HIGH-SPEED CONVERTER OF INSTANTANEOUS VALUES OF ALTERNATING VOLTAGE INTO DIRECT VOLTAGE

V. Yu. SOSNIN, M. M. SHTEIN

(Presented by the scientific and technical seminar of the Department of Information and Measurement Technology)

For purposes of magnetic measurements and monitoring of the physicomechanical properties of ferromagnets in alternating fields, the use of digital measuring equipment is promising.

In particular, it is expedient to use digital measuring instruments for point-by-point acquisition and recording of dynamic magnetization loops, and also for monitoring the structural and mechanical properties of products by the instantaneous values of the induced emf, induction, and field strength. This is possible when there is a converter of the instantaneous values of \(B\), \(H\), and \(E\) into a direct voltage.

The principal requirements imposed on such converters may be considered the following:

1) high speed;

2) stability and accuracy of the conversion coefficient, determined by the ratio of the output direct voltage to the input instantaneous value;

3) linearity of the transfer coefficient over the entire dynamic range of the input signal;

4) low ripple level of the output voltage;

5) possibility of setting the zero of the output voltage.

The converter whose circuit is shown in Fig. 1 satisfies the requirements listed above to a sufficient degree.

The converter is built on the principle of charging a storage capacitor from the input-signal source through a controlled switch. The operating principle of such converters is described quite fully in domestic and foreign literature sources [3, 4].

A KP-103 field-effect transistor is used as the controlled switch. The use of field-effect transistors as controlled switches provides a number of advantages in comparison with bipolar transistors used for the same purposes [1, 2, 3]. These include:

1) the absence of influence of the residual parameters of the transistor on the output signal of the switch load, which makes it possible to switch low-level signals \((0 \div 100\ \text{mV})\);

2) weak galvanic coupling between the control and signal circuits;

3) the high resistance of the closed transistor, which causes a smaller discharge of the storage capacitor through the resistance of the closed switch;

4) high breakdown voltages, making it possible to commutate high-level signals (tens of volts);

5) good linearity of the signal transmission coefficient.

As a buffer device between the storage capacitor and the converter load, a follower on transistors $T_2$ and $T_3$ is used. The use of a field-effect transistor in the follower makes it possible to obtain small ripples of the output voltage across the load resistance.

Fig. 1

In addition, the combination of a field-effect transistor and a silicon bipolar transistor made it possible, by appropriate selection of the operating point of transistor $T_2$ [2], to provide thermal compensation for the drift of the output voltage.

A circuit consisting of resistors $R_1$, $R_2$, and $R_3$ serves to set the zero of the follower output voltage.

The converter has the following characteristics:

1. With an internal resistance of the input-signal source equal to 300 ohms, the charge (discharge) time constant of the storage capacitor $C_3$ through an open switch is $5$ μsec.

2. The dynamic range of linear conversion of instantaneous values of the input voltage into direct voltage is from $-5$ to $+5$ V.

3. The ripple coefficient of the output voltage is not more than 1%.

4. The temperature drift of the output-voltage zero over the temperature range from $+20$ to $+40^\circ\mathrm{C}$ is not more than 10 mV.

REFERENCES

1. M. Shipley. Field-effect transistors in analog switching circuits, Elektronika, 1964, No. 32.

2. Trends in the Development of Low-Power Active Radio Components, Novosibirsk, Nauka, 1970.

3. V. A. Dolgov, E. Yu. Gonestas. Switching Devices of Automatic Control Systems, Moscow, Energiya, 1960.

4. S. S. Khrizman. Digital Measuring Instruments and Systems: Handbook, Kiev, Naukova Dumka, 1970.