Current mirrors are core structures for almost all analog mode circuits and are integral part of a signal processing elements like op amps. The performance of analog structures largely depends on their characteristics. In this paper we present some of the current mirror circuits and analyze their performances, so that one can choose a suitable current mirror for a particular application

Bansal, Bajrang

Kaushik, Himanshu

Ranjan, Prabhat

English

Interscience Research Network

International Journal of Electronics and Electical Engineering Volume 1 Issue 2 Article 9 October 2012 Current Mirror Circuits with Improved Performance Bajrang Bansal Electronics & Communication Engineering Department Jind Institute of Engineering & Technology, Jind, India, bajrangbnsl@gmail.com Prabhat Ranjan Electronics & Communication Engineering Department Jind Institute of Engineering & Technology, Jind, India, prabhattiet@gmail.com Himanshu Kaushik Electronics & Communication Engineering Department Jind Institute of Engineering & Technology, Jind, India, Himkaushik342@yahoo.co.in Follow this and additional works at: https://www.interscience.in/ijeee  Part of the Power and Energy Commons Recommended Citation Bansal, Bajrang; Ranjan, Prabhat; and Kaushik, Himanshu (2012) "Current Mirror Circuits with Improved Performance," International Journal of Electronics and Electical Engineering: Vol. 1 : Iss. 2 , Article 9. DOI: 10.47893/IJEEE.2012.1023 Available at: https://www.interscience.in/ijeee/vol1/iss2/9 This Article is brought to you for free and open access by the Interscience Journals at Interscience Research Network. It has been accepted for inclusion in International Journal of Electronics and Electical Engineering by an authorized editor of Interscience Research Network. For more information, please contact sritampatnaik@gmail.com. Current Mirror Circuits with Improved Performance  Current Mirror Circuits with Improved Performance 1Bajrang Bansal, 2Prabhat Ranjan, 3Himanshu Kaushik Electronics & Communication Engineering Department Jind Institute of Engineering & Technology Jind, India Email: 1bajrangbnsl@gmail.com, 2prabhattiet@gmail.com, 3Himkaushik342@yahoo.co.in   Abstract—Current mirrors are core structures for almost all analog mode circuits and are integral part of a signal processing elements like op amps. The performance of analog structures largely depends on their characteristics. In this paper we present some of the current mirror circuits and analyze their performances, so that one can choose a suitable current mirror for a particular application.  I. INTRODUCTION The analog circuits are implemented in the form of standard building blocks, called analog signal processing (ASP) cells [1]. The ASP cells consist of several basic analog circuit structures and may have voltage mode and/or current mode circuit structures. In Current mode circuits (CMCs) input and output signals are currents. Current mirror (CM) is one such structure, which finds use in analog circuit structures. CMs may be used as active load as they offer high impedance. They may also be used as biasing structures as they provide better insensitivity to the variations in power supply and temperature [2].  In this article, some of the CMs with improved performances are presented so that one can choose an appropriate CM for specific applications.  II. BASIC CURRENT MIRROR  CM is a two terminal structure whose output current at any instant of time is independent of voltage applied across its output terminals (Fig. 1) and depends on the input current. It allows precise copying of the current with no sensitivity to variations in power supply and temperature. The ratio of iout and iin is given by the ratio of device dimensions that can be controlled with better      accuracy [3]. CM determines the characteristics of many analog circuits [4]. Following are the performance parameters of a current mirror.   Figure 1.  Basic CM Structure Input-output resistance  CM should have zero input and infinite output resistance so that the input voltage will not vary with the input currents and the output currents will not depend on the applied output voltage. Output resistance is very important parameter for a current mirror. Output voltage swing A CM requires a minimum voltage at the output to ensure that the devices operate in saturation. This voltage is called the output compliance voltage. For a simple current mirror (Fig. 1) minimum voltage required is Vds2(sat).This output compliance voltage may be different for different topologies. Input linear range The total input current must be in the range where both transistors (Ml and M2 in Fig. 1) operate saturations. DC balance The drain source voltage of the mirror transistors MI and M2 (Fig. 1) should be equal. The error due to the mismatch in drain to source voltages is λ(Vds2-Vds1), where λ is the channel length modulation coefficient.  Device matching It requires perfect matching of the mirroring transistors Ml and M2 (Fig. 1) for accurate mirroring of the signal current. Current transfer ratio (CTR) is precisely set by the (W/L) ratios. International Journal of Electrical and Electronics Engineering (IJEEE), ISSN (PRINT): 2231 – 5284, Volume-I, Issue-2117Current Mirror Circuits with Improved Performance  The study of various CM topologies is necessary so that the circuit designers can choose an appropriate CM for particular application.  