THE METHOD OF INCREASING IN-PHASE NOISE-IMMUNITY OF DIFFERENTIAL DIFFERENCE AND INSTRUMENTATION AMPLIFIERS OF SENSOR SYSTEMS
The article considers new parametric and circuit techniques for increasing common-mode rejection ratio (CMRR) in differential difference (DDOA) and instrumentation amplifiers (IA) of sensor systems. The obligatory introduction of the relatively high-value resistor of local negative feedback into input differential stages (DS) of DDOA makes signifficanlty worse the in-phase rejections in IAs based on it. Therefore, the specific circuit solutions of input DS of DDOAs, which provide increasing the value of CMRR,are necessary. The interrelation between the range of line works of the input stages of DDOA, which must reach values of 1÷3V, and the coefficient CMRR is set. The article shows that in classical DDOA the satisfaction of conditions for cancellation of one of components of CMRR, connected with the resistances of collector junctions of the input transistors and transistors of reference current sources, is possible. If we provide the definite parametric conditions, than CMRR of DDOA will be determined only by the self-feedback gain of the input active elements. The equations are obtained, which show, that the numerical values of CMRR, depending on the Early's voltages of the input transistors of DS, are also determined by the levels of their static voltage collector-base. To minimize the systematic component CMRR, connected with the self-feedback in transistors, it is necessary to choose equal steady-state behaviors for them. It is especially important in the stages with current mirrors, where there is always the voltage separation collector-base of the input transistors. The article suggests the method of error compensation of DDOA from the in-phase signal. It is implemented in the circuit of IA on the CMOS transistors. The feature of the method is that one of three differential stages of IA, which provides entering the common negative feedback, doesn't have in-phase component of the input signals. It allows using this stage as a circuit of the inherent compensation of the in-phase errors in DDOA. In the proposed circuit of IA the transfer ratios of the in-phase signal on the output of IA decrease by a factor of 80÷100.