Chapter 18

2. Chapter 18 Tubes, Transistors, and Amplifiers

3. Objectives • Explain the operation of the vacuum tube. • Describe the workings of a cathode ray tube. • Explain the operation of the bipolar transistor. • Explain the operation of field-effect transistors. Permission granted to reproduce for educational use only.

4. Objectives • Discuss different biasing techniques. • Identify various transistor circuit configurations. • List the components of amplifier circuits and give the function for each component. • Explain amplifier operation. • Compute the gain of amplifier circuits. Permission granted to reproduce for educational use only.

5. Objectives • Perform dc load line analysis on a transistor circuit. • Discuss the advantages and disadvantages of various methods of amplifier coupling. • Describe several common thyristors. Permission granted to reproduce for educational use only.

6. Vacuum Tubes • Replaced by transistors and integrated circuits • Still found in CRTs • Result of Thomas Edison’s incandescent lamp • Electron theory and thermionic emission better understood Permission granted to reproduce for educational use only.

7. Vacuum Tubes (Cont.) • Thermionic emitters • Cathodes • Diodes • Triodes • Tetrodes • Pentodes • Cathode ray tubes (CRTs) Permission granted to reproduce for educational use only.

8. Thermionic Emitters • Materials that melt when heated to the point of emission • Tungsten is the best material because of its strength and durability • Thoriated-tungsten emitter • Most efficient emitter is oxide-coated type Permission granted to reproduce for educational use only.

9. Cathodes • Heat can be applied directly or indirectly Permission granted to reproduce for educational use only.

10. Cathodes (Cont.) • Equipment that uses batteries works best with direct heat • Less heat loss • Filament can have only a small amount of power consumed • Use an ac source with indirect heat • Less power loss • Heater voltage source and cathode can be separated Permission granted to reproduce for educational use only.

11. Diodes • Two electrodes • Vacuum tube diodes • Cathode can be heated directly or indirectly • Round piece of metal surrounding all elements in the tube is the plate Permission granted to reproduce for educational use only.

12. Diodes (Cont.) • Cathode is heated with indirect heat, causing thermionic emission • When plate is joined to positive terminal, electrons flow from cathode to plate • Electron tube allows electrons to flow in only one direction • Cathode emits largest number of electrons at a certain temperature • Plate is more positive with a higher voltage Permission granted to reproduce for educational use only.

13. Triodes • Developed by Lee DeForest • Grid inserted between cathode and plate • Control grids • Bias voltage and Cutoff bias Permission granted to reproduce for educational use only.

14. Tetrodes • Screen grids • Placed between control grid and plate • Bypassed to ground through a capacitor • To increase the speed of the electrons passing between cathode and plate, a dc voltage is applied to the screen grid • High amplification is possible Permission granted to reproduce for educational use only.

15. Pentodes • Secondary emission occurs in tetrodes • Suppressor grid is added in pentode • Suppressor grid is connected to cathode • Repels free electrons resulting from secondary emission • Electrons are driven back to the plate • High amplification factors and high plate resistance • Used as radio frequency amplifiers and audio power amplifiers Permission granted to reproduce for educational use only.

16. CRTs • Used in oscilloscope displays and older computer monitors • Older television picture tubes • Aperture controls flow of electrons into CRT • Additional grids accelerate and focus electrons (RCA) Permission granted to reproduce for educational use only.

17. CRTs (Cont.) • Magnetic deflection coils move the electron beam • Picture tube’s inside is coated with phosphor • Screen glows as electrons strike it • Tube bulb has conductive coating, collecting secondary emissions Permission granted to reproduce for educational use only.

18. Review What components have begun to replace vacuum tubes? Transistors and integrated circuits Permission granted to reproduce for educational use only.

19. Review What is thermionic emission? The process of metals and metal oxides giving up free electrons when heated, freeing the electrons Permission granted to reproduce for educational use only.

20. Review What are thermionic emitters? Materials that melt when heated to the point of emission Permission granted to reproduce for educational use only.

21. Review What is the emitter in the vacuum tube called? A cathode Permission granted to reproduce for educational use only.

22. Review In a diode, how can the cathode be heated? Directly or indirectly Permission granted to reproduce for educational use only.

23. Review What three elements make up a triode? A cathode, plate, and grid Permission granted to reproduce for educational use only.

