AN10 Power Amp Output Impedance
■In the design of power amp circuits, the need often arises for a power amp model with specified output impedance. Most often, this requirement revolves around the need to accurately predict the phase performance of power amp circuits.
■Output impedance of any op amp is modified by the feedback network present around the device. In voltage source type circuits, the effect of the network is to reduce the output impedance by a factor equal to the ratio of open loop gain to closed loop gain. In power amps, the net result is an effective output impedance of milliohm levels at frequencies below 1 kHz. Wiring and interconnections often create larger impedances than the output impedance of the closed loop power amp. Therefore, output impedance will play a minor role in the phase performance at low frequencies. At high frequencies, reactive load considerations are already addressed by capacitive load specifications given on many power amplifiers.
■Current control circuits, or current sources, include the load as a series element in the feedback loop with a sense resistor developing a voltage proportional to load current. Figure 1 shows a generalized example of just such a circuit. The load often consists of an inductive element such as a deflection yoke which can have up to 90° of phase shift at higher frequencies. Totally accurate prediction of phase in the feedback loop might at first seem to involve the series equivalent of output impedance and yoke impedance. In reality, it’s because the feedback the op amp is operating as a true current source with an impedance approaching infinity. A realistic approach to stabilizing the circuit merely involves an auxiliary feedback whose effect dominates before the combination of yoke feedback and amplifier phase approaches 180°. Output impedance is not necessary to determine stability.
■It is also important to realize that output impedance of a power op amp is not related in any way to power delivery capability or internal losses. A model of a power amp with the output resistance in series with the output will develop inordinate losses which are not observed in real world op amps.
■Output impedance is dependent on several variables such as frequency, loading and output level. Often, the impedance will rise at higher frequencies. A class C amplifier, such as PA51 or PA61, will exhibit higher impedances at lower levels due to bulk emitter resistance effects in the emitter follower outputs.
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PA02 、 PA07 、 PA08 、 PA09 、 PA10 、 PA12 、 PA19 、 PA51 、 PA61 、 PA84 |
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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2012/10/23 |
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REVD |
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AN10U;AN10 |
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190 KB |
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