Q. Explain the block diagrtam of D.C. voltmeter with direct coupled amplifier.
Sol. D.C, voltmeter with direct coupled Amplifier
The D.C. electronic voltmeters consist of an ordinary D.C. meter movement preceded by a D,C, amplifier of one or more stages. When a very high input resistance is required it is convenient to use an FET at the input stage. The output of the FET can usually be directly coupled to the input of a BJT.
Direct coupled amplifiers are normally used in low priced D.C. amplifier. Bipolar transistor Q2 along with resistors forms a balanced bridge circuit FET. Q1 serves as a source follower and is used to provide impedance transformation between the input and base of Q2. The bias on Q2 such that i2 = i3 when the input voltage Vin = 0 under that condition Vx =Vy, so that no current flow through the meter movement that is i4 = 0. The bias on Q2 is controlled by input voltage Vin thus when an unknown input which cause V1 to increase. Since Vx become greater than Vy, current i4 is no longer zero. The magnitude of this current, hence the deflection of the meter is proportional to Vin.
The value of Vin. That cause maximum meter deflection is the basic range of the instrument. This is generally the lowest range on the range switch in non-amplified models. High range can be obtained by using an input attenuator and lower ranges can be obtained by a preamplifier.
The input attenuator in fig (A) is a calibrate front panel control in the form of resistance voltage divider. The full scale voltage appears across the divider so that the voltage at each tap is a progressively lower fraction of the full input voltage.
Bridge balance is obtained by adjustment the zero set potentiometer when VIN is zero full scale calibration is obtained by adjusting the potentiometer marked calibration in series with the ammeter.
The advantages of this meter are
(1) It decreases the amount of power drawn from the circuit under test by increasing the input impedance using an amplifier with unity gain.
(2) The source follower drives am emitter follower. This combination is capable of thousand fold or more increases impedance while maintaining a voltage gain of nearly one.
(3) The input impedance of this meter is 10?, which require a power of .025 µW for a 0.5 V deflection as compared to 25 µ W for an unamplifid meter thereby giving an increased sensitivity of 100 times.
A block diagram of a meter used for measurement of small voltage and currents is shown. The input voltage is amplified and applied to a increased by a like amount. A D.C -coupled amplifier that is an amplifier with no coupling capacitors and having a well controlled D.C, gain, is used to provide dot necessary amplification. An amplifier capable of a fixed DC gain of 10 is not difficult to construct and to keep stable. A simple op-amp plus the required feedback components will do a suitable job for this application.
DC gains of much more than 10 are required to use a standard D Arsonval meter movement to measure very small currents and voltages such as microvolt and nanoampere. To amplify nano ampere to drive a milliampere meter require a gain of 106. This requires an op amp and two resistor and a simple circuit. However when gains this large are desired, all the defects of an operational amplifier become significant offset current, offset voltage and biases current become so troublesome that it is practically impossible to achieve acceptable performance with a standard op amp. Many of these defects can be reduced or eliminated by the use of trim adjustments accessible from the front panel in a similar fashion as the calibrate and zero function discussed above.