Physics of Bipolar Transistors - Analog Circuits - Lecture Slides, Slides of Computer Science

Download Physics of Bipolar Transistors - Analog Circuits - Lecture Slides and more Slides Computer Science in PDF only on Docsity! Chapter 4 Physics of Bipolar Transistors • 4.1 General Considerations • 4.2 Structure of Bipolar Transistor • 4.3 Operation of Bipolar Transistor in Active Mode • 4.4 Bipolar Transistor Models • 4.5 Operation of Bipolar Transistor in Saturation Mode • 4.6 The PNP Transistor Docsity.com Bipolar Transistor  In the chapter, we will study the physics of bipolar transistor and derive large and small signal models. Docsity.com Exponential Voltage-Dependent Current Source  A three-terminal exponential voltage-dependent current source is shown above.  Ideally, bipolar transistor can be modeled as such. Docsity.com Structure and Symbol of Bipolar Transistor  Bipolar transistor can be thought of as a sandwich of three doped Si regions. The outer two regions are doped with the same polarity, while the middle region is doped with opposite polarity. Docsity.com Injection of Carriers  Reverse biased PN junction creates a large electric field that sweeps any injected minority carriers to their majority region.  This ability proves essential in the proper operation of a bipolar transistor. Docsity.com Carrier Transport in Base tL Reverse L a + V. Biased n + Vy or ce “== “ce P r ™ PT | Xo T- “EOS a . Forward r t Biased Electron T Density = ® Docsity.com Collector Current  Applying the law of diffusion, we can determine the charge flow across the base region into the collector.  The equation above shows that the transistor is indeed a voltage-controlled element, thus a good candidate as an amplifier. BE inE S T BE SC T BE BE inE C WN nqDA I V V II V V WN nqDA I 2 2 exp 1exp          Docsity.com Parallel Combination of Transistors  When two transistors are put in parallel and experience the same potential across all three terminals, they can be thought of as a single transistor with twice the emitter area. Docsity.com Base Current  Base current consists of two components: 1) Reverse injection of holes into the emitter and 2) recombination of holes with electrons coming from the emitter. BC II  Docsity.com Emitter Current  Applying Kirchoff’s current law to the transistor, we can easily find the emitter current. B C CE BCE I I II III            1 1 Docsity.com Summary of Currents             1 exp 1 exp 1 exp T BE SE T BE SB T BE SC V V II V V II V V II Docsity.com (a) Characteristics of Bipolar Transistor Docsity.com Example: IV Characteristics le le 1.153 mA | 1.153 mA; —-—___ Vg = 800 mV 169 LA} --------------7 169 LA} Vee = 750 mV 24.6 WA. i 24.6 [LAl.--—_____ Vege = 700 mV (a) (b) 11.5 WAl----- es Vee = 800 mV 0.169 [LA] ----——_—__s Vee = 750 mV 0.025 [A] Vpe = 700 mV (c) (a) Docsity.com Transconductance  Transconductance, gm shows a measure of how well the transistor converts voltage to current.  It will later be shown that gm is one of the most important parameters in circuit design. T C m T BE S T m T BE S BE m V I g V V I V g V V I dV d g          exp 1 exp Docsity.com Transconductance and Ic  The figure above shows that for a given VBE swing, the current excursion around IC2 is larger than it would be around IC1. This is because gm is larger IC2. Docsity.com Small-Signal Model: Derivation  Small signal model is derived by perturbing voltage difference every two terminals while fixing the third terminal and analyzing the change in current of all three terminals. We then represent these changes with controlled sources or resistors. Docsity.com Small-Signal Model: VBE Change Docsity.com Small Signal Example II  In this example, a resistor is placed between the power supply and collector, therefore, providing an output voltage. Docsity.com AC Ground  Since the power supply voltage does not vary with time, it is regarded as a ground in small-signal analysis. Docsity.com Early Effect  The claim that collector current does not depend on VCE is not accurate.  As VCE increases, the depletion region between base and collector increases. Therefore, the effective base width decreases, which leads to an increase in the collector current. Docsity.com Early Effect and Large-Signal Model  Early effect can be accounted for in large-signal model by simply changing the collector current with a correction factor.  In this mode, base current does not change. Docsity.com Early Effect and Small-Signal Model C A T BE S A C CE o I V V V I V I V r     exp Docsity.com Summary of Ideas Operation in Active Mode Model Ica tog Veer UV Characteristrics Small-Signal Model / B c _| + Vx Vee o> es VE 3m Modified Small-Signal Model Docsity.com Overall I/V Characteristics  The speed of the BJT also drops in saturation. Docsity.com Example: Acceptable VCC Region  In order to keep BJT at least in soft saturation region, the collector voltage must not fall below the base voltage by more than 400mV.  A linear relationship can be derived for VCC and RC and an acceptable region can be chosen. )400( mVVRIV BECCCC  Docsity.com Deep Saturation  In deep saturation region, the transistor loses its voltage- controlled current capability and VCE becomes constant. Docsity.com PNP Equations                  A EC T EB SC T EB SE T EBS B T EB SC V V V V II V V II V VI I V V II 1exp exp 1 exp exp    Early Effect Docsity.com Large Signal Model for PNP Docsity.com PNP Biasing  Note that the emitter is at a higher potential than both the base and collector. Docsity.com Small Signal Model Example I Docsity.com Small Signal Model Example II  Small-signal model is identical to the previous ones. Docsity.com Small Signal Model Example III  Since during small-signal analysis, a constant voltage supply is considered to be AC ground, the final small-signal model is identical to the previous two. Docsity.com