Rectifier Amplifier Type AC Millivoltmeter


As the amplifier rectifier type of millivoltmeters performance is limited by the band width of the amplifier, an alternative is thought over that gave raise to the rectifier amplifier type A.C. millivoltmeter.

In this instrument initially the input signal will be rectified. The rectified D.C. will be amplified. In order to avoid the loss of signal due to the connecting cable, the rectifier element is incorporated in the probe itself. 

Selection of Diode and Amplifier 

The diode that are used with conventional type of alternating current meters cannot be used for rectification of the high frequency signals of the order of gigahertz region. The reason is their capacitance, and reverse recovery time.

For use at very high frequency rectification, Schottky barriere or point contact diodes are used When diodes are used at low forward biased voltages the rectified output will not equal the peak of the input. This results in the fact that for lower amplitudes of signal voltages, the rectified output will further be low. Therefore chopper stabilized amplifier or an amplifier stabilized for D.C. drifts is required in the A.C. millivoltmeter circuits. 

 Block Diagram of Rectifier Amplifier Type of AC Millivoltmeter

The block diagram of an A.C. millivoltmeter of the rectifier amplifier type is shown in Figure. The output of the detector probe reaches the chopper stabilized amplifier.

Block Diagram of Rectifier Amplifier Type of AC Millivoltmeter

The output of the amplifier is supplied to the digitization circuit. A display is connected to the digitization circuit. The magnitude of the input will be displayed on the display. 

Probes for Measurement  

The probe arrangement that can be used for voltage measurement with high impedance is shown in Figure.

Probe for Voltage Measurement
This probe will be used within circuits at which the impedances change and where the circuit cannot be isolated and terminated externally. 

Another probe arrangement mostly suitable for measurement of R.F. power of the order of nanowatt is shown in Figure.

Probe for Voltage Measurement

This probe is useful with high frequency circuits that can be disconnected and terminated in 50 ohm externally. The power measurement made with this probe will not yield true r.m.s. value. Mostly when modulated signals are measured the results are to be interpreted carefully. 

Measurement Method 

The type of probe is to be selected first depending on the fact whether the circuit can be disconnected and terminated, or not. After selecting the probe, the probe is to be connected to the signal source. The range selector switch of the milli-voltmeter is to be adjusted from the highest range to obtain an accurate read out from the display, which gives the magnitude of the input signal.

Applications of R.F. millivoltmeter

This type of R.F. millivoltmeter is useful at very high frequencies.
It can be used for R.F. power measurement at frequencies of the order of gigahertz. 


Voltage Range : 1.5 mV to 500 V full scale in 12 ranges in 1.5 and 5 sequence.

dB range : — 60 dB to + 60 dB in 12 decade ranges with 1 mW as '0' dB reference dissipated in a resistance of 600 ohm.

Frequency Range : 1 Hz to 5 MHz

Accuracy : ± 1% of reading, ± 1% of full scale reading

Input impedance : 5 MΩ

Predeflection : Less than 100 μV

Power requirement : 230 V, 50 Hz

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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