Bipolar junction transistors tlt8016 basic analog circuits 20052007 3 basic operation in the active region figure 4. In this experiment, we will make up the circuit using discrete. Ee105 fall 2014 microelectronic devices and circuits. A bipolar junction transistor bipolar transistor or bjt is a type of transistor that uses both electron and hole charge carriers. Early effect can be combated by using the 4transistor improved wilson current mirror configuration.
An oftenused circuit applying the bipolar junction transistor is the socalled current mirror, which serves as a simple current regulator, supplying nearly constant current to a load over a wide range of load resistances. Conceptually, an ideal current mirror is simply an ideal current amplifier with a gain of 1. The bjt is biased in the active region using the necessary biasing components. Transistor and logic gates laboratory instruction today you will learn how to use a transistor to amplify a small ac signal as well as using it as a switch to construct digital logic circuits. Generally speaking, this version is preferred over the bjt method if using discrete parts. Bjt wilson mirror a wilson current mirror is shown in fig. Zds1009 current mirror datasheet diodes incorporated. Remember that two equal size transistors at the same temperature with the same v gs or v be for a bjt have the same drain current. In chapter 8 we explored the transistor and you should recall that the bjt device is a current amplifier of sorts current controlled current source in that the collector current is. Ac analysis defining the important parameters of any system.
There are only two differences in using pnp rather than npn transistors in current mirrors. In addition to the replication, the current drop associated with. The examples tab demonstrates current mirrors, however what it fails to expand on is the efficiency of a current mirror. The word transistor is derived from the words transfer and resistor it describes the operation of a bjt i. A current mirror with a splitcollector bjt to compensate for the base currents 3 although the splitcollector mirror gain is much more accurate than what is achievable with either the basic mirror or the base current compensated structure of fig. The bipolar junction transistor transistor basics 2 the ideal bipolar junction transistor because the current gain. The transistor will operate as an amplifier or other linear circuit if the transistor is biased into the linear region. The same relationship can be obtained using the equivalent circuit shown in fig. 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. As we will go through the topic, we will know about the transistor, mainly bipolar junction transistor or bjt. Circuit ideahow the wilson current mirror equalizes the. Bipolar junction transistors are widely used for current mirroring.
The zds1009 current mirror has been developed specifically for high side, current sense plus level translation applications and as such will find a broad applications base including battery charge management, dc motor control and over current monitoring functions. The emitter current in q3 is io this current is the input to a basic current mirror consisting of q1 and q2. Characteristics of mosfet current mirrors playhookey. Mar 15, 2020 as we will go through the topic, we will know about the transistor, mainly bipolar junction transistor or bjt. Basic bjt current mirror with multiple outputs duration. Transistor m 1 is operating in the saturation or active mode, and so is m 2. The bjt is fabricated with three separately doped regions. Bjt field effect transistor bipolar junction transistor. It works in active mode actually and the collector current is a function of the base voltage. The q point is made stable in the active region of the transistor. Common emitter bjt amplifier design current mirror design. A current mirror is a circuit designed to copy a current through one active device by controlling.
We can easily use enhancement mode mosfets to form current mirrors, as shown in the circuit to the right. The npn device has one p region between two n regions and the pnp device has one n region between two p regions. This work was supported, in part, by dallas semiconductor corp. There are two types of applications of bipolar junction transistor, switching and amplification. Basic bjt current mirror with multiple outputs youtube. A wilson current mirror is a threeterminal circuit fig. This is done by providing a voltage across the baseemitter junction of the bjt and. Najmabadi, ece65, winter 2012 simplified physical structure. These semiconductor materials can either be a p type positive with an excess of holes, or a n type negative with an excess of.
Structure and operation of the basic two transistor bjt current mirror as an ic biasing block. Check out the hybridpi model for more insight, and try some sims with a small base voltage added to one of the transistors. How to use bjt bipolar junction transistor beginners. Bipolar junction transistors bjts questions and answers. Introduction the ta will show you the emitter, base and collector for the transistor. Sep 18, 2016 a bipolar junction transistor bipolar transistor or bjt is a type of transistor that uses both electron and hole charge carriers.
Current sources and active loads university of north. Ece 255, current sources, mirrors etc purdue engineering. Structure and operation of a basic bjt current mirror with multiple pnp and npn outputs. Because they have relatively low compliance voltages, they also are called wideswing current mirrors. Lecture20current mirrors 1 ee105 fall 2014 microelectronic devices and circuits prof. Npn transistor an implementation on an ic device construction is not symmetric. Use a bjt to boost up supplies to multiple base currents. Ref, is the input to the current mirror at the collector of the diodeconnected transistor q. Bjt bipolar junction transistor are widely used an amplifier, oscillator, switch etc. Current mirrors basic bjt current mirror current mirrors are basic building blocks of analog design. Bipolar junction transistors as switches this worksheet and all related. Small signal models are only useful for forward active mode and thus, are derived under this condition. The transitors have been replaced by the smallsignal tmodel, which has then been drawn as a largesignal bjt, with the smallsignal output resistances, r o in parallel. Replacing all capacitors by a shortcircuit equivalent 3.
