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computer science crazy
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16-09-2009, 11:56 PM

Common mode rejection ratio (CMRR) is important performance parameter of any difference amplifier. It is the ratio of the differential gain to the common-mode gain, generally expressed in dB. It signifies the ability of an opamp to reject common mode signal, the signal which appears simultaneously at both its input terminals. The CMRR is of great importance when it comes to noise rejection. For applications involving high CMRR, instrumentation amplifiers consisting of three opamps are generally used. It has a major disadvantage that the value of CMRR depends on the level of matching of some resistors. Matching can be achieved to a limited extent only. Current- mode instrumentation amplifiers can be used to overcome this problem. The method makes use of second generation current conveyors (CCII), which convert differential input voltage to equivalent current. This current is then converted back to voltage by a current to voltage converter. This configuration prevents the necessity of matching resistors for high common mode rejection. Current-mode instrumentation amplifier provides high CMRR even at low differential gains, and so can be used over a wide frequency range (note that, the gain-bandwidth product is a constant). But still, this configuration too, is not perfect. This is due to the transistor level mismatch in the CCII. The performance of the current-mode instrumentation amplifier can be improved by some modifications. This is described in the IEEE paper, CMRR enhancement techniques for current-mode instrumentation amplifiers, by Kimmo Koli and Kari A.I.Halonen. (IEEE transactions on circuits and systems-: fundamental theory and applications, vol. 47, no. 5, May 2000) The contents of this paper include a small introduction on the classical voltage-mode instrumentation amplifier and its improved version working on current-mode, its common-mode rejection and three techniques to improve the CMRR. The methods mentioned here are, common-mode bootstrapping, output current subtraction and use of composite conveyors. The article also discusses the effect of forward transconductance on some of the CCII based circuits. The first method of CMRR enhancement, common- mode bootstrapping, exploits the relation between common-mode rejection and power supply rejection. In this method the CCII supply voltage is forced to follow the common-mode voltage. This method has a limitation that, it cannot be used for low voltage applications. In the second method, the output current of one CCII is subtracted from the other. The current subtraction can be done either using opamps or by inverting the current using CCII. In the third method, the performance of the CCII is improved by constructing a composite current conveyor using two or more CCII
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