Balanced Bridge Transistor Voltmeters

Balanced Bridge Transistor Voltmeters Points : Balanced Bridge Transistor Voltmeters Basic configuration of a balanced bridge TVM is shown in the fig. In the circuit the bridge arms consists of resistances R₁, R₂, and the internal collector resistance of transistor Q₁ and Q₂. If a positive voltage is applied to the base of Q₁. an increased emitter current due to increased voltage drop across resistance R₁. Thus , the bridge is unbalanced and current proportional to the input voltage flows the meter which can be calibrated to read the voltage directly.

Let us suppose that resistances R_B1, R_B2, R₁ and R₂ are replaced by short circuit. The VTM remains operative, provided the value of VE is properly chosen. Now, if the ambient temperature increases, the β value of Q₁ and Q₂ increases.

When a positive voltage is applied to the base of, Q₁ its collector current becomes greater than what it was at the original temperature and the meter reads high. Note that transistor Q₂ is effective merely to maintain quiescent balance because R₁ is replaced by a short circuit. Thus, it becomes clear that some compensating action, such as a negative feedback, has to be employed to maintain accuracy of calibration.

The zero setting resistor, R₁ also provides resistance in series with the emitter of Q₁ and Q₂. If the ambient temperature increases Q₁ and Q₂ draw more emitter current but the increase in current is limited to some extent by the increased voltage drop across the emitter resistances. The negative feedback is supplemented by the action of resistor R₁ which has normally a high value. It is clear that the TVM operates effectively from a constant current source if R1 is sufficiently high in value and Voltage drop across R₁ provides negative feedback for ₁ and Q₂. Thus accuracy of calibration is maintained over a wide range of temperature.