Transformer Ratio Arm Bridges

Fig. 5.10 A simplified circuit diagram of a transformer-ratio-arm bridge.

Another kind of audio-frequency bridge, which has come to the fore recently and which is very convenient to use, is the transformer-ratio-arm bridge, shown schematically in Fig. 5.10. This bridge uses inductive ratio arms to compare the unknown directly with standard components. The voltage... 

Figure 2.16 (a) A differential three-terminal capacitor, (b) A capacitance bridge circuit with output proportional to fractional difference in capacitance, (c) A transformer ratio arm bridge. (From R S C Cobbold R S C Transducers for Biomedical Measurements Principles and Applications, copyright 1974, John Wiley and Sons, Inc.)... 

A transformer ratio arm bridge (figure 2.16(c)) can also be used to solve equation (2.27). The degree of bridge unbalance (C — C2) is a function of amplifier current which may be found from equation (2.26). 

FIGURE 6-16 Typical transformer-ratio-arm bridge transducer. (Source Huang, 1988)... 

The high-frequency limitation imposed on the operation of reactively substituted Wheatstone bridges by unavoidable stray capacitances prompted the development of the transformer ratio arm bridge (Calvert [1948]). By substituting a transformer for orthodox ratio arms, a bridge was produced for which the impedance ratio is proportional to the square of the number of turns and which was capable of accepting heavy capacitive loads with virtually no effect on the voltage ratio. 

The operation of a transformer ratio arm bridge is shown schematically in Figure 3.1.2. Briefly, voltage 180° out of phase is fed from the secondary winding of the input voltage transformer to the cell or unknown impedance and to resistance and capacitance standards. The arms of the bridge consist of a series of ratio taps of the primary windings of an output current transformer. The standard and unknown impedances are connected to the output transformer in such a way that a detector null is achieved wheu the sum of the flux induced by the unknown and standard currents in the output transformer is zero. In this condition... 

Figure 3.1.2. Transformer ratio arm bridge with dc potenticostatic control.

Figure 3.1.3. The effect of stray capacitances in the transformer ratio arm bridge.

Limitations of Potential Control. The limitations of potential control for a transformer ratio arm bridge are similar to those imposed in classical bridge meas-nrement. That is, it is not possible to apply the ac potential via a reference electrode and potentiostat circuit only to the interface of interest. The measnred impedance necessarily includes series terms associated with the lead and electrolyte resistance and the counter electrode impedance. 

Because the gains of amplifiers A and B perform the same function as the ratios in a transformer ratio arm bridge, the two techniques have many features in common. 

An active null admittance measuring instrument that incorporates many of the advantages of the transformer ratio arm technique, while obviating many of the disadvantage of passive bridges, has been reported by Berberian and Cole [1969]. Figure 3.1.4 shows a form of this bridge modified to measure impedance and to remove some of the limitations of the earlier instrument (McKubre [1976]). 

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