Also, an ideal current mirror will have zero input resistance but high output resistance. An ideal current mirror is a two port circuit that accepts an input current i in and produce an output current i out i in. These are circuits that reflect an input current to a load. Differential amplifier with active loads pnp bjt current. The dc voltage vgis a bias voltage which sets the dc output.
We will find the output resistance of the current mirror, r o by applying a voltage, v x to the output and finding the relationship between this and the current that flows in. The current into tri is set externally by other components, and as a result there is a given voltage built up across the base emitter junction of tr1. The mirrored current is a precise copy of the input current. We thus prefer to employ a unit transistor and create copies by repeating such a device. It is a semiconductor device and it comes in two general types. 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. The two bjts have equal base to emitter voltage vbe, also vb in this case and the two mosfets have equal gate to source voltage vgs, also vg in this case. Q1 q2 q3 q4 vdm2 vdm2 vee vcc i ie ie q3 q4 vodm vodm is singleended output. The output resistance is equal to the output resistance of the q 2 or m 2. Simple current mirror bjt and mosfet base current corrected simple current mirror. I o r 2 154 example current mirror with reference current source neglecting channel. Ecgr 3156 electromagnetic and electronic devices laboratory experiment 1 bjt differential pair amplifier with bjt current mirror 1 university of north carolina at charlotte. The reference current is deactivated in the small signal model, meaning that the base of q3 is simply connected to the collector of q1. Cascode current source contd zvoltage headroom consumed by a cascode mirror v x.
Transistor current mirror circuits are often used within integrated circuits as well as a number of other areas where they enable current to be balanced between. V y v v y in b, the minimum allowable voltage at node p is equal to v p,min v n. 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. On the left is a bjt current mirror and on the right is a mosfet current mirror. We neglect the early effect in the analysis and assume the transistors to have identical parameters. One of the limitations of the current mirror with 2 bjts is the early effect. Due to imbalance created by active load current mirror, only singleended output is available from common collector of q2 and q4. A pnp current mirror converts current sinks into current. This concept of mirroring a current into several other circuit elements is applied almost.
A current mirror is a circuit, which copies a reference current into one or more similar circuit elements typically transistors. The effect is known as thermal runaway, and it is possible. The gain of the current mirror can be simply varied from 1. If a voltage is applied to the bjt baseemitter junction as an input quantity and the collector current is taken as an output quantity, the transistor will act as an exponential voltagetocurrent converter.
Mosfet current mirror and cs amplifier electronic circuits 1 062 prof. Transistor current mirror circuit electronics notes. Current mirrors usually employ the same length for all of the transistors. Rout2 ro2 iv characteristics of nmos current source. Chapter 7 building blocks of integrated circuit amplifiers. 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.
Its load consists of a currentmirror active load consisting of m2 and m3. Also, it can be seen that all internal nodes have low impedance except the output node. There are two disadvantages to mosfet current mirrors as compared to their bjt equivalents. Because the limitations of the mirror depend on the limitations of the transistor, we need a transistor model that includes the early voltage of the device.
The pmos mirror5 enforces equality of the nmos drain currents, and hence that of the nmos v gss. Lecture notes microelectronic devices and circuits. In the left side, the current mirror circuit using 2n2222 bjt is shown where two identical transistor pair is used. Here we will describe the system characteristics of the bjt. Common emitter bjt amplifier design current mirror design. What is current mirror and what are its advantages. Negative feedback in a bjt current mirror all about circuits. To see the e ect of nite on the current mirror, we can assume that the two bjts are matched. First, v gs for the reference transistor will be approximately 1. Symmetry creates virtual ground at amplifier emitter connection. Current mirror bjts bipolar junction transistors electronics. This lab introduces the concept of current mirrors.
