Logarithmic operation circuit

Figure 5.4-15 is a logarithmic operation circuit with temperature compensation and offset, with an input voltage of 10MV~10V and a dynamic range of 60DB. V1 and V2 should use a pair of tubes with consistent characteristics and the same junction temperature. In order to improve accuracy, V1 and V2 should use transistors with small ICBO and large B. Because the operation accuracy and stability depend on the symmetrical characteristics of this pair of tubes, be careful when selecting.

In the figure, A1 and V1 form a basic logarithmic operation circuit, and V2 is a common-mode feedback amplifier. Its input voltage U+2 is actually the difference between the base-emitter voltage of the two tubes.

In this way, the influence of saturation current on calculation accuracy can be eliminated by utilizing the consistency of tube temperature characteristics.

U+2 is the in-phase input signal of the second amplifier A2. The closed-loop gain of A2 is (1+R3/R4), so the output voltage

That is, the output voltage is logarithmically related to the input voltage. Since U+2=RBE2-UBE1, it is amplified by (1+R2/R4) times through A2, and the output voltage range is limited to the disadvantage of UBE. If a thermistor RT is added to the feedback resistor of A2, the temperature compensation of UT can be borne by the thermistor RT, thereby achieving complete temperature compensation.

Since the operating range of the logarithmic operation circuit is about 10 to the 6th power ~ 10 to the 8th power A, and the operating current is small, the error current introduced by the offset voltage and input bias current must be carefully compensated, otherwise the operating range will be greatly reduced. Therefore, before the operation, zero adjustment must be performed carefully. The W1 potentiometer in Figure 5.4-15 is used for offset compensation.

Figure 5.4-16 is the conversion characteristic diagram of the circuit in Figure 5.4-15. The proportional coefficient is selected as 2.5V/10 times, that is, a 10-fold change in input voltage corresponds to a 2.5V change in output UO.

The high-speed functions of the potentiometers in Figure 5.4-15 are: W1 is used to adjust the offset of A1, and the output is adjusted to 2.5V when U1=10MV; W2 is used to adjust the zero-crossing point of the conversion characteristic. Since 1NO=-∞, it is obviously impossible to achieve UO=0 when U1=0, and the output voltage must correspond to a certain input voltage to be zeroed. Therefore, the current IR must be set to adjust the output to 0V when U1=100MV; WO adjusts the proportional coefficient, and adjusts the output to -5V when U1=100MV. When using, W1~W3 must be adjusted repeatedly several times.

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