Precision detector circuit

When ordinary detector diodes are used as detectors, since their forward volt-ampere characteristics are not linear, the detection distortion is quite serious under small signals. In addition, the forward voltage drop of the diode changes with temperature, so the characteristics of the detector are also affected by temperature. A precision detector composed of an operational amplifier can overcome the defects of ordinary diodes and obtain detection performance close to that of an ideal diode. Moreover, the equivalent internal resistance and temperature sensitivity of the detector are much better than those of ordinary detectors. As shown in the figure above: When Usr is negative, the amplifier is inverted, U"sc>0, D2 is cut off, and D1 is turned on. The conduction of D1 provides deep negative feedback for the amplifier. Therefore, the inverting input terminal 2 of the amplifier is a virtual point, and the detector outputs a signal from the virtual point through R2, so Usc=0. When Usr is positive, U"sc<0, so D1 is cut off. As long as U"sc reaches -0.7V, D2 is turned on. When , the forward voltage drop UD of D2 can be regarded as the output offset voltage of the amplifier, so the circuit is equivalent to an inverting input proportional amplifier, and its transmission characteristics are Usc=-(R2/R1)Usr=-Usr. As mentioned above, the transmission characteristics in the above figure are Usc=0(Usr<0); Usc=-Usr(uSR>0). Since the open-loop gain Gol of the op amp is very high, when the input signal is positive, as long as Usr. ≥UD/Gol, D2 will be turned on, and once D2 is turned on, the amplifier will be in a deep closed-loop state, and the nonlinear distortion will be very small. Starting from a small signal, there will be a good linear relationship between the input and the output. The dead zone voltage is very small, equal to 1/Gol times of the forward voltage drop UD of the diode. Assume that the feedback coefficient of the detector when D2 is turned on is F, then the internal resistance and temperature coefficient of this precision detector are those of the ordinary detector. 1/(Gol·F) times. When R2>R1, the detector also has a voltage amplification function and can amplify the signal R2/R1 times.