November 2000
Intermediate to advanced
608 pages
16h 12m
English
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7.1 CHOPPER AND LOW-DRIFT AMPLIFIERS
7.1.1 Offset and Drifts in Op Amps
In an ideal op amp the output voltage is zero when both input voltages are zero. The input currents are then also zero. In a real op amp, neither of these conditions holds. In addition to being different from zero when the input voltage is zero, the input currents are not equal to each other. Their difference is called offset current. This is due to the imbalance between input transistors [bipolar or field-effect transistors (FETs)]. This imbalance also requires an offset voltage between the input terminals for the output voltage to be zero.
We can analyze the effects of these voltages and currents by considering a simple amplifier—for example, the inverting amplifier in Figure 7.1. The output voltage is
where R′1 = R1 + Rs, β = R′1/(R′1 + R2), Ad = Ad0ωa/(s + ωa) = fT/(s + ωa), and we have assumed that Adβ is a real number. If, in addition, Adβ 
1, we can approximate
The actual sign for Vio is unknown and that of In and Ip depends on the transistor type (p–n–p, n–p–n) at the op amp input stage. Usually a worst-case condition is assumed and the contribution of offset voltage and current is added.
Equation (
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