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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?

Sometimes the minimum and maximum quiescent output voltages are not given in the data sheet, but in most cases the maximum tolerance is below 15% from the ideal voltage. From that point of view it is most profitable to use an IC similar to the classic OA as a VS and to apply the corresponding techniques for adjusting the output voltages and to reduce the offsets and the temperature dependence of these offsets. The PAAs usually do not have pins for external offset correction and we should use external components for that purpose.

Maximum output current
The maximum working output current of the PAA is not always specified, e.g., LM386. For some PAAs we have the maximum peak current or the maximum short circuit current, e.g., LM380, LM384 etc. The working values of that current can be derived from the maximum output power and the load resistance, which are always given in the data sheet.

We should differentiate between the maximum working current, the absolute output current and the short circuit current. These currents are different and can have different definitions and relations in the different ICs. Most PAAs have effective over-current or short-current protection. It is preferable to use PAAs with up to half of the maximum output current if it is specified.

The maximum output current of the IC in the VS is important. It may not correspond to the required current from the VS. If that current is too low we should add external current buffers, e.g., we can add transistors to the PAAs or the OAs. But if that current is too high we should add external current limiters and usually these limiters are in the powers supply line of the IC, e.g., we may use LM317M/T and LM337M/T to limit the output current of TDA2050 up to around 1.5A and to use the full power dissipation of the PAA.

Input/output protection of the IC
The inputs and outputs of some of the audio circuits are protected internally from voltages and current outside the safe limits. The internal protection components are usually given on the circuit diagrams published by the manufacturers. But that is not the case for all ICs.

For example, the internal protection components are not always explicitly given, or simply do not exist. In all cases we should consider the use of external protection. Usually this is implemented with additional diodes and resistors and sometimes capacitors as shown on some of the figures. Some amplifiers should be used with protective diodes in parallel with the output transistors.

Input current and offset currents
The input and offset currents of many of the PAAs are relatively low, e.g., below 1?A. Usually they are not a problem in the discussed applications.

Minimum and maximum voltage gain Many PAAs and some AOAs have minimal AC gain. If used below that gain the ICs can become unstable and oscillate. The frequency of the oscillation can be high (e.g., 0.5MHz to 10MHz) or low (a few Hertz) depending on the circuit and its implementation or the pcb.

Also it is important to note the difference between the AC and the DC gain of the amplifier. In both cases the requirements for the minimum gain can be different and should be respected in order not to have unstable operations.

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