III. CLASSIFICATION OF CM  A CM may be a 1. Simple CM 2. Standard Cascode CM 3. High swing CMs 4. Improved Swing CMs 5. Wilson CMs 6. Regulated Cascode CMs  All CMs are useful and for selecting a particular CM, it requires an in depth knowledge of these CMs.   A. Simple Current Mirror  Fig. 1 shows the basic structure for a current mirror. It uses the principle that if the gate source potentials of two identical MOS transistors are equal, the channel current should be equal. For a good current source, the MOSFETs should operate in saturation region.   For simple CM the minimum Vin value is given as inin ti2iv =v +β    (1.1)  The output compliance voltage is given as inouti2iv =β    (1.2)  The output resistance is given as  out ds2r =r     (1.3)  Output characteristic for this structure is shown in Fig. 2. It can be seen that the output current is not constant in saturation mode as it is varying with change in output voltage. This is because of its low output resistance (rds2). This low output resistance is a major drawback of this CM. The ratio of mirrored (iout) to the input current iin is [5] out 2 1in 2 1i w l=i l w    (1.4)  Bigger size transistors achieve higher output resistance which is very much desired for a good current mirror. But there will be higher output capacitance which degrades the frequency response of a CM.    Figure 2.   Output characteristics for simple CM Standard Cascode CM  B. Standard Cascode Current Mirror  The small signal output resistance is a good measure of the perfection of the current mirror. Higher performance current mirrors will attempt to increase the value of rout. The approach used to increase the output resistance is to use cascode structure. Standard cascode CM shown in Fig. 3 [6] is a good choice for increasing output resistance.                             Figure 3.   Standard Cascode CM Structure The minimum input voltage vin for this structure is given as inin ti2iv =2v +2β   (1.5) This value is double of the input compliance voltage for a conventional CM   Output compliance voltage is given as  International Journal of Electrical and Electronics Engineering (IJEEE), ISSN (PRINT): 2231 – 5284, Volume-I, Issue-2118Current Mirror Circuits with Improved Performance   inout ti2iv =v +2β   (1.6)  This is greater than the minimum voltage required for a conventional cascode, by an amount of vt, threshold voltage of a transistor. The output resistance is given as [3]  out ds2 ds4 m4 ds2 ds4 4r =r +r +g r r (1+η )   (1.7)   This can be approximated as  out m4 ds2 ds4r =g r r     (1.8)  Which is greater than the output resistance of simple CM, by gain of a transistor ( m dsg r )  Output characteristic of this structure is shown in Fig. 4.    Figure 4.   Output characteristics for Standard                                          Cascode CM  As shown in Fig. 4, the output compliance voltage is near about 1.63V. Also in saturation mode output current is not varying with output voltage. This advantage is because of its higher output resistance. The disadvantage of this structure is its higher input and output compliance voltage.  C. High Swing Cascode Current Mirror  The disadvantage of standard cascode current mirror is its higher input and output compliance voltage. This high swing cascode topology shown in Fig. 5 [3] is better alternate as it provides the same output resistance as of standard cascode but it provides low input and output compliance voltage in comparison to standard cascode structure.    Figure 5.    High Swing Cascode CM Structure      The output characteristic for this structure is shown in Fig. 6. As shown in figure, the output compliance voltage is near about 1.5V which is lower than the value for standard cascode CM. So this is a better topology than standard cascode CM.      Figure 6.   Output characteristics for high swing                               Cascode CM In Fig. 5 the W/L ratio of transistor M3 is chosen one fourth in respect to other transistors. That causes input compliance voltage as  refin ti2iv =v +2β   (1.9)  Then output compliance voltage becomes  refouti2iv =2β    (1.10)  This is same as for conventional cascode structure. International Journal of Electrical and Electronics Engineering (IJEEE), ISSN (PRINT): 2231 – 5284, Volume-I, Issue-2119Current Mirror Circuits with Improved Performance  The output resistance for this structure is same as in standard cascode structure. That is  out m4 ds4 ds2r =g r r     (1.11)  The problem with this structure is that the Vds of M1 and M2 are not equal. So the current iout will not be an accurate replica of iref due to channel length modulation. If exact mirroring of iout is desired with iref, then a slight modification of Fig. 5 will minimize this problem. The required structure is shown in Fig. 7. An additional transistor M5 is added in series with M1 so as to make the drain voltages of M1 and M2 to be equal, thus removing any error due to channel length modulation.     Figure 7.   Improved high swing Cascode CM  D. Wilson Current Mirror   This is another type of current mirror. This circuit is an n-channel implementation of the well known Wilson current mirror [7].                             Figure 8.  Wilson CM As shown in Fig. 8, the output resistance of the Wilson current mirror is increased through the use of negative current feedback [8]. This structure has large input and output compliance voltage. The output resistance for this structure is given as  ds3 m3 3 m2 ds1 m3 ds3ds3 ds2m2 ds21+r g (1+η )+g r g rrout=r +r1+g r⎛ ⎞⎜ ⎟⎝ ⎠(1.12)  The output resistance is seen to be comparable with that of standard cascode current mirror.  