrical Engineering

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Jan 9, 2024

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Lab 2: Diode Properties and Simulation In-Person Version Part 1: TRANSFER CHARACTERISTICS In this lab, we'll explore some of the input-to-output relationships (or " transfer characteristics ") of our simple P-N junction diodes. In this first part, we'll analyze the voltage- current relationship ("VI curves") for circuits which only contain diodes, such as the circuit below: To begin, we'll model this circuit in LTspice. LTSpice should be installed on each of the lab computers. To model this circuit, you'll want to take a look at the following tutorials. ( https://learn.sparkfun.com/tutorials/getting-started-with- ltspice/all Next, use LTSpice to produce a plot of the current through the diode at various voltages, using a sweep, as shown below. Include this plot in the lab write-up .

TRANSFER CHARACTERISTICS CONTINUED The LTspice graphs you've produced provide useful information: but it's important to remember that they're based on models of our circuit components. While these models attempt to be accurate, they'll never predict circuit behavior as well as our actual components! To get a "direct view", let's try to capture a similar graph with our lab equipment. First, we'll want to set up our function generator to sweep its output voltage from -1V to 1V, as we did in LTSpice. Turn on your function generator, and set it up to create the relevant sweep: 1. Select a sawtooth wave , which sweeps constantly from its minimum to its maximum value. 2. Since we don't have a constant 50Ω load, the output impedance compensation of our function generators isn't going to help us. Set the generator to High-Z mode. 3. Set the function generator to produce a 2V PP wave; resulting in a value that starts at -1V and moves to 1V. Your circuit will now look something like the circuit below: Using the oscilloscope, capture a graph of the voltage across and current through the diode, given the input above. Include these in the lab write-up . Remember, we can't visualize current directly-- so you'll likely want to employ a sense resistor, as in the circuit below:

This gives us a display that's close to the LTSpice output, but isn't quite as easy to follow, for a few reasons:  The LTspice output gives us a direct graph of voltage in vs current out . On the scope, we have to work to relate the voltage in (the sawtooth waveform) with the current out (the resultant waveform ).  In some cases, the input voltage may not be easy to read: for example, if the input waveform is distorted before being applied to the connected circuit. In these cases, it may be difficult to determine the relationship between the input voltage and our output. Fortunately, nearly every scope has a feature that will make our lives a lot easier: XY mode!In XY mode, a digital scope constantly reads the voltages present on each input, and draws a single dot , whose X position is determined by the voltage on channel one , and whose Y position is determined by the voltage on channel two-- hence "XY". These "dots" remain onscreen for a short while; as many dots accumulate, they begin to form a curve. This curve is effectively a plot of the voltage on channel one vs the voltage on channel two ! Place your oscilloscope into XY mode by following the procedure below. The TAS will demonstrate the procedure to place the new scopes into XY mode. This procedure will vary slightly from scope to scope, but should generally work on other digital scopes as well.

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Compare the graph on your oscilloscope to the graph produced by LTSpice. Take a screenshot/image of both waveforms (current and voltage) for your write up. For your own learning, consider the following discussion questions that will help prepare you for future modules: 1. How well does the graph produced by LTspice match the "graph" produced by the actual circuit? What differences do you notice be t ween them? Why might these differences occur? 2. Does it makes sense for the output impedance of your function generator affect your VI curve ? Why, or why not? Hint: Is your oscilloscope using the voltage produced by the function generator as its X axis, or the voltage delivered ? Part 2: IN CIRCUIT As you set up the experiment from the previous part, you likely noticed some distortion of the input waveform. This is quite interesting: since that voltage isn't dropping over the diode, and we know that Kirchoff's Voltage Law still applies, that voltage must be dropping over something else! Perhaps more interestingly, we can't attribute that voltage drop to our sense resistor , as we've technically been measuring the voltage across both . Switch your oscilloscope back to the normal time mode , and take another look at those input waveforms . It appears we'll need to perform a more detailed analysis in order to explain what's going on, here! Try switching your input to a sinusoidal waveform, as below: Observe the output: does the "distortion" persist? Take a screenshot/image of the voltage across the diode for your write up.

Let's try another experiment. Note: Using LTspice, try to verify the displayed waveform by performing a transient analysis of this circuit ) For your own learning, consider the following discussion questions that will help prepare you for future modules: 1. The function generator's display indicates that there will be a maximum of 1V across the diode, in either direction. If we plug this number into Ebers-Moll , would we correctly determine the maximum current through the diode? Why, or why not? 2. What might be the cause of the distortion observed? Can you adjust your LTspice model to show this distortion? Hint: It might help to consider the parasitic properties section of the supply that represents our function generator in LTspice: Part 3: RESISTOR DIODE CIRCUITS As a final-- and less guided-- experiment, let's consider the case of more significantly current limited resistor-diode circuit, like the one below: Take an image of the voltage across the resistor for your write-up.You will need to use the MATH function to subtract two measurements. Please ask the TA if you have questions.

For your own learning, consider the following discussion questions that will help prepare you for future modules: 1. How does adding a large resistor to this circuit change the voltages displayed on the scope? 2. Try to identify the points at which the diode is forward biased . What seems to be the voltage across the diode at these points? While this diode is forward biased, does this voltage seem to change significantly, or does it remain relatively constant? To get a better idea, try turning up the function generator's output amplitude by a volt or two. 3. Without physically changing the circuit, imagine what would happen if you were the swap the places of the diode and resistor. Would any of the relevant voltage drops change? Would the current through the circuit change? Part 4: INSTRUCTIONS FOR SUBMISSION 1. Open a word document at the top of the document, add your name and “in person” to denote that you did the live lab. 2. Include the screenshots from part 1 of the diode voltage and current sweeps from LTSpice. 3. Include screenshots/pictures from part 1 of the diode voltage and current on the oscilloscope. 4. Include a screenshot/picture from part 2 of the sine wave applied to the diode (voltage across the diode). 5. Include a screenshot/picture from part 3 of the voltage waveform across the resistor. 6. Label all images appropriately. 7. Answer this question in text: Explain the shape of the I-V curve measured in part 1 does not exactly agree with the LT Spice simulation.

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