Lecture 4 OUTLINE Bipolar Junction Transistor BJT General considerations Structure Operation in active mode Large signal model and I V characteristics Reading Chapter 4 1 4 4 2 EE105 Fall 2007 Lecture 4 Slide 1 Prof Liu UC Berkeley Voltage Dependent Current Source A voltage dependent current source can act as an amplifier If KRL is greater than 1 then the signal is amplified Vout AV KR L Vin EE105 Fall 2007 Lecture 4 Slide 2 Prof Liu UC Berkeley Voltage Dependent Current Source with Input Resistance The magnitude of amplification is independent of the input resistance rin EE105 Fall 2007 Lecture 4 Slide 3 Prof Liu UC Berkeley Exponential Voltage Dependent Current Source Ideally a bipolar junction transistor BJT can be modeled as a three terminal exponential voltagedependent current source EE105 Fall 2007 Lecture 4 Slide 4 Prof Liu UC Berkeley Reverse Biased PN Junction as a Current Source PN junction diode current is independent of the reverse bias voltage It depends only on the rate at which minority carriers are introduced into the depletion region We can increase the reverse current by injecting minority carriers near to the depletion region EE105 Fall 2007 Lecture 4 Slide 5 Prof Liu UC Berkeley BJT Structure and Circuit Symbol A bipolar junction transistor consists of 2 PN junctions that form a sandwich of three doped semiconductor regions The outer two regions are doped the same type the middle region is doped the opposite type EE105 Fall 2007 Lecture 4 Slide 6 Prof Liu UC Berkeley NPN BJT Operation Qualitative In the forward active mode of operation The collector junction is reverse biased The emitter junction is forward biased current gain EE105 Fall 2007 Lecture 4 Slide 7 IC IB Prof Liu UC Berkeley Base Current The base current consists of two components 1 Injection of holes into the emitter and 2 Recombination of holes with electrons injected from the emitter I C I B EE105 Fall 2007 Lecture 4 Slide 8 Prof Liu UC Berkeley BJT Design Important features of a well designed BJT large Injected minority carriers do not recombine in the quasi neutral base region Emitter current is comprised almost entirely of carriers injected into the base rather than carriers injected into the emitter EE105 Fall 2007 Lecture 4 Slide 9 Prof Liu UC Berkeley Carrier Transport in the Base Region Since the width of the quasi neutral base region WB x2 x1 is much smaller than the minority carrier diffusion length very few of the carriers injected from the emitter into the base recombine before they reach the collector junction depletion region Minority carrier diffusion current is constant in the quasi neutral base The minority carrier concentration at the edges of the collectorjunction depletion region are 0 EE105 Fall 2007 Lecture 4 Slide 10 Prof Liu UC Berkeley Diffusion Example Redux Linear concentration profile constant diffusion current Non linear concentration profile varying diffusion current x p N 1 L J p diff EE105 Fall 2007 p N exp dp qD p dx N qD p L J p diff Lecture 4 Slide 11 x Ld dp qD p dx qD p N x exp Ld Ld Prof Liu UC Berkeley Collector Current IC A E qD n n i2 N BW B IC V BE I S exp VT V BE exp 1 VT where IS A E qD n n i2 N BW B The equation above shows that the BJT is indeed a voltage dependent current source thus it can be used as an amplifier EE105 Fall 2007 Lecture 4 Slide 12 Prof Liu UC Berkeley Emitter Current Applying Kirchhoff s Current Law to the BJT we can easily find the emitter current 1 I E I C I B I C 1 EE105 Fall 2007 Lecture 4 Slide 13 Prof Liu UC Berkeley Summary of BJT Currents IC IB IE EE105 Fall 2007 V BE I S exp VT 1 V BE I S exp VT 1 V BE I S exp VT 1 Lecture 4 Slide 14 Prof Liu UC Berkeley Parallel Combination of Transistors When two transistors are connected in parallel and have the same terminal voltages they can be considered as a single transistor with twice the emitter area EE105 Fall 2007 Lecture 4 Slide 15 Prof Liu UC Berkeley Simple BJT Amplifier Configuration Although the BJT converts an input voltage signal to an output current signal an amplified output voltage signal can be obtained by connecting a load resistor with resistance RL at the output and allowing the controlled current to pass through it EE105 Fall 2007 Lecture 4 Slide 16 Prof Liu UC Berkeley BJT as a Constant Current Source Ideally the collector current does not depend on the collector to emitter voltage This property allows the BJT to behave as a constant current source when its base to emitter voltage is fixed EE105 Fall 2007 Lecture 4 Slide 17 Prof Liu UC Berkeley Constraint on Load Resistance If RL is too large then VX can drop to below 0 8V so that the collector junction is forward biased In this case the BJT is no longer operating in the active mode and so I C I B There exists a maximum tolerable load resistance EE105 Fall 2007 Lecture 4 Slide 18 Prof Liu UC Berkeley BJT I V Characteristics EE105 Fall 2007 Lecture 4 Slide 19 Prof Liu UC Berkeley Example EE105 Fall 2007 Lecture 4 Slide 20 Prof Liu UC Berkeley BJT Large Signal Model A diode is placed between the base and emitter terminals and a voltage controlled current source is placed between the collector and emitter terminals EE105 Fall 2007 Lecture 4 Slide 21 Prof Liu UC Berkeley BJT vs Back to Back Diodes Figure b presents a wrong way of modeling the BJT EE105 Fall 2007 Lecture 4 Slide 22 Prof Liu UC Berkeley
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