JJJuly09LessonThree - Key Lecture Concepts for CoE225/EE...

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Key Lecture Concepts for CoE225/EE 271 (Mostly Digital Electronics) 1 LESSON THREE: Multi-Diode and Op-Amp Circuits with Time Varying Inputs, i.e. Analog Signals. Lesson Overview: The major learning objective of this lesson is to master the principles for analysis of circuits with time-varying input waveforms. You will also learn about some of the applications of diode circuits. The lesson begins with a study of rectifier diode circuits that can convert time-varying inputs, such as sinusoids available from electrical outlets, to DC voltages used by a load. The loads might be e.g. computer circuits and the amplifiers of analog audio and video voltage waveforms. An analog waveform is one that varies smoothly and continuously with time, e.g. a sine wave that has a varying amplitude and frequency. The conversion to DC depends on “large” capacitors which can keep the voltage on the output node nearly constant, even as current is drawn from the node by the load. Analog signal waveforms often must be modified and amplified. A general purpose electronic component for doing this is the operational amplifier, an active component with 15 to 25 transistors that costs less than a dollar because it is mass-manufactured using integrated circuit technology. It is considered to be active versus passive because it is powered by applied DC voltages. In section C, some basic analog circuits will be presented that use passive components, e.g. resistors and diodes, along with the operational amplifier to add, subtract, amplify, and rectify various analog input waveforms. The important concepts of output and input impedance of an amplifier relative to the load and the output impedance of the signal source also will be presented. The lesson will conclude with an analysis of a resistor/diode analog circuit that serves as a DC voltage source, minimizing changes in the desired constant DC voltage for the load with changes in current drawn by the load. You will learn how the very steep breakdown characteristic of diodes in the third quadrant is used to keep the DC voltage more constant. A) Diode Circuits with Time Varying Inputs: Half-Wave and Full-wave Rectifiers. The operation of circuits with time varying inputs, such as the full wave rectifier circuit in fig3.1a, can be understood by analysis of the circuits taking "time snapshots" of the input at several key moments. Fig3.1b shows the output, v 0 (t), obtained by snapshot analysis under the sine wave input, v i (t). [The two-volt input has a frequency of 1 MHZ.] Note that the circuit produces an output that has only positive values. The input is said to be rectified because the output has only one polarity, positive in this case. Rectification of a sine wave is the first step to obtain a DC voltage from a time-varying input. To understand how this diode rectifier works, we will study first two similar two-diode circuits in fig3.1c and fig3.1d . To simplify the analysis of the circuits in
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JJJuly09LessonThree - Key Lecture Concepts for CoE225/EE...

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