Thus, as in the opamp version of this circuit, the diode provides its own bias. Conceptually, an ideal current mirror is simply an ideal current amplifier with a gain of 1. Ee105 fall 2015 microelectronic devices and circuits prof. Index terms current mirror, widlar current source, bjt memristor circuit, power analysis, noise analysis, total harmonic distortion. A method and apparatus for compensating a base current of a bipolar junction transistor by replicating operating conditions of the bjt in a compensating circuit.
Prof, ece department, ssn college of engg for a noninfinite early voltage and vcb1 vbe3 current is given by where bjt wilson mirror a wilson current mirror is shown in figure. Agenda operating point transistor dc bias configurations design operations various bjt circuits. Another variation of the basic current mirror circuit is referred to as the wilson mirror or wilson current mirror. 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. Pdf applications of current mirrors in analog signal. This work was supported, in part, by dallas semiconductor corp. Experiment 4 current mirrors the main objective for this part will be to construct a simple current mirror and test its parameters by adding or adjusting resistors. We can easily use enhancement mode mosfets to form current mirrors, as shown in the circuit to the right. 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. Structure and operation of the basic two transistor bjt current mirror as an ic biasing block. The differences turn out to be device property changes that impact various aspects of performance.
This page contains list of freely available ebooks, online textbooks and tutorials in bjt circuits. Current mirror bjts bipolar junction transistors electronics. Free bjt circuits books download ebooks online textbooks. Voltage references and biasing stanford university. Differential amplifier with active loads pnp bjt current. Specially, for lateral transistor, the base current in some processes may be as large as. It may be used as a wilson current source by applying a constant bias current to the input branch as in fig. Check our section of free ebooks and guides on bjt circuits now.
Current mirror circuit designs using bjt and mosfet. Use a bjt to boost up supplies to multiple base currents. The input of the current mirror is a current mode terminal. Lecture20 current mirrors 1 ee105 fall 2014 microelectronic devices and circuits prof. On the left is a bjt current mirror and on the right is a mosfet current mirror. Analog integrated circuits fundamental building blocks current mirrors 5 figure 5. The two bjts have equal base to emitter voltage vbe, also vb in this case and the two mosfets have equal gate to. Variations in cmos devices versus lithography is quite s. Bipolar junction transistors are widely used for current mirroring. These currents are used to load other stages in circuits and they are designed in such a way so that current is constant and independent of loading. T3 acts as a regulating valve which increases the current through the current mirror as input a gets more power while t4 kind of does the opposite by draining current from the mirror with increasing input b power. Analogue electronicscurrent mirrorswilson wikibooks.
These are circuits that reflect an input current to a load. Direct scaling of the width also faces difficulties. Within the circuit, a third transistor is introduced. Current mirrors usually employ the same length for all of the transistors. Current mirrors neglecting channel length modulation i d k n 2 w. Summary of key concepts voltage source easily synthesized from reference current source using mosfet in diode configuration current source easily synthesized from current source using current mirror circuit. In the final section, we saw some examples of how current mirrors are incorporated into analog integrated circuits.
This makes the saturation current is and current gain. Ecgr 3156 electromagnetic and electronic devices laboratory experiment 1 bjt differential pair amplifier with bjt current mirror 1 university of north carolina at charlotte. Need for temperature compensation of current mirror. R b serves no purpose except to provide a path for the base current. Simple current mirror bjt and mosfet base current corrected simple current mirror widlar current source.
Both of them worked as desired but, when heated or cooled, the current through the right side the side where the output is taken from decreased or increased significantly with. Ee105 fall 2015 microelectronic devices and circuits. As with bjt current sources circuits, mosfet current sources require a large output resistance for excellent stability. Widlar current mirror design using bjtmemristor circuits. This transistor, shown as tr3 in the diagram keeps the collector of tr1 at a voltage equivalent to two diode drops below the rail voltage vcc. The transistor q1 and resistor r are used to set up the reference current, iref, the current the opposing side, iout, will mirror. Vincent chang intro current gain output resistance base current compensation concept current transfer ratio multiple outputs npn mirror. T1 and t2 are pnp transistors and form together a current mirror. Pdf current mirrors are one of the most common building blocks both in analog and mixedsignal vlsi circuits.
To alter the bjt mirror ratio, we use the fact that the saturation current of the bipolar transistor is proportional to its emitter area ae and can be. Applications of current mirrors in analog signal processing. 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. Currently this section contains no detailed description. An example of a general current mirror is shown in figure 1. The basic bjt current mirror configuration is shown in fig. I am currently learning about current mirror configurations. A wilson current mirror is a threeterminal circuit fig. Since transistors in the current mirror are processed on the same ic, all physical. An example application of the current mirror is the transconductance op amp.
It should be noted that the wilson current mirror is not a constant current source. This is done by providing a voltage across the baseemitter junction of the bjt and. A read is counted each time someone views a publication summary such as the title, abstract, and list of authors, clicks on a figure, or views or downloads the fulltext. Xperiment 1 bjt differential pair amplifier with bjt current mirror o bjectives in this. Current ratioing is achieved by only scaling the width of transistors. Small signal model of the mos cascode current mirror the input resistance in order to calculate the input and the output resistances, thevenins and nortons theorems must be considered. This is a pity, as current outputs offer advantages in. Characteristics of mosfet current mirrors playhookey. Circuit for pnp current mirror using simple device with dotmodel statement shown figure 1 shows a schematic for a pnp current mirror. As we mentioned on the page discussing errors in current mirrors, the basic current mirror circuit shown to the left has, among other problems, a mismatch between i ref and i o caused by the unavoidable presence of base current for the two transistors flowing through r.
I understand that if the transistor q1 is connected in a diode connected bjt configuration and the transistor q1 and q2 have identical transistor parameters, the current flowing through both the transistors will be the same. Bipolar junction transistors bjt general configuration and definitions the transistor is the main building block element of electronics. What is the difference between a mos current mirror and a. Us7116174b2 base current compensation circuit for a. The goal of this activity is to study the bjt current source or current mirror. Concept of current mirror circuit analog electronics. The mirrored current is a precise copy of the input current. If the supply voltage, v cc changes, the output current. Transistor basics the bipolar junction transistor pdf. Although current mirrors and circuits such as the howland current source are taught in analog circuit courses, a surprising number of engineers tend to think exclusively in terms of voltage when defining the output of precision analog circuitry. A differential amplifier built with a current mirror. Basic current mirror basically, current mirrors cms are used to mirror a reference current multiple times from one designated source into another consuming circuits.
It is a semiconductor device and it comes in two general types. 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 we know that in a transistor operating in its active mode, the collector current is equal to base current multiplied by. Due to imbalance created by active load current mirror, only singleended output is available from common collector of q2 and q4. A mosfet circuit rigged up as a current mirror used. The pmos mirror5 enforces equality of the nmos drain currents, and hence that of the nmos v gss. 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. Here we will describe the system characteristics of the bjt. Ee105 fall 2014 microelectronic devices and circuits. Bjt current mirror mosfet current mirror multisim live. An output current of the compensating circuit is fractionally related to the base current and thus can be supplied to an operational circuit comprising the bjt to compensate the base current. Differential amplifier with active loads active load basics pnp bjt current mirror.
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