Bjt circuit configurations, bjt design, base resistance and emitter current crowding in bjts, punchthrough breakdown in bjts, gummelpoon bjt equivalent circuit accounting for the leakage currents. The q point is made stable in the active region of the transistor. We have two current equations and three currents i. The bipolar junction transistor bjt is constructed with three doped semiconductor regions separated by two pn junctions. For more information, several excellent texts exist for beginners, including fundamentals of microelectronics by razavi and the art of electronics by horowitz and. The transconductance g m of a mosfet is geometry dependent whereas that of the bjt is not. A signal of small amplitude if applied to the base is available in the amplified form. Real iv is limited by breakdown of the base collector junction. Because of recombinations of the minority and majority carriers, the equations for the currents can be divided into three regions. Here we will describe the system characteristics of the bjt. Gramp, gmin, and gmindc place a conductance in parallel with both the baseemitter and basecollector pn junctions.
It covers the measurements of static and small signal parameters. Bipolar junction transistors bjt general configuration and definitions the transistor is the main building block element of electronics. The bjt is biased with a current source with high output impedance and a capacitor connects the emitter to ground cap provides an ac short at the emitter for small timevarying signals but is an open circuit for dc signals can redraw the circuit with an equivalent circuit that replaces the bjt with its hybrid. Laboratory exercise 2 dc characteristics of bipolar junction transistors bjt the aim of the exercise the main aim of this laboratory exercise is to understand principles of operation of bipolar junction transistors bjt. The hbt can handle the singles of very high frequencies of several hundred ghz generally it is used in ultrafast circuits and mostly used in radiofrequency. Measure ib, the base current and compare that to the collector current.
An npn transistor has an n type emitter, a p type base and an n type collector while a pnp transistor. The bipolar junction transistor bjt, the first type of transistor to be massproduced, is a combination of two junction diodes, and is formed of either a thin layer of ptype semiconductor sandwiched between two ntype semiconductors an npn transistor, or a thin. A bipolar junction transistor is a three terminal semiconductor device consisting of two pn junctions which is able to amplify or magnify a signal. Depending on how the junction is biased, current may easily flow between the diode terminals forward bias, v d v on or the current is essentially zero reverse bias, v d bjt formula summary equations are for the npn bjt. The three terminals of the bjt are the base, the collector and the emitter. Real shows finite slope due to base width modulation dependent on the applied v. The bjt bjt device equations figure 1 shows the circuit symbols for the npn and pnp bjts. Transistors are three terminal active devices made from different semiconductor. Bjt bipolar junction transistor are widely used an amplifier, oscillator, switch etc. For example, with biasing point of i d 50 ma, with k0 n 120 av2, with wl 1, g m 0. Ac analysis defining the important parameters of any system.
Free bjt circuits books download ebooks online textbooks. These semiconductor materials can either be a p type positive with an excess of holes, or a n type negative with an excess of. Bjt transistor modeling 4 a model is a combination of circuit elements, properly chosen, that best approximates the actual behavior of a semiconductor device under specific operating conditions. Bjt structure and operational modes recall that the semiconductor diode is simply a pn junction. Bipolar junction transistor operationtypesapplications. The bipolar junction transistor bjt is an active device. Click on the tab below to simply browse between the. Speed wb, l are critical dimensions for improving speed performance note exponent of 2 in. Note that, just as f or the case of the commonemitter amplifier, the bypass capacitor introduces a zero at. This set of characteristics obeys the same exponential. The middle section base is narrow and one of the other two regions emitter is heavily doped. The transconductance of a bjt is much larger than that of a mosfet. Laboratory exercise 2 dc characteristics of bipolar. A bipolar junction transistor consists of two backtoback pn junctions, who share a thin common region with width, wb.
