Overview – The Ideal DiodeHalf-Wave RectifierUNIVERSITY OF CALIFORNIA, BERKELEY EE40: Introduction to Microelectronic Circuits Diodes Background Overview – The Ideal Diode An ideal diode allows current to flow in only one direction. Every diode has a characteristic “threshold voltage” – VT. An ideal diode has an infinite resistance when the voltage across it, VD, is less than VT and zero resistance when the VD is greater than VT. The threshold voltage concept comes from the fact that a diode is just a pn junction; the threshold voltage is defined by the concentration of donors and acceptors in the junction. The I-V graph for an ideal diode should look like: Figure 1 Ideal Diode IV Curve and Schematic Real diodes do not exhibit this ideal behavior. This lab will explore the actual behavior of diodes. Diode Logic Diode logic (DL) is a family of logic circuits that perform logic operations based on the ability of the diode to conduct current in only one direction, under forward bias. Figure 2 contains the schematics for OR and AND gates implemented in DL. VCC is a voltage source that defines the HI logic level, which should be greater than the inherent diode voltage drop. The logic level LO is defined by 0 volts, or “ground”. In the OR circuit, the output C is raised HI whenever either A or B are HI. In the AND circuit, output C is LO whenever either A or B are LO. Figure 2 OR, AND gates in Diode Logic 1Half-Wave Rectifier The half-wave rectifier is a circuit that allows only part of a sinusoidal input signal to pass. The circuit is simply the combination of a single diode in series with a resistor, where the resistor is acting as a load (see figure 3). Figure 3 Half-Wave Rectifier Schematic Figure 4 Half-Wave Rectifier, Voltage vs Time When the input voltage is less than the threshold voltage, the output voltage is 0. This is because when input voltage is less than the threshold voltage (and thus the voltage across the diode is less than the threshold voltage), zero current flows through the diode. By KCL, no current flows through the load resistor resulting in no voltage drop via ohm’s law. If, on the other hand, the input voltage greater than the threshold voltage, essentially any current can pass through the diode (real diodes only approximate this vertical line). The voltage drop across the diode will just be its threshold voltage and the current through it will be defined by the load. 2Name:_____________________ Student ID:_____________________ Section:_____________________ Date:_____________________ UNIVERSITY OF CALIFORNIA, BERKELEY EE40: Introduction to Microelectronic Circuits Diodes Prelab 1. Suppose Vin is a sine wave that has amplitude voltage Va, frequency f, and offset = 0V. Suppose the threshold voltage of the diode is VT = 500mV. Describe and draw what you will see at the output. Make sure to include the amplitude, frequency, and offset in terms of the input and VT. + R Vout - + Vin - 2. Explain where the threshold voltage of a diode comes from and how you will measure the threshold voltage of a diode using the half-wave rectifier circuit. 33. Provide the truth table (include output voltage) and identify the logic function of the following circuits. A, B are the inputs and C is the output. Assume large signal diode model. Logic X Logic Y 4. Load Line: The I-V characteristics of a diode are plotted on the graph. Find the circuit current I’ and diode voltage V’ using the load line method.
View Full Document
Unlocking...