bjt - ELEC 101 Bipolar Transistor 1 2009/10 Spring Bipolar...

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Unformatted text preview: ELEC 101 Bipolar Transistor 1 2009/10 Spring Bipolar Transistor (BJT) 1 A Short History of Transistor 1945 - Transistors as amplifiers William Bradford Shockley, John Bardeen , Walter Houser Brattain in Bell Telephone Laboratories The three of them won the 1956 Nobel Prize in Physics for inventing the transistor "for their researches on semiconductors and their discovery of the transistor effect" Shockley: “one of century’s most important scientists”, Times Magazine Shockley hired the brightest scientists and engineers. However he is not a good leader and manager. The famous “traitorous eight” left him and started their own companies. Two of the eight, Bob Noyce and Gordon Moore created Intel. ELEC 101 2 Bipolar Transistor 2 2009/10 Spring Transistor Basic There are two types of standard BJT: NPN and PNP C B NPN or B E E C E B B C PNP NPN E C PNP The leads are labeled Collector (C), Base (B) and Emitter (E) The N and P refers to the material that is used to construct the transistor, n-type and p-type semiconductor The base is typically very thin ! C B E E N P N B C P N P ELEC 101 3 Bipolar Transistor 3 2009/10 Spring BJT operation When VBE > 0.7V, B-E junction is in forward-biased mode, large amount of electrons are injected into base - Some electrons recombine with the holes in the base - Some electrons leave base toward VBE, hence form the base current iB - Most electrons are swept into the collector as the base is very thin and there is an electric field that attracts electrons to go to the collector (B-C junction in reverse-biased), hence form the collector current iC ELEC 101 Bipolar Transistor 4 2009/10 Spring C 4 BJT characteristics B E Voltage Control Current Control IC is non-linearly dependence on VBE and very sensitive to it (an exponential function) IC current is more or less proportional to base current As a result, it is basically a current-control current source. ELEC 101 5 Bipolar Transistor 5 2009/10 Spring BJT model C C B IC I=IB IB E B IE E The base-emitter junction behaves like a diode Collector current IC is linear related to the base current IB, i.e., IC = IB where is the current gain For Silicon BJT, can range from 20 to 200. can go up to 1000 with SiGe technology. ELEC 101 6 Bipolar Transistor 6 2009/10 Spring Modes of Operation C IC C B I=IB IB E B IE E Operation modes: (1) VB < 0.7V, IB = 0, transistor is off (act like a turned OFF switch) (2) VB > 0.7V, IB is large: transistor fully on (act like a turned ON switch) (3) VB > 0.7V, IB is small: active mode with transistor partly on (act like an amplifier) ELEC 101 7 Bipolar Transistor 7 2009/10 Spring BJT as a switch White LEDs White LEDs IC Vsupply (High Voltage) IB VBE > 0.7V (1) VB = 0.7V, IB = 0, transistor is off, LED light is OFF (2) VB > 0.7V, IB is large: transistor fully on, LED light turns ON Vsupply (High Voltage) ELEC 101 8 Bipolar Transistor 8 2009/10 Spring BJT as an amplifier 5V IC IB Collector resistor 170 Vo Vin VBias ~ 0.7V Base resistor 10k (1) Bias the BJT to VB = 0.7V, IB is small (limited by base resistor), transistor is ON (2) Input signal will be amplified at the output. Gain is negative ! (3) Gain can be adjusted by the base and collector resistors. ...
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