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CHAPTER 4

CHAPTER 4. The Bipolar Transistor. OBJECTIVES. Describe and Analyze: Transistor architecture Transistor characteristics Transistors as switches Transistor biasing Transistor amplifiers Troubleshooting techniques. Introduction.

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CHAPTER 4

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  1. CHAPTER 4 The Bipolar Transistor

  2. OBJECTIVES Describe and Analyze: • Transistor architecture • Transistor characteristics • Transistors as switches • Transistor biasing • Transistor amplifiers • Troubleshooting techniques

  3. Introduction The key characteristic of a bipolar transistor is that a small amount of power in the base-emitter circuit can control a larger amount of power in the collector-emitter circuit.

  4. Inside the Transistor A small voltage applied base to emitter causes charge carriers to flood into the base. Almost all of those carriers are then swept into the collector. Some of them come out the base due to electron-hole recombination at defects in the crystal structure.

  5. Alpha () and Beta () Alpha is a key parameter of BJTs:  = IC / IE Beta is a secondary parameter:  = IC / IB With a little algebra you can get the relationship: • =  / (1 - ) Using the values  = 0.99 and  = 0.98, you get  = 99 and  = 49 respectively. So you can see that small changes in alpha cause large changes in beta. Two transistors with the same part numbers can have two very different values of beta.

  6. Calculations with Beta Suppose you measure the currents in a transistor and find that IB = 0.2 mA and IC = 10 mA. Calculate the value of IC for a base current of 1mA. Find beta from data:  = IC / IB = 10 / 0.2 = 50 Use calculated beta to find new IC: IC =   IB = 50  1mA = 50 mA

  7. Transistor Switches Compared to mechanical switches, transistors used as switches: • Last much longer • Can turn on and off much faster

  8. Calculations for a Transistor Switch Suppose you want to use a transistor ( = 50) to turn on an LED. The LED requires 40 mA for full brightness. You will use a 12 Volt DC power supply. What value should you use for a base resistor? Base current needed: IB = IC /  = 40mA / 50 = 0.8 mA For reliability, over-drive the base by a factor of 2: Actual IB = 2  calculated IB = 2  0.8mA = 1.6 mA Calculate base resistor: RB = 12V / 1.6mA = 7.5 k

  9. Power Gain of a Transistor Switch Suppose a transistor ( = 100) is being used to turn a 100 Watt load on and off. The load uses 50 Volts DC. What is the power gain? Find IC using I = P / V: IC = 100W / 50V = 2A Find IB using beta: IB = IC /  = 2A / 100 = 20 mA Find base-drive power assuming VBE = 0.7 Volts: P = V  I = 0.7V  .02A = 14 mW Power gain (AP) = PLOAD / PBASE: AP = 100W / .014W = 7143

  10. Transistor Data: Stress Limits • Power Dissipation, Maximum: VCE IC + VBE  IB • Collector Current, Maximum: IC(MAX) • Base Current, Maximum: IB(MAX) • VCE Maximum: VCEO • VCB Maximum: VCBO • VBE Reverse-Biased, Maximum: VEBO • Junction Temperature, Maximum: TJ(MAX)

  11. Transistor Data: Signal • DC Beta: hFE hFE = IC / IB where currents IC and IB are DC • AC Beta: hfe hfe = iC / iB where currents iC and iB are signal currents • Gain-Bandwidth Product: fT fT is the frequency at which hfe = 1

  12. Gain & Amplification Voltage Gain: AV = VOUT / VIN Current Gain: AI = IOUT / IIN Power Gain: AP = POUT / PIN = AV AI Amplification requires power gain.

  13. Classification of Amplifiers Class A: Can be done with one device Class B: Requires two devices, one for each half cycle Class C: Requires resonant circuit to restore signal shape to a sine-wave

  14. Base-Bias Simple one-resistor base-biasing is not practical due to the large variations in beta between devices.

  15. The Base-Biased Amplifier:Input Impedance Zin = rb ||   r’e where r’e = 25mV / IE (approximately) and rb = Rb for single resistor biasing

  16. The Base-Biased Amplifier:Output Impedance Output Impedance: Zout = Rc

  17. The Base-Biased Amplifier:Voltage Gain Voltage Gain: Av = rc / re where Rc = Rc || RL and re = r’e

  18. Characteristic Curves • With Ib fixed, collector is a constant-current source. • With fixed steps in Ib, the space between the Ic lines shows change in beta with collector current.

  19. Troubleshooting • The base-emitter and base-collector junctions in a transistor can be checked for opens and shorts by measuring resistance with a DMM or VOM. • You can usually distinguish the base-emitter from the base-collector because the resistance will read lower from base to collector. Actually, what your meter is showing you is the voltage drop across the junction. • The above test also lets you separate NPN transistors from PNP transistors.

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