Electronic Voltmeter Using Transistor

Electronic Voltmeter Using Transistor – FET and BJT

a) Description of the Circuit: 

The circuit of a FET voltmeter is shown in Figure. It consists of an input attenuator. The input attenuator is a potential divider consisting of resistances 9 MΩ, 900 kΩ and 100 kΩ. A selector switch is used for range selection. The gate is connected to the pole of the selector switch through a 1 MΩ resistance. This is the gate limiting resistance. The field effect transistor works as the first stage of the voltmeter.

It works as a source follower. It offers a high input impedance and low output impedance. The source of the FET is directly connected to the base of the transistor (BJT). So the transistor T2 with its emitter resistance of 10 k Q acts as the load for the source follower.

Transistor T2, its emitter resistance, the variable resistance of 2.5 k Ω, and the 2.2 k Ω resistor (in series with the 2.5 k Ω resistor) act as a bridge circuit. The D.C. micro ammeter is connected with a variable resistance in series across the diagonal points of the bridge, in the position of a null indicator. The bridge can be balanced to give a null indication in the micro ammeter. The 2.5 k Ω variable resistor termed as zero adjustment control is used for this purpose.
Electronic Voltmeter using FET and BJT
The drain of the FET and the collector of the BJT are connected to the supply through the switch S. The range selector switch has three positions. In the first position it directly connects the input voltage at test terminals to the gate of the FET.

In the second position it connects the voltage developed across the part of the potential divider i.e. 1 MΩ. In the third position it connects the voltage developed across the part of the potential divider i.e. 100 k Ω. Thus three different ranges are possible with a multiplication factor of 10.

(b) Working of the Voltmeter: 

The working of the above voltmeter circuit can be split in to three sections namely:

(i) Initial adjustments. (ii) Calibration. (iii) Measurement.

i. The initial adjustments:

The switch S is to be closed. This applies supply to the circuit. Now the test terminals are to be shorted. That is we are giving no input signal to the voltmeter. The range switch is to be kept in 1st position. The potentiometer of 2.5 k Ω is to be adjusted to get zero current in the micro ammeter. That is we are adjusting the bridge for null balance.

ii. The calibration:

Having ensured that the null indicator, i.e. the micro ammeter is reading with shorted test terminals, a '0' we have to calibrate the instrument. For this purpose, the short at test terminals is removed. A standard voltage of 500 mV is to be applied at the test terminals, keeping the range selector switch in the 1st position, i.e. 500 mV, position. The micro ammeter has to indicate full scale deflection. The calibration potentiometer can be adjusted (10 k Ω) to set the full scale deflection. After making this adjustment the voltmeter is ready for measurement.

iii. The measurement:

We choose the range of voltage to be measured by setting the range selector switch. Then we apply the voltage using the test terminals to the voltmeter. Application of the voltage will increase the drain current. Hence there will be a change in the voltage drop across the emitter resistor of T2. This causes disturbance in the balance of the bridge. Hence there will be current through the micro ammeter. As the micro ammeter has already been calibrated to give full scale deflection for 500 mV, the reading can be had directly if the range selector is in first position.

If the range selector is in the second position the reading in volts is to be multiplied by 10. In the third position of the switch the reading is to be multiplied by 100. Thus voltages from few millivolts to 50 V can be measured with this voltmeter.

(c) Disadvantages of the FET Input Voltmeter:

1. This voltmeter can only measure steady voltages (D.C. voltage ).
2. The balance of the bridge is effected by the parameter variation of the transistors.
3. The zero setting is to be ensured before every measurement.

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