The input compliance voltage is given as  out outin t2 32i 2iv =2v + +β β  (1.13)  The output compliance voltage is given as  out outout t2 32i 2iv =v + +β β  (1.14)  The output characteristic for this structure is shown in Fig. 9.                     Figure 9.  Output characteristics for Wilson CM As shown in this Fig. 9, the output compliance voltage is approximated 1.63V. Also in saturation region its output current is not varying with output voltage just because of its higher output resistance. The advantage of Wilson CM is its higher output resistance. Its main drawback is the asymmetrical biasing, which causes large DC matching error. This can be removed by adding one transistor in series with transistor M1.   E. Regulated Cascode Current Mirror  As explained earlier, the well desired property of good current mirror is its high output resistance. An improvement in Wilson CM can be made if somehow its output resistance is increased. This can be achieved by International Journal of Electrical and Electronics Engineering (IJEEE), ISSN (PRINT): 2231 – 5284, Volume-I, Issue-2120Current Mirror Circuits with Improved Performance  using negative current feedback circuit. The resulted circuit is shown in Fig. 10 [9].    Figure 10.  Regulated Cascode CM  The output compliance voltage for this structure is given as out outout2 32i 2iv = +β β   (1.15)  The output resistance for this structure is given as  out ds2 m3 ds3 m1 ds1r r g r g r≅    (1.16)  So this structure achieves an output resistance on the order of  2 3m dsg r   The output characteristic of this structure is shown in Fig. 11.      Figure 11.  Output characteristics for Regulated                                           Cascode CM  As can be seen from this Fig. 11, output compliance voltage for this structure is 1.5V. Also in saturation mode, output current is constant as it should be ideally. IV.         SIMULATION RESULTS  The circuits have been simulated using MCNC 1.25um CMOS process level 2 spice parameters. The purpose was to obtain higher output resistance and low output compliance voltage. The performance comparison of various CM structures is shown in table I.  TABLE I.  COMPARISON OF VARIOUS CM STRUCTURES Characteristics Simple CM StandarCascodeCM High  Swing   CM Wilson CM RegulateCascode CM Input Compliance Voltage Low    High Medium High High Output Compliance Voltage Low    High Medium High High Output Resistance Low    High High High Highest Area (No. of  Transistors) 2     4 4 3 4   As output resistance is very important performance parameter of a CM, table II summarizes this parameter for different CM structures.  TABLE II.  OUTPUT RESISTANCES OF VARIOUS CM Current Mirror Output Resistance (rout) Simple CM ds2r  Standard Cascode CM ds2 ds4 m4 ds2 ds4 4r +r +g r r (1+η )  High Swing Cascode CM m4 ds4 ds2g r r  Wilson CM ds3 m3 3m2 ds1 m3 ds3ds3 ds2m2 ds21+r g (1+η )+g r g rr +r1+g r⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠ International Journal of Electrical and Electronics Engineering (IJEEE), ISSN (PRINT): 2231 – 5284, Volume-I, Issue-2121Current Mirror Circuits with Improved Performance  Regulated Cascode CM ds2 m3 ds3 m1 ds1r g r g r   V. CONCLUSION In this article, various possible CM structures are presented along with their merits and demerits. Any one of them can be selected as per requirement. The most important performance parameters for a CM are its compliance voltages and output resistance. The most frequently used CMs have been considered for comparison of the performance parameters. They are simple CM, standard cascode CM, high swing CM, Wilson CM, Regulated cascode CM. From comparison of various parameters of these structures, it can be seen that Regulated cascode CM structure is best suited structure, as it provides medium input and output compliance voltage and very high output resistance.     REFERENCES  [1] M. Ismail and T. Fiez, “Analog VLSI Signal and Information Processing,” New York: McGraw-Hill 1994. [2] B. Razavi, “Design of Analog CMOS Integrated Circuits,” New York: Tata McGraw-Hill 2002. [3] P. E. Allen and D. R. Holberg, “CMOS analog circuit design,” New York: Oxford University Press, 2002. [4] S. S. Rajput and S. S. Jamuar, “A high performance current mirror for low voltage designs,” Proc. APCCAS-2000/IEEE, Tianjin pp. 170-173, China, Dec 2000. [5] I. M. Filanovsky, “Current Mirrors with Limiting of Linear Dynamic Range,” University of Alberta, Edmonton, Alberta, Canada, T6G 2El. [6] D.A. Johns and K. Martin, “Analog Integrated Circuit Design, ” John Wiley & Sons, Inc., New York, 1997. [7] G. R. Wilson, “A Monolithic Junction FET-npn Operational Amplifier,” IEEE J. Solid-State Circuits, Vol. SC-3, N0. 5, pp. 341-348, Dec. 1968. [8] Z. Wang, “Analytical determination of output resistance and DC matching errors in MOS current mirrors,” IEE PROCEEDINGS, Vol. 137, Pt. G, No. 5, OCTOBER 1990.                             [9] E. SACKINGER and W. GUGGENBUHL, “A versatile building           block: the CMOS differential difference amplifier,” IEEE J., 1987,           SC-22, (2), pp. 287-294.        International Journal of Electrical and Electronics Engineering (IJEEE), ISSN (PRINT): 2231 – 5284, Volume-I, Issue-2122

Current Mirror Circuits with Improved Performance

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Current Mirror Circuits with Improved Performance

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