Current mirror layout strategies for enhancing matching. Spencer, member, ieee abstract a pedagogical analysis of the modified mos wilson current mirror using signal flow graphs sfgs, masons gain. Eecs140 analog circuit design lectures on current sources cascode source cont. Mar 06, 2017 operation of the basic mosfet current mirror. There is variety of current mirror circuits available, each of them having their own advantage and applications 4. The simple two transistor implementation of the current mirror is based on the fundamental relationship that two equal size transistors at the same temperature with the same v gs for a mos or v be for a bjt have the same drain or collector current. Lecture20current mirrors 10 mos current mirrors changing the mirror ratio mirror ratio can be changed by modifying wl ratios of the two transistors forming the mirror. Mosfet current mirror and active load electronic circuits 1 2 prof. The fundamental current mirror with mos transistors the test schematic oglsimplamos. This is not a current mirror, because the relationship between vx and iref is not clearly defined. From this it will become clear if the gm of m1 is relevant or not. Current mirrors current mirrors are important blocks in electronics. They are widely used in several applications and chips, the operational amplifier being one of them.
Note that the gate voltage of each mosfet is the same i. Wu,uc berkeley ref ref copy i w l w l i 1 1 1 1 2 th n ox ref x v c w l i v. A wilson current mirror is a threeterminal circuit fig. What is current mirror and what are its advantages. Spencer, member, ieee abstract a pedagogical analysis of the modified mos wilson current mirror using signal flow. Wooyoung choi various bias techniques for mosfet circuits how do we make.
Conceptually, an ideal current mirror is simply an ideal inverting current amplifier that reverses the current direction as well. Unfortunately, this is not really true for mosfets. Mosfet current mirror and cs amplifier electronic circuits 1 062 prof. Eecs240 lecture 8 4 current mirror better approach. The implementation of the current mirror circuit may seem simple but there is a lot going on. In the current mirror circuit shown on the left, the reference current is set by the resistor. Lectures on current sources university of california. The reference current source is also known as golden current source. The principal limitation on the use of the wilson current mirror in mos circuits is the high minimum voltages between the ground connection in fig. 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. In the cascode current mirror, if we choose wl ratios of all the transistors equal, we have the potential at a equal to 2v.
Feb 17, 2018 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. Cascode current mirror wilson current mirror wideswing current mirror 4. Direct scaling of the width also faces difficulties. The current mirror circuit of claim 1 wherein the means for causing the voltage at the input node to be substantially equal to the voltage at the output node comprises a transistor having a first electrode connected to the input node, a second electrode connected to the gates of the first and second mos transistors, and a third electrode connected to receive a voltage substantially equal to. Ee105 fall 2014 microelectronic devices and circuits. 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. Characteristics of mosfet current mirrors playhookey. Size the transistors in the mirror for a current gain equal to unity, a 30. Simple current mirror bjt and mosfet base current corrected simple current mirror widlar current source. Analog integrated circuits fundamental building blocks current mirrors 5 figure 5. The current mirror load differential amplifier is not a good example for common mode. Your current mirror will pull current out of an external circuit comprised of a 10k. A current mirror circuit for mirroring current in cmos integrated circuit technology includes a current mirror arrangement formed of first and second pchannel mos transistors mp32,mp33, a variable input current source i cs, a first source follower transistor mn34, a second source follower transistor mp35, a current sinking transistor mn31, and a load circuit 212.
If we try building a current mirror with discrete, offtheshelf bjts of the same type, the circuit will work. Feb 18, 2018 in next videos, we will cover cascode current mirror, wilson current mirror etc. 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. 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 fundamental current mirror with mos transistors the test schematic oglsimpla mos. This thinking is starting to change as new frontiers in ultralow power begin to be exploited through precise control of subthreshold leakage currents. A pedagogical exercise in signal flow graphs, masons gain rule, and drivingpoint impedance techniques ronald g. Remembering the drain current to gate source voltage relationship for a mos transistor. The transistors wont be wellmatched, but the circuit will function, although imperfectly.
Iowa state university ames, ia 50011, usa dallas semiconductor corp. Lowvoltage wilson current mirrors in cmos i in v in 2i b i b i out v out 2i b i in i b i out v out v in i in 2i b i b i out v out v in highswing wilson highswing superwilson lowvoltage superwilson shown above are lowvoltage, highswing analogs of the wilson and superwilson mirrors. Mos current mirror the motivation behind a current mirror is to duplicate a scaled version of the golden current to other locations. We thus prefer to employ a unit transistor and create copies by repeating such a device. Lecture20 current mirrors 10 mos current mirrors changing the mirror ratio mirror ratio can be changed by modifying wl ratios of the two transistors forming the mirror. Current mirror concept generation of required v gs current mirror circuitry ee105spring2008 lecture20,slide19 prof. Ch 9 cascode stages and current mirrors 47 bad mos current mirror example this is not a current mirror since the relationship between vxand iref is not clearly defined. A cmos current mirror with lower than vos3a, input voltage requirement is presented. Differential amplifier with mos diode loads smallsignal analysis of the commonmode of the differential amplifier the commonmode gain of the differential amplifier with a current mirror load is ideally.
Consider a mosfet connected in diode configuration. The current mirror circuit gains its name because it copies or mirrors the current flowing in one active device in. Current mirrors usually employ the same length for all of the transistors. Transistor current mirror circuit electronics notes.
