\Transistor voltmeter (TVM) : a simple diagram of TVM using two transistors in the bridge configuration is as shown.
TVM is provided by using transistor bridge circuits. In bridge circuit, large negative feedback would help to minimize drift due to variation in βlco and due to power supply fluctuations. The resistor and internal collector resistance of transistor constitute the bridge arms. With a positive voltage applied to the base of transistor there would be an increase in the emitter current. In turn, the emitter of the transistor would reduce on account of the increased voltage drop across. In this way, the bridge would be unbalanced so that there would be a current in the meter.
With an increase in the ambient temperature, the beta value of both the devices would increase. If the transistors are perfectly matched the current increase through would be equal to the current increase through so that the bridge would remain balanced. The saturation current through each transistor would also increase balanced. Even though power supply fluctuations result in variation of values, the change in bias is same for both the transistors, so that the bridge would remain balanced. But in order to maintain calibration accuracy, a large amount of negative feedback would be needed, in addition to a stable balance condition.
It may be noted that the zero set control, also provides a resistance in series with the emitter of the devices and would draw more emitter current, this increase would be somewhat limited by increased voltage drop across the emitter resistance. This negative feedback is augmented by the action of which usually has a very high value. It may be noted that the transistor voltmeter is effectively driven from constant current source if is sufficiently large in value and that the potential drop acrss provides the requisite negative feedback for the device and. It is thus possible to maintain the calibration accuracy over a relatively wide temperature range.
Advantages
1. A TVM does not require any warm up time
2. A ATVM is comparatively compact and is light in weight and well suitable for field work.
3. A TVM may be designed as a battery operated instrument.
Disadvantages
1. Sensitivity is low compared to VTVM.
2. Excessive drift is encountered in operation and acceptable accuracy can only be obtained at a specific ambient temperature.