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Electronics Transistors

Electronics Transistors. Dr. Walid Mohiyeldin Ibrahim IT Department. What is a Transistor?. Vacuum tubes. Purpose Used as signal amplifiers and switches Advantages High power and frequency operation Operation at higher voltages Less vulnerable to electromagnetic pulses Disadvantages

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Electronics Transistors

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  1. ElectronicsTransistors Dr. Walid Mohiyeldin Ibrahim IT Department

  2. What is a Transistor?

  3. Vacuum tubes • Purpose • Used as signal amplifiers and switches • Advantages • High power and frequency operation • Operation at higher voltages • Less vulnerable to electromagnetic pulses • Disadvantages • Very large and fragile • Energy inefficient • Expensive

  4. Invention • Evolution of electronics • In need of a device that was small, robust, reliable, energy efficient and cheap to manufacture • 1947 • John Bardeen, Walter Brattain and William Schockly invented transistor • Transistor Effect • “when electrical contacts were applied to a crystal of germanium, the output power was larger than the input.”

  5. Transistors First Transistor Different types and sizes FET and BJT Transistor BJT (PNP) Electrical Diagram Modern Electronics

  6. Purpose • To amplify and switch electronic signals on or off (high or low) • Modern Electronics Microprocessor Motor Controllers Cell Phones

  7. General Applications

  8. Doping • Process of introducing impure elements (dopants) into semiconductor wafers to form regions of differing electrical conductivity Negatively charged Semiconductor Positively charged semiconductor

  9. Doping Effects • P-type semiconductors • Created positive charges, where electrons have been removed, in lattice structure • N-type semiconductors • Added unbound electrons create negative charge in lattice structure • Resulting material • P-N junction

  10. P-N junction Forward Biasing Reverse Biasing

  11. BJT Doping • Collector (C): Larger in size and moderately doped • Base (B): Very thin layer and lightly doped. • Emitter (E): Smaller than (C)and heavily doped.

  12. P-N junction • Controls current flow via external voltage • Two P-N junctions (bipolar junction transistor, BJT) • Controls current flow and amplifies the current flow

  13. BJT Introduction • Bipolar Junction Transistors (BJT) consists of three “sandwiched” semiconductor layers • The three layers are connected to collector (C), emitter (E), and base (B) pins • Current supplied to the base controls the amount of current that flows through the collector and emitter

  14. BJT Introduction • Transistors are equivalent to a dam with a variable gate that controls the amount of water flow • A small amount of energy is required to operate the gate. • Amplification is achieved by a small amount of energy can be used to control the flow of a large amount of current.

  15. In the hydrodynamic analogy, • The emitter correspond to the river above the dam • The collector correspond to the river below the dam. • The base terminal corresponds to the control input that varies the flow through the dam.

  16. BJT Schematic NPN • NPN • BE forward bias • BC reverse bias • PNP • BE reverse bias • BC forward bias PNP

  17. BJT Characteristic Curves Transfer Characteristic • Characteristic curves can be drawn to show other useful parameters of the transistor • The slope of ICE / IBE  is called the Transfer Characteristic (β)

  18. BJT Characteristic Curves Input Characteristic • The Input Characteristic is the base emitter current IBE against base emitter voltage VBE • IBE/VBE shows the input Conductance of the transistor. • The increase in slope of when the VBE is above 1 volt shows that the input conductance is rising • There is a large increase in current for a very small increase in VBE.

  19. BJT Characteristic Curves Output Characteristic • collector current (IC) is nearly independent of the collector-emitter voltage (VCE), and instead depends on the base current (IB) IB4 IB3 IB2 IB1

  20. BJT Operating Regions

  21. BJT Operating Regions

  22. BJT Applications BJT Switch • Offer lower cost and substantial reliability over conventional mechanical relays.  • Transistor operates purely in a saturated or cutoff state (on/off) • This can prove very useful for digital applications (small current controls a larger current)

  23. BJT Applications BJT Amplifier

  24. BJT Applications BJT Amplifier

  25. Basic Transistor Operation • To operate the transistor properly, the two pn junction must be correctly biased with external dc voltages. • The figure shown the proper bias arrangement for both npn and pnp transistor for active operation as an amplifier.