24. Review What is the bias voltage? Voltage applied to the control grid in a triode Permission granted to reproduce for educational use only.

25. Review The tetrode adds a fourth element to the elements of the triode. What is the fourth element? Screen grid Permission granted to reproduce for educational use only.

26. Review Why is high amplification possible in the tetrode? The control grid is placed very close to the cathode Permission granted to reproduce for educational use only.

27. Review What is the fifth element called in the pentode? Suppressor grid Permission granted to reproduce for educational use only.

28. Review What is the function of the suppressor grid? Repels free electrons resulting from secondary emission Permission granted to reproduce for educational use only.

29. Review What kind of vacuum tubes are used in oscilloscopes and older computer monitors? Cathode ray tubes (CRTs) Permission granted to reproduce for educational use only.

30. Transistors • Point contact resistor had two wires fused on germanium crystal • Instant circuit operation with no large amounts of power • Amplify current • Create ac signals at different frequencies • Used as switching devices Permission granted to reproduce for educational use only.

31. Transistors (Cont.) • Bipolar junction transistors (BJTs) • Impure crystals • Two junctions • Current device Permission granted to reproduce for educational use only.

32. Transistors (Cont.) • NPN bipolar transistors • P-type crystal between two N-type crystals • PNP bipolar transistors • N-type crystal between two P-type crystals • Emitters • Bases • Collectors Permission granted to reproduce for educational use only.

33. Transistors (Cont.) • Most applications require one voltage source • Forward biased • Reverse biased • 95%–98% of current flow is emitter to collector • Rest of current goes from emitter to base Permission granted to reproduce for educational use only.

34. Field-Effect Transistors (FETs) • Voltage devices • Control current in source-drain circuit by amount of potential applied to the gate • Junction field-effect transistors (JFETs) • Metal oxide semiconductor field-effect transistors (MOSFETs) Permission granted to reproduce for educational use only.

35. Parts of a JFET • Source • Drain • Gate • Channel Permission granted to reproduce for educational use only.

36. JFETs • When small potential is applied to gate, large current through P channel exists • When large potential is applied to gate, small current exists Permission granted to reproduce for educational use only.

37. MOSFETs • Depletion-mode MOSFETs • Enhancement-mode MOSFETs • Thin film of insulation between gate and channel area Permission granted to reproduce for educational use only.

38. Review What does a BJT consist of? Three layers of impure semiconductor crystals Permission granted to reproduce for educational use only.

39. Review What are the three crystals called? The base, emitter, and collector Permission granted to reproduce for educational use only.

40. Review What kind of transistor has arrows in its schematic that never point in? NPN transistor Permission granted to reproduce for educational use only.

41. Review What is the emitter-base circuit called when the positive terminal of the battery is connected to the P-type base? Forward biased Permission granted to reproduce for educational use only.

42. Review What does a FET control? The current in the source-drain circuit by the amount of potential applied to the gate Permission granted to reproduce for educational use only.

43. Review What are the three main parts of the JFET? Source, drain, and gate Permission granted to reproduce for educational use only.

44. Review In a JFET, what is the channel? The path from the source to the drain Permission granted to reproduce for educational use only.

45. Review What are the two types of MOSFETs? Depletion-mode MOSFET and enhancement-mode MOSFET Permission granted to reproduce for educational use only.

46. Transistors as Amplifiers • Amplification accomplished with vacuum tubes or semiconductor devices • Gain • Control circuits, can control larger amounts of voltage or current • Can switch current on or off Permission granted to reproduce for educational use only.

47. Biasing • Two junctions • Emitter junction • Collector junction • Forward and reverse biases are needed for a transistor amplifier, one kind for each junction Permission granted to reproduce for educational use only.

48. Single Battery Circuit • The collector is connected to the positive terminal • Most negative point in the circuit is the ground, directly connected to negative terminal • Voltage at the base is less positive than positive terminal of the source Permission granted to reproduce for educational use only.

49. Methods of Bias • Fixed bias • Single battery bias • Emitter biasing • Self bias • Degeneration Permission granted to reproduce for educational use only.

50. Review What is an amplifier? An electronic circuit that uses a small input signal to control a larger output signal Permission granted to reproduce for educational use only.