November 2000
Intermediate to advanced
608 pages
16h 12m
English
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3.5 DIFFERENTIAL AND INSTRUMENTATION AMPLIFIERS
3.5.1 Differential amplifiers
Voltage amplifiers yield an output voltage proportional to that at their input. A differential amplifier processes the voltage difference between two input terminals, neither of which is connected to the reference voltage of its power supply. In addition, we will show later that it is best for amplifier input terminals to have high and similar impedance to ground. Common resistive sensor bridges supplied by a grounded voltage or current source cannot have any output terminal grounded. Therefore, differential amplifiers suit them. Differential amplifiers were first proposed by B. H. C. Matthews in 1934—using vacuum tubes—for electrophysiological studies.
Figure 3.32 (a) Differential amplifier based on a single op amp and four matched resistors. (b) Adding a resistor increases the input common mode voltage range.
Figure 3.32a shows a simple differential amplifier. If we first assume that the op amp has a frequency-dependent differential gain Ad and negligible common mode gain (Ac = 0), then the (Laplace transform of the) output voltage is (reference 1, Section 2.4.2)
where β = R1/(R1 + R2) is the feedback factor for the op amp. To illustrate the differential properties of the circuit, it is convenient ...
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