AC Bridge Circuits
As we saw with DC measurement circuits, the circuit configuration
known as a bridge can be a very useful way to measure unknown values
of resistance. This is true with AC as well, and we can apply the very
same principle to the accurate measurement of unknown impedances.
To review, the bridge circuit works as a pair of two-component voltage
dividers connected across the same source voltage, with a
null-detector meter movement connected between them to indicate a
condition of "balance" at zero volts:
[IMAGE]
Any one of the four resistors in the above bridge can be the resistor
of unknown value, and its value can be determined by a ratio of the
other three, which are "calibrated," or whose resistances are known to
a precise degree. When the bridge is in a balanced condition (zero
voltage as indicated by the null detector), the ratio works out to be
this:
[IMAGE]
One of the advantages of using a bridge circuit to measure resistance
is that the voltage of the power source is irrelevant. Practically
speaking, the higher the supply voltage, the easier it is to detect a
condition of imbalance between the four resistors with the null
detector, and thus the more sensitive it will be. A greater supply
voltage leads to the possibility of increased measurement precision.
However, there will be no fundamental error introduced as a result of
a lesser or greater power supply voltage unlike other types of
resistance measurement schemes.
Impedance bridges work the same, only the balance equation is with
complex quantities, as both magnitude and phase across the components
of the two dividers must be equal in order for the null detector to
indicate "zero." The null detector, of course, must be a device
capable of detecting very small AC voltages. An oscilloscope is often
used for this, although very sensitive electromechanical meter
movements and even headphones (small speakers) may be used if the
source frequency is within audio range.
One way to maximize the effectiveness of audio headphones as a null