An example application of the current mirror is the transconductance op amp. As the base emitter voltage on both transistors is the same, the current in one transistor will exactly mirror that of the second, assuming that both transistors are accurately matched. A current mirror is a circuit designed to copy a current through one active device by controlling. Voltage references and biasing stanford university. Pdf applications of current mirrors in analog signal. The goal of this activity is to study the bjt current source or current mirror. Current mirror examples, proper scaling, bipolar current mirrors duration. Index termscurrent mirror, widlar current source, bjtmemristor circuit, power analysis, noise analysis, total harmonic distortion. Simple current mirror bjt and mosfet base current corrected simple current mirror widlar current source. Current mirror circuit well matched transistors important. The simple current mirror can be obtained from the one transistor current source by using a second transistor in diode connection that generates the necessary gatesource voltage of the transistor in the output stage. Your bottom circuit is correct but bib betaib for the two controlled sources in the middle is nonsense. Otherwise, the mirror would still have a voltage limitation, but would be an ideal current source as long as qout was.
This makes the saturation current is and current gain. Basic current mirror basically, current mirrors cms are used to mirror a reference current multiple times from one designated source into another consuming circuits. Analogue electronicscurrent mirrorswilson wikibooks. The current is converted into a vbe by q1, then q2 turns this voltage back into a current again. Severalstandardbjtampli ercon gurationsarediscussed belowandareanalyzed. Current mirrors neglecting channel length modulation i d k n 2 w l.
Yes, we can get the same result with bjt current mirrors by controlling the relative emitter areas of the mirror transistors, but its easier to manage with mosfets. It may be used as a wilson current source by applying a constant bias current to the input branch as in fig. A wilson current mirror is a threeterminal circuit fig. 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. It is of particular interest for current sense applications for feedback purposes. Ece 255, current sources, mirrors etc purdue engineering. Its just that i watched a video on youtube with two transistor current mirror and there the guy said that if a voltage spike appears at the base of the first transistor, this will increase the base current which will increase the collector current and since the collector is connected to the base, the voltage at the base will decrease, thus providing some negative feedback against voltage.
A basic bjt current mirror circuit courtesy of sedra and. Zds1009 current mirror datasheet diodes incorporated. Ee105 fall 2014 microelectronic devices and circuits. Current mirror circuit designs using bjt and mosfet.
Xperiment 1 bjt differential pair amplifier with bjt current mirror o bjectives in this experiment the students will be. Bjt current mirror mosfet current mirror multisim live. The current source iref sets the drain current in m3 which is mirrored into the drain of m2. Use a bjt to boost up supplies to multiple base currents. Thus, as in the opamp version of this circuit, the diode provides its own bias. Direct scaling of the width also faces difficulties.
Widlar current mirror design using bjtmemristor circuits. Current ratioing is achieved by only scaling the width of transistors. By applying a negative feedback simply joining the base and collector the transistor can be reversed and it will begin acting as the opposite logarithmic. V tn 2 v gs1 v gs2 q 1 and q 2 are matched same k n and v tn i o i ref wl 2 wl 1 ifc ons iderghaltmu, i ovaryslightwv o output resistance r o. Instead of the programming resistor, a potentiometer is used to control the current flow in the live simulation.
Bipolar junction transistors bjt general configuration and definitions the transistor is the main building block element of electronics. A basic bjt current mirror circuit courtesy of sedra and smith. Vincent chang current mirror with multiple outputs pnp current mirrors npn current mirrors s e v v i c i s. 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.
Lecture20 current mirrors 12 current mirrors input and output resistances the input resistance of the current mirror is set by the diode connected transistors. R b serves no purpose except to provide a path for the base current. The current mirror circuit is simulated using proteus models. A fantastic discussion on the cascode current mirror and its improved version can be found in 1. 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. Wilson current mirror bjt based addition of a diodeconnected transistor in series with q2 can reduce the effect of. Ee105 fall 2015 microelectronic devices and circuits. Note that at the q2 side, more current mirrors needs to be used to provide current ki d2 ki d1. The npn current mirror, in its most basic form, is, in effect, an inverting current buffer in the sense that it changes current sources of any impedance into current sinks of high impedance. How to build a current mirror circuit chapter 5 discrete semiconductor circuits pdf version.