  26. Basic Transistor Operation (cont.) Illustration of BJT action: C B E

  27. Basic Transistor Operation (cont.) Transistor Currents: • The directions of the currents in npn transistor and pnp transistor are shown in the figure. • The emitter current (IE) is the sum of the collector current (IC) and the base current (IB) • IB << IE and IC • The capital letter – dc value (1)

  28. Transistor Characteristic & Parameters DC Beta ( ) and DC Aplha ( ) • The ratio of the dc collector current (IC) to the dc base current (IB) is the dc beta ( ) = dc current gain of transistor • Range value : 20< <200 • Usually designed as an equivalent hybrid (h) parameter, on transistor data sheet – • The ratio of the dc collector current (IC) to the dc emitter current (IE) is the dc alpha ( ) – less used parameter in transistor circuits • Range value-> 0.95< <0.99 or greater , but << 1 (Ic< IE )

  29. Transistor Characteristic & Parameters (cont.) Current and Voltage Analysis: • The current and voltage can be identified as following: • Current: Voltage: dc base current, dc voltage at base with respect to emitter, dc emitter current, dc voltage at collector with respect to base, dc collector current, dc voltage at collector with respect to emitter, forward-biased the base-emitter junction reverse-biased the base-collector junction Transistor current & voltage

  30. Transistor Characteristic & Parameters (cont.) Current and Voltage Analysis: • When the BE junction is forward-biased, like a forward biased diode and the voltage drop is • Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the voltage across is: …….(1) • Also, by Ohm’s law: ……..(2) • From (1) ->(2) : • Therefore, the dc base current is: (3) (4)

  31. Transistor Characteristic & Parameters (cont.) Current and Voltage Analysis: • The voltage at the collector with respect to the grounded emitter is • Since the drop across is: • The dc voltage at the collector w.r.t the emitter: where • The voltage at the collector w.r.t the base: (5) (6)

  32. Basic Transistor Operation • To operate the transistor properly, the two pn junction must be correctly biased with external dc voltages. • The figure shown the proper bias arrangement for both npn and pnp transistor for active operation as an amplifier.

  33. Basic Transistor Operation (cont.) Illustration of BJT action: C B E

  34. Characteristics of Transistors • Cutoff Region • Not enough voltage at B for the diode to turn on. • • No current flows from C to E and the voltage at C is Vcc. • Saturation Region • The voltage at B exceeds 0.7 volts, the diode turns on and the maximum amount of current flows from C to E. • The voltage drop from C to E in this region is about 0.2V but we might assume it is zero in this class.

  35. Characteristics of Transistors • Active Region • As voltage at B increases, the diode begins to turn on. • A small amounts of current start to flow through into the doped region. • A larger current proportional to IB, flows from C to E. • As the diode goes from the cutoff region to the saturation region, the voltage from C to E gradually decreases from Vcc to 0.2V.

  36. Transistor Characteristic & Parameters (cont.) Current and Voltage Analysis: • When the BE junction is forward-biased, like a forward biased diode and the voltage drop is • Since the emitter is at ground (0V), by Kirchhoff’s voltage law, the voltage across is: …….(1) • Also, by Ohm’s law: ……..(2) • From (1) ->(2) : • Therefore, the dc base current is: (3) (4)

  37. Transistor Characteristic & Parameters (cont.) Current and Voltage Analysis: • The voltage at the collector with respect to the grounded emitter is • Since the drop across is: • The dc voltage at the collector w.r.t the emitter: where • The voltage at the collector w.r.t the base: (5) (6)

  38. Bipolar Transistor Biasing

  39. Bipolar Transistor Biasing

  40. Bipolar Transistor Biasing

  41. Example • Determine Operation Q-point in figure below and find the maximum peak value of base current for linear operation (Active Region). Assume βDC=200.

  42. Solution • Operation Q-point is defined by values of IC and VCE. ) = 6.93V • Q-point is at IC=39.6mA and VCE=6.93V. Since IC(cutoff)=0, we need to know IC(sat) to determine variation in IC can occur and still in linear operation. • Before saturation is reached, IC can increase an amount equal to: IC(sat) – ICQ = 60.6mA – 39.6mA = 21mA.

  43. Questions?

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