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lab 7 2010 transistor bias circuits and thermistor

# lab 7 2010 transistor bias circuits and thermistor - LAB 7...

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L AB 7: B IPOLAR T RANSISTOR B IASING - T EMPERATURE S ENSOR I Objective and Background: This laboratory has four objectives: 1. To experiment with three types of transistor bias circuits studied in class: base bias, voltage divider bias and collector-feedback bias; 2. To diagnose faulty transistor bias circuits using MultiSim 3. To calibrate a catheter thermistor by measuring the resistance vs. temperature curve; 4. To build and test a temperature sensor in which the thermistor is used as the bottom resistor in a voltage divider bias circuit. Analyzing your results will allow you to compare the three bias circuits for different transistors. Recall that the purpose of the bias circuit is to provide dc voltages at the transistor leads such that the base-emitter junction is forward-biased and the base-collector junction is reverse- biased. The bias circuit sets up the proper quiescent conditions, that is the dc I E and V CE (Q point). Base bias, the simplest form of bias circuit is satisfactory for switching operations but unsatisfactory for linear applications because of the circuit dependency on β . Voltage divider bias is widely used because it requires only one dc supply. The analysis of the circuit is simple when the voltage divider current is much larger (> 10 times) than the base current. Collector- feedback bias uses negative feedback to stabilize the Q point. The circuit can be analyzed by writing KVL around the base loop. The thermistor that you will use is an NTC device. Its resistance decreases nonlinearly with temperature. II Base Bias Prelab: Consider the circuit shown on the following graph. 1. Based on the value of the supply source and that of the collector resistance, estimate the saturated collector current I C(sat) . Use I C(sat) and the data sheet for the 2N3904 transistor (see previous lab) to select the value of β (noted h FE in the data sheet) that you will use to analyze the circuit. 2. Apply the method of analysis studied in class to compute V RB (voltage drop across base resistance), V B , I B , I C , V RC , and V C Write the values on your prelab answer sheet and in the table that appears in the “Procedures” section.

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