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Chapter 17

Electronics Fundamentals. Circuits, Devices, and Applications. Floyd. Chapter 17. Base-Collector junction. Base-Emitter junction. Summary. Bipolar junction transistors (BJTs).

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Chapter 17

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  1. Electronics Fundamentals Circuits, Devices, and Applications Floyd Chapter 17

  2. Base-Collector junction Base-Emitter junction Summary Bipolar junction transistors (BJTs) The BJT is a transistor with three regions and two pn junctions. The regions are named the emitter, the base, and the collector and each is connected to a lead. There are two types of BJTs – npn and pnp. Separating the regions are two junctions. C (Collector) C n p p n B (Base) B n p E (Emitter) E

  3. Summary BJT biasing For normal operation, the base-emitter junction is forward-biased and the base collector junction is reverse-biased. For the npn transistor, this condition requires that the base is more positive than the emitter and the collector is more positive than the base. For the pnp transistor, this condition requires that the base is more negative than the emitter and the collector is more negative than the base. BC reverse- biased + + pnp + npn BE forward- biased +

  4. Summary BJT currents A small base current (IB) is able to control a larger collector current (IC). Some important current relationships for a BJT are: IC I IB I IE I

  5. Summary Voltage-divider bias Because the base current is small, the approximation is useful for calculating the base voltage. After calculating VB, you can find VE by subtracting 0.7 V for VBE. R1 RC VC Next, calculate IE by applying Ohm’s law to RE: VB VE R2 RE Then apply the approximation Finally, you can find the collector voltage from

  6. Summary Voltage-divider bias Example: Calculate VB, VE, and VC for the circuit. Solution: 3.02 V +15 V VE = VB- 0.7 V = 2.32 V R1 RC 27 kW 2.2 kW 2N3904 R2 RE 6.8 kW 1.0 kW 9.90 V

  7. IIN = 0 = 0 V = VCC IIN > IC(sat)/bDC Summary The BJT as a switch BJTs are used in switching applications when it is necessary to provide current drive to a load. VCC VCC In switching applications, the transistor is either in cutoff or in saturation. RC RC In cutoff, the input voltage is too small to forward-bias the transistor. The output (collector) voltage will be equal to VCC. VOUT When IIN is sufficient to saturate the transistor, the transistor acts like a closed switch. The output is near 0 V.

  8. Selected Key Terms Bipolar junction transistor (BJT) Class A amplifier Saturation A transistor with three doped semiconductor regions separated by two pn junctions. An amplifier that conducts for the entire input cycle and produces an output signal that is a replica of the input signal in terms of its waveshape. The state of a transistor in which the output current is maximum and further increases of the input variable have no effect on the output.

  9. Selected Key Terms The non-conducting state of a transistor. Cutoff Q-point Amplification Common-emitter (CE) Class B amplifier The dc operating (bias) point of an amplifier. The process of producing a larger voltage, current or power using a smaller input signal as a pattern. A BJT amplifier configuration in which the emitter is the common terminal. An amplifier that conducts for half the input cycle.

  10. Selected Key Terms A type of FET that operates with a reverse-biased junction to control current in a channel. Junction field-effect transistor (JFET) MOSFET Depletion mode Enhancement mode Metal-oxide semiconductor field-effect transistor. The condition in a FET when the channel is depleted of majority carriers. The condition in a FET when the channel has an abundance of majority carriers.

  11. Selected Key Terms An FET amplifier configuration in which the source is the common terminal. Common-source Oscillator Feedback A circuit that produces a repetitive waveform on its output with only a dc supply voltage as an input. The process of returning a portion of a circuit’s output signal to the input in such a way as to create certain specified operating conditions.

  12. Quiz 1. The Thevenin circuit shown has a load line that crosses the y-axis at a. +10 V. b. +5 V. c. 2 mA. d. the origin. 5.0 kW +10 V

  13. Quiz 2. In a common-emitter amplifier, the output ac signal will normally • have greater voltage than the input. • have greater power than the input. • be inverted. • all of the above.

  14. Quiz 3. In a common-collector amplifier, the output ac signal will normally • have greater voltage than the input. • have greater power than the input. • be inverted. • have all of the above.

  15. Quiz 4. The type of amplifier shown is a a. common-collector. b. common-emitter. c. common-drain. d. none of the above. VCC R1 C1 R2 RE

  16. Quiz 5. A major advantage of FET amplifiers over BJT amplifiers is that generally they have a. higher gain. b. greater linearity. c. higher input resistance. d. all of the above.

  17. Quiz 6. A type of field effect transistor that can operate in either depletion or enhancement mode is an a. D-MOSFET. b. E-MOSFET. c. JFET. d. none of the above.

  18. Quiz 7. For an FET, transconductance is the ratio of a. drain voltage to drain current. b. gate-source voltage to drain current. c. gate-source current to drain voltage. d. drain current to gate-source voltage.

  19. Quiz 8. A transistor circuit shown is a a. D-MOSFET with voltage-divider bias. b. E-MOSFET with voltage-divider bias. c. D-MOSFET with self-bias. d. E-MOSFET with self bias. +VDD RD R1 R2

  20. Quiz 9. A Colpitts or Hartley oscillator both have a. positive feedback. b. amplification. c. a closed loop gain of 1. d. all of the above.

  21. Quiz 10. If you were troubleshooting the circuit shown here, you would expect the gate voltage to be a. more positive than the drain voltage. b. more positive than the source voltage. c. equal to zero volts. d. equal to +VDD +VDD RD R1 R2

  22. Quiz Answers: 1. c 2. d 3. b 4. a 5. c 6. a 7. d 8. b 9. d 10. b

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