Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Chapter 17

Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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. n n p p p n E (Emitter) B (Base) C (Collector) E B C Separating the regions are two junctions. Base-Collector junction Base-Emitter junction

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary BJT biasing For normal operation, the base-emitter junction is forward- biased and the base collector junction is reverse-biased. npn BE forward- biased BC 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

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary BJT currents A small base current (I B ) is able to control a larger collector current (I C ). Some important current relationships for a BJT are: I I I IBIB IEIE ICIC

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage-divider bias Because the base current is small, the approximation is useful for calculating the base voltage. R1R1 R2R2 RCRC RERE VBVB VEVE After calculating V B, you can find V E by subtracting 0.7 V for V BE. Next, calculate I E by applying Ohm’s law to R E : Then apply the approximation Finally, you can find the collector voltage from VCVC

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary Voltage-divider bias Calculate V B, V E, and V C for the circuit. R1R1 R2R2 RCRC RERE V E = V B  0.7 V = 27 k  6.8 k  1.0 k  2.2 k  +15 V 2N V 2.32 V 9.90 V

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Summary The BJT as a switch BJTs are used in switching applications when it is necessary to provide current drive to a load. In cutoff, the input voltage is too small to forward-bias the transistor. The output (collector) voltage will be equal to V CC. In switching applications, the transistor is either in cutoff or in saturation. RCRC V CC RCRC V OUT I IN = 0 = V CC I IN > I C(sat) /  DC When I IN is sufficient to saturate the transistor, the transistor acts like a closed switch. The output is near 0 V.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Bipolar junction transistor (BJT) Class A amplifier Saturation Selected Key Terms 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. A transistor with three doped semiconductor regions separated by two pn junctions. 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Cutoff Q-point Amplification Common- emitter (CE) Class B amplifier The dc operating (bias) point of an amplifier. Selected Key Terms A BJT amplifier configuration in which the emitter is the common terminal. The non-conducting state of a transistor. An amplifier that conducts for half the input cycle. The process of producing a larger voltage, current or power using a smaller input signal as a pattern.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Junction field- effect transistor (JFET) MOSFET Depletion mode Enhancement mode Metal-oxide semiconductor field-effect transistor. A type of FET that operates with a reverse- biased junction to control current in a channel. Selected Key Terms 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Common-source Oscillator Feedback A circuit that produces a repetitive waveform on its output with only a dc supply voltage as an input. An FET amplifier configuration in which the source is the common terminal. Selected Key Terms 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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. +10 V 5.0 k 

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 2. In a common-emitter amplifier, the output ac signal will normally a.have greater voltage than the input. b.have greater power than the input. c.be inverted. d.all of the above.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 3. In a common-collector amplifier, the output ac signal will normally a.have greater voltage than the input. b.have greater power than the input. c.be inverted. d.have all of the above.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 4. The type of amplifier shown is a a. common-collector. b. common-emitter. c. common-drain. d. none of the above. R1R1 R2R2 RERE C1C1 V CC

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz 8. A transistor circuit shown is a a. D-MOSFET with voltage-divider bias. b. E-MOSFET with voltage-divider bias. c. D-MOSFETwith self-bias. d. E-MOSFET with self bias. RDRD +V DD R1R1 R2R2

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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.

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall 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 +V DD RDRD +V DD R1R1 R2R2

Chapter 17 Electronics Fundamentals Circuits, Devices and Applications - Floyd © Copyright 2007 Prentice-Hall Quiz Answers: 1. c 2. d 3. b 4. a 5. c 6. a 7. d 8. b 9. d 10. b