### Balanced Bridge Voltmeters

Such voltmeters can be used for measurement of both dc and ac voltages and are very popular. These voltmeters are available in both versions, vacuum tube type and transistorized type and are described below:

### (a) Balanced Bridge Triode VTVM.

The circuit diagram of a balanced bridge triode VTVM is shown in figure.

In this instrument two identical triodes T1 and T2 are used. Plate voltage is applied to both   of the triodes through two, identical resistors RL and a variable resistance RQ. Resistance R0 is so adjusted that plate voltages of both of the triodes become equal when there is no voltage applied on the grid of triode T1. So, in this condition, meter connec­ted between the plates of the two triodes reads zero. The bias battery ECC biases the cathodes of both of the triodes positive and so the bias of the grid of both of the triodes becomes negative with respect to their respective cathodes.

When a positive dc voltage is applied to the grid of triode T1, potential of grid with respect to cathode increases and so the plate current increases. Due to increase in plate current of triode T1 the voltage drop across the resistor RL connected in series with triode T1 increases and, therefore, plate voltage of triode T1 falls. The increased current also flows through the resistor Rk which is common to both of the tubes T1 and T2. An increase in current through resistor Rk causes increase in voltage drop across it and, therefore, cathode of triode T2 becomes more positive. It results in increase of -negative grid bias of tube T2 and therefore, plate current of triode T2 is reduced. With this voltage drop across resistor RL connected with plate of triode T2 falls and, therefore, plate voltage of triode T2 increases. Thus with the application of dc voltage on the grid of triode T1 voltages of the two triode plates do not remain same and a current flows through the PMMC meter, which is proportional to the applied dc voltage. So PMMC meter can be directly calibrated to read the applied input dc voltage. Range of this instrument can be extended by using a range selector switch and a potential divider network. For measuring ac voltage with this voltmeter, it is first rectified and then applied to this voltmeter. A resistance is connected in series with the meter for the purpose of setting the full scale reading of the meter accurately.

The balanced bridge VTVM has the following advantages over an ordinary VTVM (i.e. a VTVM using a single tube):

1. Effect of variations in the tube characteristics is reduced.
2. Fluctuations in the power supply have a smaller effect on the measuring circuit.
3. There is a lesser tendency for the meter zero to shift during the meter operation.

### (b) Balanced Bridge TVM.

The circuit diagram for a balanced bridge TVM is shown in figure.