AC Electronic Voltmeter
• The IC Op-amp voltage-follower voltmeter described
previously was a dc instrument.
• Connecting a rectifier in series with the meter circuit of this
instrument, as shown in Figure below, converts it into a half-
wave rectifier voltmeter.
• The output from the voltage-follower is exactly the same as
the input. So the voltage fed to the meter circuit is simply a
half-wave rectified version of the input voltage EB from the
attenuator.
• The difference between the non-electronic instrument and
electronic ac voltmeter circuit is, of course, that the electronic
instrument has a very high input impedance.
• The coupling capacitor (C1) is usually provided at the input of
an ac voltmeter to block unwanted dc voltages.
• The voltage drop (VF) across the rectifier is a source of error in
the circuit above. VF can be taken into account in design
calculations when the instrument is indicating full scale.
• However, at other points on the scale an error occurs due to
VF. Also, the rectifier voltage drop is not always exactly 0.7V,
as usually assumed for a silicon diode, and it varies with
temperature change.
• To avoid these errors, the voltage follower feedback
connection to the inverting terminal is taken from the cathode
of rectifier D1 instead of from the amplifier output as shown
in Figure below.
• The result is that the half-wave output precisely follows the
positive half-cycle of the input voltage.
• There is no rectifier voltage drop fro input to output. The
circuit is known as a precision rectifier.
• Note that capacitors C2, C3, and C4 are connected across the
attenuator resistors. These are normally employed with the
attenuator on ac electronic voltmeters in order to
compensate for the input capacitance of the amplifier.
• Low-level ac voltage should be accurately amplified before
being rectified and applied to a meter circuit. Amplification is
combined with half-wave rectification in the circuit shown in
Figure below.
• With the diode omitted, the op-amp circuit is non-inverting
amplifier. Inclusion of D1 causes the positive half-cycles of the
𝑅2 +𝑅3
input to be amplified by a factor 𝐴𝑣 = .
𝑅3
• The amplification is precise and here again there is no rectifier
voltage drop involved.
• The peak voltage applied to the meter circuit is AvEp and again
the rectifier voltage drop does not enter into the calculations.
Voltage-to-Current Converter
• The circuit below is a voltage-to-current converter with half-
wave rectification.
• The circuit functions exactly as explained in previous section,
with the exception that only the +ve half-cycles of the ac input
are effective in passing current through the meter.
• The meter peak current is Ip=Ep/R3, and the average meter
current is Iav=0.5(0.637Ip)
• A full-wave bridge rectifier is employed in the circuit shown
below
• When the input voltage is positive, the op-amp output voltage
is positive . Diode D1 and D4 are forward biased so that
current flows through the meter from top (+ve) to bottom (-
ve).
• When the input is negative, D2 and D3 are forward biased.
Once again current passes through the meter from the +ve to
the –ve terminal.
• Whether the input is +ve or –ve, the meter peak current is
again limited to Ip=Ep/R3.
• The average meter current in the full-wave rectifier circuit is
Iav=0.637Ip
• A half-bridge rectifier could be useful in an electronic
voltmeter that uses a transistor amplifier.
• Figure below shows a half-bridge rectifier connected to the
output stage of a transistor amplifier.
• Output transistor Q3 passes direct current through resistor R5,
producing a dc voltage at the collector of the transistor which
might be above or below the zero voltage level.
• Capacitors C1 and C2 blocks DC from the meter circuit and pass
the AC when an ac voltage appears at the amplifier’s output.
Current Measurement with Electronic Instruments
• The basic circuit of an analog electronic ammeter for
measurement of very low currents is shown in Figure below.
• The small voltage drop across shunt resistor Rs is amplified
before being applied to the deflection instrument. This
approach is just as applicable to the measurement of low-
level alternating currents as it is to direct-current
measurements.
• For alternating currents, an ac electronic voltmeter using
rectifiers is used instead of a dc instrument.
• For medium or high current measurements there is no need
to use electronic amplifiers.
