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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 

In Part 1, we gave an overview of voltage splitters and their various implementations. In this instalment, we will tackle some features of audio power amplifiers.

Notes about some features of audio power amplifiers
The table gives some of the power audio amplifiers (PAAs) that are appropriate for voltage splitters (VSs). We should make short notes about some features of the PAAs and op amps (OAs) used in VSs. That is important in order to understand better the circuits proposed below. In all cases the datasheets and application notes of the involved ICs should be considered carefully.

Table: Some audio power amplifiers appropriate for voltage splitters. Note: for the abbreviation, please see the list of the abbreviations for the article. *—the gain can be or is adjusted with external components (sometimes wider range that specified in the table is possible). **—Please see the datasheets and the application notes (in all cases).

The input stage of the PAAs and OAs
The input stage and the parameters associated with that stage are very important to obtain stable and reliable operation of the VSs. Generally speaking there are three types of input stages of the PAAs and OAs:

 • differential amplifiers based on PNP transistors,
 • differential amplifiers based on NPN transistors,
 • input stage based on single transistor (usually NPN) which is comparing the input signal and the signal from the negative feedback from the output.
Each of the inputs mentioned above has some advantages and disadvantages and particularities. For example with the differential amplifiers based on PNP transistors we can go typically to the input ground, which is usually not possible for the second and third type of input stages.

The input stage and the parameters associated with that stage are very important to obtain stable operation of the VS. Some of the inputs of the ICs have effective built-in protection but others are not well protected and should be taken into consideration.

Power supply range
The power supply range of the PAAs and OAs can have a lot of particularities. For example, some of the ICs are specially designed for a single power supply, e.g., LM380, LM386, LM324, etc. Many of these ICs automatically keep the DC output voltage around half of the applied power supply. Care should be taken about the DC levels of the input pins of these ICs.

If these circuits are used with a dual power supply, we should take some precautions in order not to damage them, e.g., the load may have a particular configuration. Other PAAs as TDA2030/40/50 and others can work with single and dual, symmetrical and non-symmetrical power supplies.

Power dissipation
The power dissipation of the PAA in the VS should be evaluated in every particular case. That parameter depends mainly on the type, size and symmetry of the loads driven by the VS.

In the case of symmetrical loads, the average power dissipation from the VS can be very low. In the case of heavy unsymmetrical loads, the average power dissipation from the VS can be high and close to the power dissipation of the DC amplifier. Most of the PAAs have internal thermal protection but that is not the case for all amplifiers.

Switch ON and OFF particularities
Some of the PAA can have features as mute control, standby mode, etc. These features can be used and the VSs based on these amplifiers can have switch ON and OFF, power reduction modes, standby mode, etc. If we use these special modes we should take into considerations the transitional processes during the activation and deactivation on the modes.

Offset of the output voltage
Ideally, in single polarity operations the PAA—e.g., LM386, LM380, LM384, etc.—should divide the power supply in exactly two equal parts. In practice we may find a significant difference between both output voltages, e.g., more than 10%. In some cases we are in need of additional external components to adjust the output voltages to the required voltage levels and stability.

The DC output offset of the simple PAA can be significant, e.g., with a power supply of 18V the amplifier LM380 can have an output voltage of 9V ±1V, or the deviation of the output voltage from the ideal middle 18V/2 = 9V is around ±1V without external adjustment.

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