Lab_6 - EE 101A / Winter 2010 Lab #6 (For weeks of 2/22 and...

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EE 101A / Winter 2010 Lab #6 (For weeks of 2/22 and 3/1) Page 1 of 9 1. Motivation: Making our converter a “better” voltage source We’ve spent several weeks building an AC/DC voltage converter to use as a power supply, which is really a voltage source. For the first few labs, we focused on converting what comes out of the wall into a constant voltage of around 15 V. In Lab 5, we added a neat feature that allowed variable output voltage. However, we also determined that the output resistance was too large for it to be a high quality power supply. Now we’d like to build an output stage that will allow our circuit to connect to various loads with minimal degradation in performance. We explored the concept of loads in Lab 4. We revisit this concept because anything to which we connect our converter can be thought of as a load. A “good” voltage source can deliver its advertised voltage to as wide a range of loads as possible. One way to measure the “quality” of a voltage source is called the source resistance R S . This is the effective resistance seen looking into the output of the source. Question 1: Let’s say we have a 10 V source with R S = 1 k as shown below. Calculate the voltage that appears across each of the following loads. Question 2: An ideal voltage source is capable of driving any load. What is its R S ?
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Lab #6 (For weeks of 2/22 and 3/1) Page 2 of 9 Figure 1. Circuit schematic as of Lab 6 Question 3: Fig. 1 above shows our current circuit. Let’s review the limitations of this circuit, so that we can better understand why we need an output stage, and what requirements it should satisfy. The outputs are between W and B , so we’ll be connecting our load ( R L ) across those two nodes. Identify 2~3 problems with the circuit. Making R L fairly small (hundreds of ohms) will help you see these problems. Hint 1: What happens when the load draws more than ~10mA of current? (we saw this in lab 4) Hint 2: What kind of R S does this circuit present to the load? Is it constant? Is it low/high? As you probably figured out, the type of load we put across the output has an effect on the internal current/voltage levels of our circuit. With the circuit as it is right now, you found that the output voltage V WB can fluctuate quite a bit depending on the load, and that R S can vary as well. What we need is a voltage buffer , which isolates the load from the rest of the circuit, and presents a fairly low R S . A good voltage buffer is also able to deliver significant current to the load. And finally, a voltage buffer should have a gain of about unity. A DC shift is acceptable and sometimes unavoidable. Ask your TA if any of these points is unclear. 6
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This note was uploaded on 03/30/2010 for the course EE 101A taught by Professor Wong during the Winter '08 term at Stanford.

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Lab_6 - EE 101A / Winter 2010 Lab #6 (For weeks of 2/22 and...

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