AC millivoltmeter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Measuring technology
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The device consists of an input emitter follower (transistors V1, V2), an amplifying stage - (transistor V3) and an AC voltmeter (transistors V4, V5, diodes V6-V9 and microammeter P1).
(click to enlarge)
The main parameters:
- Range of measured voltages, mV.......3...5000
- Operating frequency range, Hz.......30...30 000
- Frequency response unevenness, dB....... 1
- Input resistance, mΩ within 10, 20, 50 mV.......0,1
- within 100 mV ... 5 V ....... 1,0
- Measurement error, %......10
The measured alternating voltage from connector X1 is fed to the input emitter follower through a voltage divider (resistors R1, R2* and R22), with which this voltage can be reduced by 10 or 100 times. A 10-fold decrease occurs when switch S1 is set to X 10 mV (the divider is formed by resistor R1 and resistor R22 connected in parallel and the input resistance of the emitter follower). Resistor R22 serves to fine-tune the input resistance of the device (100 kOhm). When switch S1 is set to X 0,1 V, 1/100 of the measured voltage is supplied to the input of the emitter follower. The lower arm of the divider in this case consists of the input resistance of the follower and resistors R22 and R2*.
At the output of the emitter follower, another voltage divider is included (switch S2 and resistors R6-R8), which makes it possible to attenuate the signal that is fed further to the amplifier.
The next stage of the millivoltmeter - the voltage amplifier of the AF on the transistor V3 (gain of about 30) - provides the ability to measure low voltages. From the output of this stage, the amplified AF voltage is fed to the input of an AC voltage meter with a linear scale, which is a two-stage amplifier (V4, V5) covered by negative feedback through a rectifier bridge (V7-V10). A microammeter P1 is included in the diagonal of this bridge.
The non-linearity of the scale of the described voltmeter in the range of marks 30 ... 100 does not exceed 3%, and in the working area (50 ... 100) - 2%. When calibrating, the sensitivity of the millivoltmeter is adjusted by the resistor R13.
The device can use any low-frequency low-power transistors with a static current transfer coefficient k21e = 30 ... 60 (at an emitter current of 1 mA). Transistors with a large coefficient h21e should be installed in place of V1 and V4. Diodes V7-V10 - any germanium from the D2 or D9 series. The KS168A zener diode can be replaced by two KS133A zener diodes by turning them on in series. The device uses capacitors MBM (C1), K50-6 (all others), fixed resistors MLT-0,125, trimmer SPO-0,5. Switches S1 and S2 (sliding, from the Sokol transistor radio) are modified so that each of them becomes two-pole in three positions: in each row, the extreme fixed contacts (two moving contacts) are removed, and the remaining moving contacts are rearranged in accordance with the diagram switching.
The adjustment of the device is reduced to the selection of the modes indicated on the diagram by resistors marked with an asterisk, and the graduation of the scale according to the exemplary device.
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