ITT Technical Institute ET215 Devices 1 Unit 6 Chapter 3, Sections 3.7-3.9
Chapter 3 Section 3.7 The Bipolar Transistor as a Switch Objectives: Explain how a transistor can be used as a switch 1. Compute the saturation current for a transistor switch 2. Explain how a transistor switching circuit with hysteresis changes states
The Bipolar Transistor as a Switch A switch is a two-state device that is either open or closed. In Part A, the transistor is in cutoff because the baseemitter pn junction is not forward-biased, which opens the circuit. In Part B, the transistor is in saturation because the base-emitter pn junction is forward-biased, which closes the circuit.
The Bipolar Transistor as a Switch Conditions in Cutoff As mentioned before, a transistor is in cutoff when the base-emitter pn junction is not forward-biased. Neglecting leakage current, all of the currents are zero, and V CE is equal to V CC.. V CE(cutoff) = V CC
The Bipolar Transistor as a Switch Conditions in Saturation When the emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated. Since V CE is very small at saturation, the entire power supply voltage drops across the collector resistor. An approximation for the collector current is I C(sat) = V CC / R C The minimum base current needed to produce saturation is I B(min) = I C(sat) / β DC
The Bipolar Transistor as a Switch Example: (a) For the transistor switching figure shown, what is VCE when Vin = 0V? (b) What minimum value of I B is required to saturate this transistor if β DC is 200? (Assume V CE(sat) = 0V). (c) Calculate the maximum value of R B when Vin = 5 V.
The Bipolar Transistor as a Switch Solution: (a) When V in = 0 V, the transistor is in cutoff (acts like an open switch) and V CE = V CC = 10 V (b) Since V CE(sat) = 0 V, I C(sat) V CC / R C = 10 V / 1.0 kω = 10 ma I B(min) I C(sat) / β DC = 10 ma / 200 = 0.05 ma (this is the value of I B necessary to drive the transistor into saturation. Further increase in I B will drive the transistor deeper into saturation, but will not increase I C ) (c) When the transistor is saturated, V BE = 0.7V. The voltage across R B is V RB = V IN - V BE = 5 V 0.7V = 4.3 V the maximum value of R B needed to allow a minimum current of 0.05 ma is (Ohm s Law) R B = V RB / I B = 4.3 V / 0.05 ma = 86 Ω
The Bipolar Transistor as a Switch Improving the One-Transistor Switching Circuit Another improvement for basic switching circuits is the addition of hysteresis (meaning that there are two threshold voltages depending on whether the circuit is already high or already low). See Figure
Chapter 3 Section 3.8 Transistor Packages and Thermal Identification Objectives: Identify various types of transistor package configurations 1. List three broad categories of transistors 2. Recognize various types of cases and identify the pin configurations
Transistor Packages and Thermal Identification Transistor Categories: 1. General-Purpose 2. Power Devices 3. RF Devices Although certain types have unique packaging, you will find an overlap in categories
Transistor Packages and Thermal Identification General-Purpose / Small-Signal Transistors 1. Used for low- or medium-power amplifiers or switching circuits 2. Generally packages are either plastic or metal cases (TO-xx pins 3-51 f)
Transistor Packages and Thermal Identification General-Purpose / Small-Signal Transistors Sometimes, many transistors can be within one case
Transistor Packages and Thermal Identification Power Transistors 1. Used to handle large currents (typically more than 1 amp and/or large voltages) 2. Application may include final stage of an amplifier to drive speakers 3. Most cases requires a heat sink to remove generated heat (notice part g)
Transistor Packages and Thermal Identification Power Transistors
Transistor Packages and Thermal Identification RF Transistors 1. Designed to operate at extremely high frequencies 2. Commonly used in communications circuits 3. Unusual shape is designed to optimize certain high-frequency parameters
Objectives: Chapter 3 Section 3.9 Troubleshooting Troubleshoot various faults in transistor circuits 1. Define floating point 2. Use voltage measurements to identify a fault in a transistor circuit 3. Use a DMM to test a transistor 4. Explain how a transistor can be viewed in terms of a diode equivalent 5. Discuss in-circuit and out-of-circuit testing 6. Discuss point-of-measurement in troubleshooting 7. Discuss leakage and gain measurements
Troubleshooting Troubleshooting a Biased Transistor Common Faults: 1. Open bias resistors 2. Open or resistive connections 3. Shorted connections 4. Opens or shorts internal to the transistor itself Floating point refers to a point in the circuit that is not electrically connected to ground or a solid voltage
Troubleshooting Troubleshooting a Biased Transistor
Troubleshooting Troubleshooting a Biased Transistor
Troubleshooting Testing a Transistor with a DMM: 1. A good digital multimeter can be used as a fast and simple way to check a transistor for open or shorted junctions. 2. Recall that a good diode will show extremely high resistance with a reverse bias and a very low resistance with a forward bias. 3. Many DMMs have a diode test position that can be used. 4. DMMs without this feature, can use the ohms scale to check the resistance.
Troubleshooting Transistor Testers: 1. Transistor checkers will perform a comprehensive and/or automatic test(s) of the transistor curves.
Troubleshooting Point-of-Measurement in Troubleshooting:
Troubleshooting EXAMPLE: Point-of-Measurement in Troubleshooting: 1. The transistor is in cutoff as indicated by the 10 V on the collector leads. 2. The base bias voltage of 3 V appears on the PC board but not on the transistor lead as indicated by the floating point measurement. This shows that there is an open external to the transistor between the two measured base points. 3. Check the solder joint on the base lead.
Troubleshooting Leakage and Gain Measurements 1. Leakage normally can be ignored 2. A faulty transistor may have more than na currents 3. Use a transistor tester 4. Gain can be determined by the transistor tester and should match the manufactures β DC 5. Most testers provide an in-circuit β DC test so that the transistor need not be removed form the circuit.