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Utilise audio amps for voltage splitting (Part 2)

Posted: 17 Mar 2014 ?? ?Print Version ?Bookmark and Share

Keywords:voltage splitters? power audio amplifiers? PAA? op amps? transistors?

Many PAAs and AOs have the possibility of connecting external passive components to provide appropriate frequency compensation for the selected gain, e.g., TBA820M. For other ICs we should use external frequency compensation (FCs).

PAAs similar to the operational amplifiers
Many audio power amplifiers such as TDA2030, TDA2030, TDA2050 and LM1875 are, roughly speaking, powerful operational amplifiers. They have differential inputs and a single output. We can use external components to obtain practically any AC and DC gain and any frequency compensation (FC).

The problem is that when we add simple external FC we reduce the slew rate and the bandwidth of the amplifier. As a result, we increase the time of reaction of the VS to the change of the loads. Usually that is not a problem and can be compensated with appropriate capacitors in parallel with the voltages produced by the VS, but the IC should be stable with that capacitive load.

Adding external transistors to the PAA
Transistors are low priced and widely used electronic components. When in need of higher output current and power dissipation from the VS it is preferable to add external transistors to the PAA. That makes the VS more resistant to negative events from the loads. Also the VS can provide larger output current and power dissipation. The transistors can have significant power dissipation.

Usually the additional transistors work with collector-to-emitter voltages above 3V and can have significant gain in the VS. Also the THD and IMD of the VS are not important here and the quiescent current of these transistors can be very low. The voltage drops over the output transistors of the IC and over the additional transistors is usually significant and do not limit the gain of the ICs.

AC and DC gain
Normally the PAA are used as AC amplifiers in the audio range. But many of them can also be used as DC amplifiers. One of the limiting factors in that application is the power dissipation. Also in all applications we should take care about the minimum gain when the PAA is stable and the external FC to keep the amplifier stable.

Quality of the external capacitors
The external capacitors are not part of the IC but they interact with the internal components of the IC and should be mentioned briefly here. Care should be taken about the quality of the capacitors connected to the output of the ICs, between the pins for the power supplies of the ICs, and in the feedback and the frequency compensation circuits.

Some capacitors can prevent oscillation of the ICs, the others can provoke oscillation always or under some conditions. That is due to the parasitic inductances and resistances of the capacitors and to other inherent parameters.

About the author
Petre Petrov has worked as a researcher and assistant professor in the Technical University of Sofia (Sofia, Bulgaria) and as a lecturer and expert lecturer in Morocco. Now he is working as electronics engineer in Bulgaria.

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