VOLTMETER WITH IMPROVED LINEARITY

Radio, 1998, 1

The linearity of the scale of a simple AC voltmeter can be improved by using half-wave transistor rectifier with (Fig. 1). The proposed circuits are designed as a modification of the voltmeter described in "Radio", 1985, № 11.

Fig. 1.
VT1 - germanium USSR transistor GT310G (Transition frequency (f_t) = 160MHz; Forward current transfer ratio (h_FE) = 60...180);
VD1 - silicon diode KD522A;
PA1 - microammeter with full scale deflection of 50 uA

The device uses a microammeter with full scale deflection of 50 uA and with the resistance of 2 kilohms. The upper limit of the measuring range of the voltmeter is 1 V, and in the range of 0.2...1 V the measurement error due to the nonlinearity of the scale is less than 1% of the full scale range.

By selection of the resistor R3 values the voltmeter is calibrated at the middle of the scale, and by selection of the resistor R1 values - in the end of the scale. The linearization of the right side of the scale achieved due to the base current, which creates a voltage drop across the resistor R2. Because of this the gain of the rectifier are reduced to the end of the scale as required.

If the measurement range is 3...5 Volts and above, then it is better to use a silicon transistor in the rectifier. This voltmeter for the range of 10 V with the transistor KT3102D, has an error due to the nonlinearity of the scale at range of 1 V about 2%, and in the range of 2...10 V - less than 0.1% of the full scale range. Parameters of the circuit components are: R1 - 20 kilohm, R2 - 180 kilohms, and R3 - 62 kilohms.

A voltmeter with a full-wave rectifier can be implemented with transistors of the same p-n-p type as shown in Fig. 2. The resistance of the resistors R4, R6 is roughly determined from the relation: R < U_n/2I_n, where U_n - the measurement range, and I_n - the full scale deflection current of the microammeter.

Fig. 2.
VT1 - germanium USSR transistor GT310G (Transition frequency (f_t) = 160MHz; Forward current transfer ratio (h_FE) = 60...180);

The input impedance of this voltmeter is greater than the input impedance of the bridge circuit with the four transistors, because the measured network is loaded with two resistors in the bases of the transistors, rather than four resistors. Also the two transistors of the same structure is easier to find.

It is possible to extend the limits of measurement by using an additional voltage divider, that provides the current much more than the base current. The use of additional resistors shifting the balance achieved in the calibration, and the linearity of the scale gets worse.

V. KHVALYNSKY

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