Amplifiers – Signal feedback – Phase shift means in loop path
Patent
1989-06-20
1991-03-12
Mullins, James B.
Amplifiers
Signal feedback
Phase shift means in loop path
330260, 330294, H03F 134
Patent
active
049995847
DESCRIPTION:
DESCRIPTION
FIGS. 3a and 3b show a block diagram of an adaptive differential amplifier according to the invention particularly suited for a stable and substantially interference-free measuring of very weak (1...100/.mu.V) biosignals. In the amplifier of FIGS. 3a and 3b, an integral signal z is counted from an output signal y, whereby the integral signal obtained is directly proportional to the amplitude and duration of an input signal S. In the embodiment of FIG. 3a, the integral signal z is applied back to the input terminal B with a phase shift of -180.degree. (-1/.tau..intg.ydt). In the embodiment of FIG. 3b, the integral signal (1/.tau..intg.ydt) is applied back to the input terminal A with a phase shift of 0.degree..
FIG. 4 shows a more detailed circuit diagram of the realization of the block diagram of FIG. 3a. Herein an integrating feedback 1a is effected by means of a conventional inverting integrator connection formed by an operational amplifier OP and a resistance R and a capacitor C. In this connection, the resistance R is connected to the output of the differential amplifier G for receiving an output signal y thereof, and one terminal of the resistance R is connected to the negative terminal of the operational amplifier OP, the positive terminal thereof being earthed. The capacitor C is arranged as a feedback component between the negative input terminal and the output terminal of the operational amplifier OP. An output signal z of the operational amplifier is applied through a resistance R.sub.g to a non-inverting input B of the differential amplifier G. A resistance R.sub.g equal in magnitude is connected between one input A of the differential amplifier and an earth connection. By giving the integration time constant (.tau.=RC) a suitable value, it is possible to determine the range of the rate of change of signals (interferences) such that signals slower than these start to attenuate rapidly. In other words, the slower the interference, the more efficient the attenuation. On the other hand, an increase in the impedance of the signal source automatically shifts the border of the attenuation range towards higher rates of signal change and vice versa, i.e. the amplifier adapts itself to such changes. This kind of adaptive differential amplifier also retains the good CMRR properties thereof, for the input terminal impedances remain resistive and nearly equal in magnitude. This is based on the fact that the output impedance of the operational amplifier OP is very small, so that the effect of the feedback loop on the input terminal impedance is practically insignificant.
In cases where the internal impedance of the input signal source S is very low, it is possible to connect a resistance RO in series with the signal source S between the input terminals A and B, said resistance being chosen so that it is considerably lower than the resistance R.sub.g of the input terminals but clearly higher than the internal impedance of the signal source S.
One important field of application of an adaptive differential amplifier of the above type is a real-time measuring of the profile of a muscle tonus (EMG signal) during free movement performance. The registration of these phenomena is necessary e.g. in the training of different sports as well as in physical and rehabilitative treatments. Previously the measurement of such kinesiological phenomena has not been possible in any greater degree.
FIG. 5 shows a block diagram of the third embodiment of the adaptive differential amplifier according to the invention, in which the attenuation properties increase with an increase in the rates of signal change, so that the operation thereof is a mirror image of the embodiment shown in FIG. 3a. In this embodiment, a derivative of the output signal y of the differential amplifier G is formed instead of the integral thereof, which derivative is summed to the non-inverting input B of the differential amplifier G with a phase shift of -180.degree. (-.tau..multidot.dy/dt). According to the principle shown in FIG. 3b, this derivative feedba
REFERENCES:
patent: 3972006 (1976-07-01), Ruegg
patent: 4243918 (1981-09-01), Meise
patent: 4494551 (1985-01-01), Little, III et al.
patent: 4543536 (1985-09-01), Pederson
Kempler William B.
Mullins James B.
Noraxon Oy
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