Transistor Voltmeters with Cascaded Transistors

Transistor Voltmeters with Cascaded Transistors Points : Transistor Voltmeters with Cascaded Transistors A circuit using a single transistor may be used for the measurement of voltage but such a circuit suffers from a big disadvantage that its sensitivity is low. To offset this, a TVM uses a pair of transistors connected in cascade. A simplified diagram of a TVM using two cascaded transistors is shown in fig. Here the two transistors Q₁ and Q₂ are direct coupled in a complementary configuration (one in a PNP and other is a NPN transistor). They form a direct coupled amplifier. The advantage of a complementary configuration is that it minimizes the number of circuit components.

The input voltage is applied through network of resistances. The range switch selects the resistor that is in series with the transistor for the desired voltage range.

The values of these resistors are typical. Note that for the 0 to 1v range, the value of resistance is 200,000 η. this means that neglecting the small value of resistance offered by transistor Q the sensitivity 200,000 Ω/v or 200Ω/v. Thus a current of 1/200.000 A or 5 micro Ampere is drawn from the circuit. The values of other resistance arc so chosen that the same current (5 µ A) is drawn at other ranges.

As mentioned above, the sensitivity of this a arrangement is 200k Ω/v, which is of an order greater than that of a Vom ( multimeter), although an ordinary. VTVM provides much greater sensitivity on its low voltage range.

A other problem with this TVM is that excessive drift is encountered in operation and acceptable accuracy can only be obtained at a specific ambient temperature.

Variable resistor R₂ is the zero adjustment resistor at controls the bucking current supplied by V to buck out the quiescent current.