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lab1 - ECE 323 Project 1 Fall 2009 Write-up Due 9/25 1...

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ECE 323: Project 1, Fall 2009 Write-up Due: 9/25 1 University of Massachusetts Department of Electrical and Computer Engineering ECE 323 Electronics I Fall 2009 Project 1: Design of a Peak Detector Employing Diodes and OPAMPs Charlie Circuitmaker realized he could use the diodes he was learning about in ECE 323 to build an AM radio. You are going to follow the steps he took to design, simulate, and build three increasingly complex versions of the peak detector. First, Charlie used a simple diode peak detector. Charlie decided he needed a more complicated design because the “diode drop” of approximately 0.7V caused a problem when using small signals. Then he tried the “super diode” circuit from Sedra and Smith. This circuit worked well at low frequencies, but behaved oddly at high frequencies. Dr. Designsalot, Charlie’s mentor, suggested a small change to this circuit that improved the results dramatically. The pre-lab write up including the hand calculations and PSPICE simulations outlined in the pages that follow is due on September 25 th . The work to be included in the lab write up is presented in textboxes in the lab description that follows. Late labs will not be accepted.
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ECE 323: Project 1, Fall 2009 Write-up Due: 9/25 2 1.0 Design Specifications Your designs will meet the same specifications that Charlie used: Parameter Value Diodes 1n914 Operational Amplifiers 741 Power Supply +/- 5V Load Resistance 4.7k Ω Signal Frequency 5kHz Input Amplitude 2V pp Voltage Ripple 25mV pp 2.0 Simple Diode Peak Detector Charlie Circuitmaker started with the circuit in Fig. 1. He knew that he could adjust the RC time constant by varying the resistance and capacitance. For paper and pencil calculations, he assumed that those values were selected so that the signal would decay by 10% during one cycle. He could then approximate this decay as a straight line. He modeled the diode as turning on at 0.7V and no resistance. 3.0 “Super Diode” Peak Detector 3.1 Design and Hand Calculations Charlie’s plot of the output voltage showed that the simple diode peak detector would not work with small waveforms. After a bit more thought, he found this was true independent of his resistance and capacitance values. He had of course done all of the assigned reading in Sedra and Smith, so he knew that this was a common dead zone problem that was sometimes solved with a “super diode” (section 3.5.5 and 13.9.) Charlie used the “super diode” in his second topology shown in Fig. 2. Using Charlie’s approximations, sketch the waveforms for two cycles at the input and output when driving this circuit with a 1V pp sine wave at 5kHz. Figure 1: Simple Diode Peak Detector
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ECE 323: Project 1, Fall 2009 Write-up Due: 9/25 3 It took Charlie a little while to figure out how the “super diode” worked. He started by taking the case where the input is higher than the output. Because the positive input is higher than the negative input of the opamp, it will drive V drive high. The diode conducts in this direction, so C 1 is charged and V out
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