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Unformatted text preview: Transistor-Based Ramp Generator Lab 5: Analog-to-Digital Conversion ECE 327: Electronic Devices and Circuits Laboratory I Abstract For the analog-to-digital conversion lab, we need a resetable ramp generator. Here, we explore building a ramp generator from a current source. We show two implementations that both use a PNP current source. Contents 1 Introduction 1 2 Transistor Top Regulation 2 Resistor-Biased Ramp Generator . . . . . . 3 Diode-Biased Ramp Generator . . . . . . . 4 A Parts 5 1 Introduction Our goal is to build a ramp generator like 10 V 0 V Reset The signals available to us are 10 V, 0 V, and a reset signal. The output should be a ramp train that resets to 0 V when the reset signal is asserted, and we assume that the reset signal will be asserted when the ramp reaches 8 V or sooner (i.e., the ramp generator has an 8 V compliance ). We implement the ramp generator with a pnp current source that drives a capacitor that can be discharged with a switch. Sawtooth generation: This ramp generator can be used to produce a sawtooth wave by connecting the ramp generators reset input to a short pulse that is asserted at regular intervals. As long as the ramp generator does not saturate (i.e., go out of compliance) between pulses, the output will be a sawtooth. PWM and Ramp Slope: The ramp generator will be used with a comparator to generate a pulse-width- modulated (PWM) signal. (i) The ramp and PWM signal reset low on a 30 kHz clock with period T 33 s. (ii) The PWM signal transitions high whenever the ramp crosses the 28 V input from below. (iii) The PWM signal drives a light-emitting diode (LED) that transmits to a detector equipped with a Schmitt trigger which gives it hysteresis . So PWM transitions must be spaced at least 6 s apart to be detected (i.e., 6 s is the minimum negative pulse width and positive pulse width ). So, after a reset, the ramp must not cross 2 V before 6 s, and the ramp must cross 8 V no later than ( T 6 s) 27 s. Hence, the ramp slope r must be so that . 297 V s 8 V 27 s 8 V T 6 s r 2 V 6 s . 33 V s . (1.1) In practice, the actual ramp will be an exponential (due to capacitor and switch leakages) and the actual ramp slope may need to be slightly increased....
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- Fall '08