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handbook of modern sensors

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5.2 Amplifiers 161<br />

(A)<br />

(B)<br />

Fig. 5.9. (A) Instrumentation amplifier with two operational amplifiers; (B) low-cost instrumentation<br />

amplifier with one operational amplifier.<br />

should be remembered, however, that the input <strong>of</strong>fset voltage will be amplified with<br />

the same gain. The CMRR primarily depends on matching values <strong>of</strong> resistors R.<br />

At very low frequencies, it is the reciprocal <strong>of</strong> the net fractional resistor mismatch,<br />

[i.e., CMRR=10,000 (−80 dB) for a 0.01% mismatch]. At higher frequencies, the<br />

impedance mismatch must be considered, rather than the resistor mismatch. To balance<br />

the impedances, a trimpot and a capacitor C 1 may be used. Also, remember that<br />

IA as a rule requires a dual (plus and minus) power supply.<br />

When cost is a really limiting factor and no high-quality dc characteristics are<br />

required, a very simple IA can be designed with just one operational amplifier and<br />

two resistors (Fig. 5.9B). The feedback resistor R a is connected to the null-balance<br />

terminal <strong>of</strong> the OPAM, which is the output <strong>of</strong> the front stage <strong>of</strong> the monolithic circuit.<br />

The amount <strong>of</strong> the feedback through R a depends on the actual circuit <strong>of</strong> an OPAM<br />

and somewhat varies from part to part. For the TLC271 operational amplifier (Texas<br />

Instruments), a gain <strong>of</strong> the circuit may be found from<br />

A ≈ 1 + R a<br />

(R a is in k), (5.11)<br />

2k<br />

which for values indicated in Fig. 5.9B gives a gain <strong>of</strong> about 50. The connections and<br />

values <strong>of</strong> the external components are different for different types <strong>of</strong> the operational<br />

amplifier. In addition, not all OPAMs can be used in such an unusual circuit.<br />

5.2.4 Charge Amplifiers<br />

The charge amplifier (CA) is a very special class <strong>of</strong> circuits which must have extremely<br />

low bias currents. These amplifiers are employed to convert to voltage signals from<br />

the capacitive <strong>sensors</strong>, quantum detectors, pyroelectric <strong>sensors</strong>, and other devices

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