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Transistor Worksheet C B E Dual Subscripts C B Q1 NPN E When using dual subscripts, the first subscript represents where the (+) lead (red) on a VOM would go, the second subscript represents where the black (-) lead would go. For example, VAB is read as “What is the voltage at point A with respect to Point B?” VE VB VC VBE VEB VBC VCB VCE VEC 2V 2.7V 3V 5V 5.6V 12V 6.3V 7.0V 10V 3.8V 4.5V 12V Now, a PNP transistor…. C B E VE VB VC VBE VEB VBC VCB VCE VEC -3.8V -4.5V -12V -1.2V -1.8V -15V 0V -0.6V -10V -1V -1.6V -5V The Load Line VCE IC The load line for a circuit is drawn as follows: 1. Determine the MAXIMUM voltage VCE across the transistor. This occurs when the transistor is OFF, and is equal to the source voltage at the collector (Vcc). Plot this on the „Y‟ axis. 2. Determine the MAXIMUM current IC through the transistor. This occurs when the transistor is all the way ON (Saturated). Plot this on the X axis. 3. Draw a straight line between the points defined above. This is the load line for the circuit, and describes the transistor‟s range of operation. The Operating Point is plotted on the load line as follows: 1. Calculate VCE and IC as done on the previous page. 2. Plot the VCE on the Y axis and IC on the horizontal axis. Draw lines straight up and straight across to intersect the load line at a point somewhere between maximum current and maximum voltage. This is the operating point. VCC RC Q1 NPN RB Solution Method: 1. Assume VE = 0V 2. VB = VE + 0.7V 3. VRB = VCC – VB 4. IB = VRB / RB 5. IC = Beta x IB = IE 6. VRC = IC x RB 7. VQ = VCC – VRC 8. PQ = VQ x IC Problems – Basic Circuit VCC RC RB BETA VB VRB IB IC VRC VCE PQ 10V 1K 33K 20 12V 1K 33K 20 10V 1.5K 28K 15 12V 1.5K 36K 25 10V 0.7K 30K 22 Plot the load line and operating point for each problem above: Voltage Divider Bias Vcc RB1 Rc V out Q1 RB2 Re Solution Method 1. VB = ((RB2)/(RB1+RB2)) x Vcc Voltage Divider Formula 2. VE= VB – 0.7 Diode Theory 3. IE = VE/RE Ohm‟s Law 4. Ic = IE Transistor Approximation 5. VRC = Ic x Rc Ohm‟s Law 6. VQ = Vcc – VRC – VRE KVL 7. PQ = VQ x IE Watt‟s Law Problems for the circuit above: RB1 RB2 RC RE VCC VCE IC PQ 52K 10K 6.8K 2K 10V 60K 8K 6.5K 2.2K 10V 47K 12K 7K 3K 10V 50K 18K 6.8K 2K 12V 50K 8K 7K 1.8K 10V Plot load lines and operating points for the above problems… Testing Transistors In order to verify that a transistor is good, use the following procedure: For NPN transistors: 1. Place the RED ohmmeter lead at the base. 2. Place the BLACK lead at the emitter 3. Measure a LOW resistance. 4. Reverse the leads. Measure a HIGH resistance. 5. Place the RED lead again at the base. 6. Place the BLACK lead at the collector. 7. Measure a LOW resistance. 8. Reverse the leads. 9. Measure a HIGH resistance. These will be the measurement(s) if the transistor is GOOD. Note that the device cannot be in a circuit when this test is performed – the base must be lifted from the circuit to avoid a parallel path, and of course power to the circuit must be OFF. For PNP transistors: 1. Place the BLACK ohmmeter lead at the base. 2. Place the RED lead at the emitter 3. Measure a LOW resistance. 4. Reverse the leads. Measure a HIGH resistance. 5. Place the BLACK lead again at the base. 6. Place the RED lead at the collector. 7. Measure a LOW resistance. 8. Reverse the leads. 9. Measure a HIGH resistance. These will be the measurement(s) if the transistor is GOOD. Note that the device cannot be in a circuit when this test is performed – the leads must be lifted from the circuit to avoid a parallel path, and of course power to the circuit must be OFF. Term 2 Electronics Test Name :_______________________ For the following circuit: 1. Given the following parameters, solve for the unknowns. VCC = 12 V RC = 3.3 K ohms RB = 560K ohms Beta = 120 VCC RC Q1 NPN RB VE = __________ VB = __________ VRB = __________ IB = __________ IC = __________ IE = __________ VRC = __________VCE (VQ) = _______ 2. For the circuit above, draw the load line and indicate the operating point. VCE IC 2. For the transistor below, label the terminals (emitter, collector, base) and indicate the type of transistor (PNP, NPN).
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