Linear AC voltmeter

Radio, 1985, 11

The use of diode rectifiers in AC voltmeters with a low lower limit of measurement range (0.5...1 V) leads to significant nonlinearity of the scale because of the nonlinearity of current-voltage characteristics of diodes. The use of electronic amplifiers in rectifier circuits allowing to linearize the scale but requires a power supply.

At the same time a linear-bridge rectifier [1, 2] is known, its shoulders are formed by junctions of the collector-emitter of the four transistors, this four transistors is getting the bias voltage from the signal source. The operation of this rectifier is described in detail in [2], a circuit diagram of an AC voltmeter on this basis is shown in Fig. 1. The input (measured) voltage is applied directly to the emitters of the transistors VT1-VT4 (the one of the diagonals of the bridge formed by them), the proportional to this input voltage current is measured by microammeter, connected across collectors of the transistors VT1-VT4 (the other diagonal of the bridge).

Schematic diagram of linear AC voltmeter

Fig. 1.
VT1, VT2 - germanium USSR n-p-n transistors MP38 (Transition frequency (ft) = 2MHz, Forward current transfer ratio (hFE), min/max = 25...55);
VT3, VT4 - germanium USSR p-n-p transistors MP39 (Transition frequency (ft) = 500KHz, Forward current transfer ratio (hFE), min/max >=12);
R1..R4 = 10k, R5+RPA1 = 5k or 10k (see text)

Testing of this voltmeter showed that the absolute value of the losses in the rectifier at input voltage above 0.1 V is almost independent of its level and is approximately 40 mV for a load resistance of 10 kilohms (the upper limit of measurement is 1 V) and 60 mV for a load of 5 kilohms (the upper limit of measurement is 0.5 V). The components for this testing was used the same as described in fig.1, value of resistors R1-R4 is 10k, microammeter with full-scale deflection of 100 uA. In other words, the scale of the voltmeter was linear in the voltage range of 0.1...0.5 V and 0.1...1 V.

The voltmeter was tested with an input sinusoidal signal with a frequency of 10...10000 Hz. The transfer characteristics (the dependence of the microammeter readings PA1 of the input voltage) for the upper limits of measurement of 0.6 V and 1 V are shown in Fig. 2a and 2b. Here is 1 - the characteristics of an ideal voltmeter, 2 - the described voltmeter, 3 - a voltmeter based on a diode bridge with germanium diodes D2B made in USSR.

The transfer characteristics of the linear voltmeter

Fig. 2a.

The transfer characteristics of the linear voltmeter

Fig. 2b.

1 - the characteristics of an ideal voltmeter, 2 - the described voltmeter, 3 - a voltmeter based on a diode bridge with germanium diodes D2B made in USSR.

Any germanium transistors with a static current gain hFE not less than 50 can be used in this device. Microammeter PA1 - any one with a full scale deflection of 50...100 mA. The upper limit of the measurement is set by selection of the resistor R5.

A voltmeter that does not require a power supply can be used, for example, to measure AC voltage or a current in a circuit that are not galvanically isolated with power supply unit. The disadvantage of this voltmeter - a slightly increased current consumption from the measured circuit (due to the shunting effect of the basic circuits of transistors VT1-VT4), and the relative complexity of creating a multirange voltmeter.

V. OVSIENKO

References:

  1. Author's certificate № 345573. A.K. Ovsnenko. Single-phase rectifier.- Bulletin "The discoveries, inventions...", 1972, №22.
  2. Ovsnenko A. K., Kachan Y. G., Kovalenko M. A. The Linear Rectifier of Small Signals.- Measurement Techniques, 1975. №6, с. 48.

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