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Bipolar Junction Transistors (1). Dr. Wojciech Jadwisienczak. EE314. Introduction Your goal is to explain the transistor. It is assumed that EE314 students to which this presentation is aimed, have not a clue to how these little Buggers work and/or how to use them.

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## Bipolar Junction Transistors (1)

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**Bipolar Junction Transistors (1)**Dr. Wojciech Jadwisienczak EE314**Introduction Your goal is to explain the transistor. It**is assumed that EE314 students to which this presentation is aimed, have not a clue to how these little Buggers work and/or how to use them. A real problem with previous explanations: for the sake of "fidelity" authors' include confusing details until the concept, or thread--of how they actually work & how to use them--is lost. The following presentation is comprised of several different explanations. You should read chapter 13 and this presentation several times, because any insight gained from one will help in understanding another.**History of BJT**• First BJT • Basic symbols and features • A little bit of physics… • Currents in BJT’ • Basic configurations • Characteristics Chapter 13: Bipolar Junction Transistors pp. 584-624**First - BJTs**The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research. Reference: Bell Labs Museum B. G. Streetman & S. Banerjee ‘Solid State Electronic Devices’, Prentice Hall 1999.**Interesting story…**Picture from previous slide shows the workbench of John Bardeen and Walter Brattain at Bell Laboratories. They were supposed to be doing fundamental research about crystal surfaces. The experimental results hadn't been very good, though, and there's a rumor that their boss, William Shockley, came near to canceling the project. But in 1947, working alone, they switched to using tremendously pure materials. It dawned on them that they could build the circuit in the picture. It was a working amplifier! John and Walter submitted a patent for the first working point contact transistor. Shockley was furious and took their work and invented the junction transistor and submitted a patent for it 9 days later. The three shared a Nobel Prize. Bardeen and Brattain continued in research (and Bardeen later won another Nobel). Shockley quit to start a semiconductor company in Palo Alto. It folded, but its staff went on to invent the integrated circuit (the "chip") and to found Intel Corporation. By 1960, all important computers used transistors for logic, and ferrite cores for memory.**Point-Contact Transistor –**first transistor ever made**Qualitative basic operation of point-contact**transistor Problems with first transistor…**First Bipolar Junction Transistors**W. Shockley invented the p-n junction transistor The physically relevant region is moved to the bulk of the material**force – voltage/current**water flow – current - amplification Understanding of BJT**Basic models of BJT**npn transistor Diode Diode pnp transistor Diode Diode**BJTs – Basic Configurations**Fluid Flow Analogy Difference between FET (field effect transistor) and BJT Technology of BJTs pnp BJT npn BJT**BJTs – Practical Aspects**Heat sink**More accurate physical description…**pnp BJT -iC iE -VCE -iB • Injected h+ current from E to B • 2. e- injected across the forward-biased EB junction (current from B to E) • 3. e- supplied by the B contact for recombination with h+ (recombination current) • 4. h+ reaching the reverse-biased C junction • 5,6.Thermally generated e- & h+ making up the reverse saturation current of the C junction**Now, you can try…**npn BJT**npn BJTs – Operation Modes**Forward & reverse polarized pn junctions Different operation modes:**npn BJTs – Operation Modes**• When there is no IB current almost no IC flows • When IB current flows, IC can flow • The device is then a current controlled current device Operational modes can be defined based on VBE and VBC**BJT-Basic operation**pnp BJT npn BJT (n+), (p+) – heavy doped regions; Doping in E>B>C**BJTs – Current & Voltage Relationships**Operation mode: vBE is forward & vBC is reverse Einstein relation The Shockley equation IES–saturation I (10-12-10-16A); VT=kT/q -thermal V (26meV) D – diffusion coefficient [cm2/s] m – carrier mobility [cm2/Vs] The Kirchhoff’s laws It is true regardless of the bias conditions of the junction Useful parameter the common-emitter current gain for ideal BJT b is infinite**BJTs – Current & Voltage Relationships**Useful parameter the common-base current gain for typical BJT a is ~0.99 The Shockley equation once more If we define the scale current A little bit of math… search for iB Finally…**BJTs – Characteristics**Schematic Common-Emitter Output Input VBC<0 or equivalently VCE>VBE If VCE<VBE the B-C junction is forward bias and IC decreases Remember VBE has to be greater than 0.6-07 V Example 13.1**BJTs – Load line analysis**Common-Emitter Amplifier smaller vin(t) Input loop if iB=0 if vBE=0**BJTs – Load line analysis**Common-Emitter Amplifier Output loop Example 13.2**Circuit with BJTs**Our approach: Operating point - dc operating point Analysis of the signals - the signals to be amplified Circuit is divided into: model for large-signal dc analysis of BJT circuit bias circuits for BJT amplifier small-signal models used to analyze circuits for signals being amplified Remember !**Large-Signal dc Analysis:Active-Region Model**Important: a current-controlled current source models the dependence of the collector current on the base current VCB reverse bias ? VBE forward bias ? The constrains for IB and VCE must be satisfy to keep BJT in the active-mode**Large-Signal dc Analysis:Saturation-Region Model**VCB forward bias ? VBE forward bias ?**Large-Signal dc Analysis:Cutoff-Region Model**VCB reverse bias ? VBE reverse bias ? If small forward-bias voltage of up to about 0.5 V are applied, the currents are often negligible and we use the cutoff-region model.**Large-Signal dc Analysis**Procedure: (1) select the operation mode of the BJT (2) use selected model for the device to solve the circuit and determine IC, IB, VBE, and VCE (3) check to see if the solution satisfies the constrains for the region, if so the analysis is done (4) if not, assume operation in a different region and repeat until a valid solution is found This procedure is very important in the analysis and design of the bias circuit for BJT amplifier. The objective of the bias circuit is to place the operating point in the active region. Bias point – it is important to select IC, IB, VBE, and VCE independent of the b and operation temperature. Example 13.4, 13.5, 13.6**Large-Signal dc Analysis: Bias Circuit**VBB acts as a short circuit for ac signals From Example 13.6 Remember: that the Q point should be independent of the b (stability issue) VBB & VCC provide this stability, however this impractical solution Other approach is necessary to solve this problem-resistor network**Large-Signal dc Analysis: Four-Resistor Bias Circuit**Solution of the bias problem: 3 1 Thevenin equivalent 4 2 Equivalent circuit for active-region model Input Output

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