When input is applied as shown below the base current starts to vary up and down, hence collector current also varies as i c. It is a semiconductor device and it comes in two general types. How to use bjt bipolar junction transistor beginners. The hybrid pi model of a bjt is a small signal model, named after the plike equivalent circuit for a bipolar junction transistor. Bipolar junction transistors, bjt in active mode, a simple bjt circuit, ebersmoll model for a pnp transistor, ebersmoll model, ic vce characteristics. In the active mode, the collectorbase junction is reverse biased and the baseemitter junction is forward biased. Bipolar junction transistor is a three terminal device commonly termed as bjt was invented in bell laboratories in 1948 by william shockley, brattain, john bardeen which revolutionized not only the world of electronics but also our day to day life. Chapter 3 frequency response of basic bjt and mosfet. The bjt is biased in the active region using the necessary biasing components. Keeping this in mind, we are going to introduce the bipolar junction transistor as nothing more than two diodes placed back to back with the center region common. Ece 255, mosfet small signal analysis purdue university. We also discuss the design parameters available with each of the two devices, such as i c in the bjt, and i d and v ov in the mosfet, and the tradeoffs encountered in deciding on suitable values. T equation indicates that improvement goes as factor squared.
It consists of an input impedance, r p, an output impedance r 0, and a voltage controlled current source described by the transconductance, g m. Bjt characteristics theory the transistor is a two junction, three terminal semiconductor device which has three regions namely the emitter region, the base region, and the collector region. The hetero bipolar junction transistor is also a type is the bipolar junction transistor. In addition it contains the baseemitter capacitances, the junction capacitance. Letting the impedance of c g and c d be equal to zero, the equivalent resistance seen by c s is m cs s s g r 1 2 1. What region of operation is the transistor operating in. This is quite often the case and tends to confuse new students of electronics. The presentation of the material in this chapter parallels but does not rely on that for the mosfet in chapter 4. The base current i b controls the collector current i c regions of bjt operation. It is a currentdriven device mosfet is voltage driven, the output current is equal to the input current times a factor which is called gain. Bipolar junction transistors bjt and its applications. It uses different semiconductor materials to the emitter and base region and produces heterojunction. The values of i2 and i3 are dramatically di erent than the ones obtained earlier. Transistor action the term transistor action refers to the control of the large collectoremitter linking current by the smaller base back injection current in forward active operation, the origin of current gain in a bjt two features of the device are essential for transistor action.
Logic gate can turn loads on bjt in saturation or off bjt in cutoff i. Development of the large signal model of a bjt ebers moll model input. The junction between the ntype collector and the p type base is called the collectorbase junction cbj. Thus it is the pole of highest magnitude among all the poles and. Bjt models using the bjt model starhspice manual, release 1998. Lecture10 bjt switching characteristics, small signal model bjt switching characteristics. The miller theorem and the frequency response of the. Note for the cbj, the anode is the base, and the cathode is the collector. The analysis or design of a transistor amplifier requires knowledge of both the dc and ac response of the system. Ithe bipolarjunction transistor bjt model in hspice is an adaptation of the. To facilitate this comparison, typical values for the important parameters of the two devices are first presented. Bjt amplifier circuits pdf 23p this note covers the following topics. The forward biased junction in the bjt follows the same curve as we saw for the forward biased diode. When more than one equation is given, either can be used.
Contacts are made to all three regions, the two outer regions called the emitter and collector and the middle region called the base. A bjt is not the same as two diodes connected backtoback although it does have two pn junctions. Bipolar junction transi stors bjts i ntroduction in this chapter, we study the other major threeterminal device. Recommended software programs are sorted by os platform windows, macos, linux, ios, android etc. Therefore voltage across r 3 varies as the collector current is passing through it. Applications of bipolar junction transistor or bjt. For transistor amplifiers the resulting dc current and voltage establish an operating point on the characteristics that define the region that will be employed for. For the pnp device, reverse the directions of all current labels and reverse the order of subscripts involving node labels, i. Experiment 9 bipolar junction transistor characteristics 3 of 6 figure 2. This set of characteristics obeys the same exponential relationship as the diode, has the same turn on voltage 0. Soon after its invention it replaced vacuum tubes that were ubiquitous electronics device in all.
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