It is a current driven device mosfet is voltage driven, the output current is equal to the input current times a factor which is called gain. Small signal model of a bjt just as we did with a pn diode, we can break the bjt up into a large signal analysis and a small signal analysis and linearize the non linear behavior of the ebers moll model. This is problematic for a couple of reasons, most importantly the. Implementation using discrete transistors the bjt di erential pair is an integral part of op amp integrated circuits. The bipolar junction transistor bjt as a switch bipolar. Problems for amplifier section university of california. An example application of the current mirror is the transconductance op amp. For switching applications transistor is biased to operate in the saturation or cutoff region. Electronic circuits 1 highspeed circuits and systems laboratory lect.
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. In this setup, the output current i out is directly related to i ref, as discussed next. The mirrored current is a precise copy of the input current. Applications of bipolar junction transistor or bjt. Widlar current mirror design using bjtmemristor circuits. If a voltage is applied to the bjt baseemitter junction as an input quantity and the. For its analysis, we assume identical transistors and neglect the early e. It is not very widely known, but the collector current of the bjt actually depends not on base current, but on the voltage across be junction.
Current mirror circuit designs using bjt and mosfet. Because of the op amp, these circuits are sometimes called gainboosted current mirrors. Additionally we have darlington pairs and a pchannel mirror. Bjt are called bipolar because both holes and electrons are involved in the. Bjt circuit practice problems you should do a bunch of these to get ready for quizzes, exams, etc. Important attributes for current sources include high output resistance with a wide range of voltage compliance and rejection of external variations such as power supply or temperature. This paper presents a description of basic current mirror cm, widlar current mirror, fourth circuit element. Vincent chang current mirror with multiple outputs pnp current mirrors. Why cant we replace diode connected bjts by simple diodes in a current mirror circuit. Discretecircuit bias c e c c2 c c1 c e c c2 c c1 bias with single supply bias with two supplies. The bjt symbols and their corresponding block diagrams are shown on figure 1. The bjt current mirror operates by the same method as the mosfet version, but with a few key differences.
Because the transistor operation is carried out by two types of charge carriers majority and minority carriers, an ordinary transistor is called bipolar. Figure 3 shows a mirror using negative feedback to increase output resistance. Setting all dc sources to zero and replacing them by a shortcircuit equivalent 2. In any practical circuit, you would lay out the diode as a matched, interdigitated, diodetied bjt. Applications of bipolar junction transistor or bjt history. Scribd is the worlds largest social reading and publishing site. It is of particular interest for current sense applications for feedback purposes. Bjt transistor modeling 5 the ac equivalent of a transistor network is obtained by. Ee105 fall 2015 microelectronic devices and circuits prof. When input is applied as shown below the base current starts to vary up and down, hence collector current also varies as i c.
Jan 06, 2019 this is just one portion of an endless history of bjt. In addition to the replication, the current drop associated with cmos vs ttl is not clearly explained. The bjt mirror has two important differences from that of the mosfet. A currentsteering circuit using mosfet courtesy of sedra and. In contrast, unipolar transistors, such as fieldeffect transistors, only use one kind of charge carrier. With discrete parts and for low frequencies, its easier to just throw an opamp at it. However, due to the quadratic term, a very good approximation is achieved for much lower values of. Jan 29, 2017 structure and operation of the basic two transistor bjt current mirror as an ic biasing block.
Bipolar junction transistors bjts questions and answers q1. Therefore voltage across r 3 varies as the collector current is passing through it. Analogue electronicscurrent mirrorswilson wikibooks, open. Every mirror configuration that can be built with npn transistors can also be built with pnp transistors, and the general behavior, advantages, and disadvantages of each configuration will be the same.
Bipolar junction transistors also known as bjts can be used as an amplifier, filter, rectifier, oscillator, or even a switch, which we cover an example in the first section. Just as we connected collector to base with bipolar junction transistors, we can connect drain to gate of a mosfet to cause the device to operate as a forwardbiased diode. This is a common collector amplifier emitter follower. Chapter 16 analog integrated circuits chapter outline 16. Analogue electronicscurrent mirrorswilson wikibooks. Vmin always trades with noise lowest possible noise. It may be used as a wilson current source by applying a constant bias current to the input branch as in fig. Bjt differential amplifier similarly for bjt a d g m r c. The basic current mirror can also be implemented using mosfet transistors, as shown in figure 2. The goal of this activity is to study the bjt current source or current mirror. Ee105 fall 2015 microelectronic devices and circuits. The output resistors in q1 and q2 are present in this model, but they are greyed out as they do not play a part in the analysis.
There are also a number of secondary performance issues with mirrors, for example. Jan 29, 2017 structure and operation of a basic bjt current mirror with multiple pnp and npn outputs. Drain, gate, source 20080724 v1 c e b q1 rbias 10k rload 3k d1 simple diode current sinking mirror current limitor this is a lesser version of the transistor as diode configuration. The first trick to use bipolar junction transistor as a current mirror circuit is to construct an exponential voltage to current converter using the transistor. Lecture08 current sources university of california, berkeley.
584 1063 169 77 279 772 818 788 1151 1190 1104 1072 577 1013 470 1274 1397 687 986 290 457 653 1150 88 1454 826 307 396 1345 590 24