Problem calculate the mirror ratio for the mos and bjt current mirrors in fig. For example the mos transistor is generally modeled as a voltage controlled current source and can not be used. Bulk driven technique is used to reduce the threshold of pmos used in low voltage current mirror circuits lvcm. Or it can consist of a current controlled current source cccs. State of the system when there is no signal current and voltages in all elements. To best understand this important circuit building. Bias current sources what makes a current source a current source. Pdf compensation in mos current mirror circuits researchgate. Pdf a new low voltage pmos bulk driven current mirror circuit. Wooyoung choi for q 1 and q 2 i o i ref only if v ds1 v ds2 v 0v gs 0 ogs o ref vv ii r mismatches between i ref and i o.
Similar to its bipolar counterpart, mos cascode can be thought of as stacking a transistor on top of a current source. The only way to clearly define vx with iref is to use a diode connected mos since it provides squarelaw iv relationship. Measure id3 with either an ammeter, or by measuring the voltage drop across r1 and applying ohms law. Current and voltage sources 1 analog design for cmos vlsi systems franco maloberti current mirrors a current mirror gives a replica attenuated or amplified, if necessary of a bias or signal current. This work was supported, in part, by dallas semiconductor corp. The current mirror is used to provide bias currents and active loads to. Wu, uc berkeley the only way to clearly define vx with iref is to use a diode. It may be used as a wilson current source by applying a constant bias current to the input branch as in fig. Nmos current mirrors are used as current sinks and pmos current mirrors are used as current sources. Current mirror layout strategies for enhancing matching performance maofeng lan, anilkumar tammineedi, and randall geiger, fellow, ieee abstract this paper proposes new current mirror layout strategies to reduce the matching sensitivity to the linear parameter gradients. Figure 4 shows the complete differential amplifier implemented using a pair of inverter amplifier with pmos current load, and 200ua current souce. 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. Current ratioing is achieved by only scaling the width of transistors. To assemble and characterize mosfet current mirrors.
An example application of the current mirror is the transconductance op amp. The basic idea of the fcopamp is to apply cascode transistors to the input differential pair but using transistors opposite in type from those used in the input stage. Applications of current mirrors in analog signal processing. Three types of bidirectional mos current mirror amplifier are considered. This work proposes a new low voltage current mirror circuit using bulk driven technique. Analysis of the modified mos wilson current mirror. Variations between individual mos transistors, even of the same type, are too great. The mirrored current is a precise copy of the input current.
Current mirror circuits generally consist two main transistor, although other devices such as fets can be used. Us4855618a mos current mirror with high output impedance. Identical transistors by definition have the same wl and process technology constants. 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. Mosfet differential amplifier twoweek lab background the mosfet is by far the most widely used transistor in both digital and analog circuits, and it is the backbone of.
Pdf applications of current mirrors in analog signal. Aboushady university of paris vi 2 2 1 gs th n ox ref v l c w i. Cmos current mirrors with reduced input and output voltage. Each operates at all current levels with an output. Think of a mos diode, what is the small signal impedance of a mos diode. Current mirrors consist of two branches that are parallel to each other and create two approximately equal currents. Level 3 using the current mirror from level 2, use. Abstracta pedagogical analysis of the modified mos wilson current mirror using signal flow graphs sfgs, masons gain rule, and drivingpoint impedance.
Current scaling similar to their bipolar counterpart, mos current mirrors can also scale iref up or down i1 0. The current being copied can be, and sometimes is, a varying signal current. Subthreshold mosfet behavior for analog applications current leakages are ever present in electronic systems, and many forms of leakage are considered by engineers to be unusable. A pnp current mirror converts current sinks into current. Bias current source is this a good bias current source. Nmos transistors ald 1106 n mos fet transistors see attached data sheet 3. Some current mirror circuits may use more than two transistors to enable the level of performance to be improved. The generation of this reference current source is itself very complex and requires complex circuit which should be independent of power supply, temperature and process variation. This is why these circuits are called current mirrors. The only way to clearly define vx with iref is to use a diodeconnected mos since it provides squarelaw iv relationship. The fundamental current mirror with mos transistors. V d bipolar diode exponential diode connected transistor quadratic v t v d i d v ds v gs v t after we reach the point v d v t, the transistor will always be in sat. Pdf this paper investigates the state of the art compensation techniques and explores various possibilities for the enhancement of bandwidth.
May 29, 2015 nmos current mirrors are used as current sinks and pmos current mirrors are used as current sources. Mos current mirrors with reduced input and output voltage requirements are derived. The proposed current mirror uses mos current dividers to sample the output current and a feedback action is used to force it to be equal to the input current. Cascode amplifiers and cascode current mirrors ece 102, fall 2012, f. In next videos, we will cover cascode current mirror, wilson current mirror etc. Eecs140 analog circuit design lectures on current sources simple source cont. The input of the current mirror is a current mode terminal. In the simple current mirror, both transistors have the same v gs. Lecture08 current sources university of california, berkeley. Subthreshold mosfet behavior for analog applications. The simple mos current mirror 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. Cascode stages and current mirrors ee105 spring 2007. They used suitably biased triode region transistors and have no hard limit on their dynamic range.
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