Abstract
Chapter 7 describes a dc model for an op amp and gives examples of dc resistive circuit design based on the model. Such a treatment is instructive but unrealistic for all but simple circuits operating at relatively low frequencies. In almost all practical applications of op amps, the frequency responses of surrounding circuitry and of op amps themselves must be considered.
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Notes
- 1.
However, if the source is capacitance coupled to the amplifier, the required dc bias-current compensation drastically reduces the input impedance.
- 2.
Another is slew rate, discussed in Section 17.5.
- 3.
We assume here that the bandwidth of the feedback amplifier exceeds that of the op amp alone, which is almost always the case (except for a follower). Because the gain-bandwidth product for a feedback amplifier equals that for the op amp alone, and because the dc gain of a stable feedback amplifier is smaller than that of the op amp, the bandwidth of the amplifier must be larger than that of the op amp.
- 4.
The output swing of an op amp is defined as if the supply is symmetric, whether or not that is actually the case.
- 5.
It also is possible to use a circuit called an automatic gain control (AGC), which keeps the rms amplitude of an input within specified bounds. You might learn about AGCs in a subsequent course.
- 6.
See Section 8.10.7 in Chapter 8.
- 7.
The output stage of an op amp is typically a push-pull configuration of two transistors, where one transistor amplifies the positive parts of an applied voltage and the other amplifies the negative parts. There is actually small crossover voltage range near zero where there is current through both supplies. However, this current is usually quite small, relative to a typical load current.
- 8.
Actually, this is only the power delivered by the supply to the output stage. A typical op amp is a three-stage amplifier, and even if no input is applied (even if \(i_0 = 0\)), some dc power is required to keep the op amp in an active (ready) state.
- 9.
Direct coupling is used almost exclusively in integrated circuits.
- 10.
The 14.14 mV source voltage shown is peak. The meter reading is rms.
- 11.
Virtually all manufacturers of op amps have extensive web sites describing their offerings.
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© 2011 Springer Science+Business Media B.V.
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Glisson, T.H. (2011). Operational Amplifiers II: AC Model and Applications. In: Introduction to Circuit Analysis and Design. Springer, Dordrecht. https://doi.org/10.1007/978-90-481-9443-8_17
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DOI: https://doi.org/10.1007/978-90-481-9443-8_17
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