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Utilising audio amps for voltage splitting

Posted: 06 Mar 2014  Print Version  Bookmark and Share

Keywords:power supply  voltage splitters  power audio amplifier  operational amplifier  Zener diodes 

There are times when it is necessary to split a single power supply into two or more parts which are not necessarily equal. This is the case with 6-, 12-, 15-, 24-, 36- or 48-V power supplies from dry batteries or car accumulators. Although accurate specialised voltage splitters (VSs) exist, they may be not readily available or too expensive for the projects. Also sometimes the required current or power from the VSs cannot be provided by the IC designed to work as the VS.

Fortunately for many applications when we are in need of a VS, we can use the low-cost power audio amplifier (PAA) such as an LM386, LM380, LM384, TBA820M, TDA2002, TDA2003, TDA2030, TDA2040, TDA2050, LM1875 and many more to split the power supply. This is especially the case for VSs for the test bench and for experimental systems.

We can build around the PAAs simple, low-cost and non-switching VSs for the power supply of the electronic equipment. The power audio amplifiers listed above and many more are acquired in large quantities for a lot of projects. That makes their price of usage and replacement low and affordable.

Also the PAAs have been produced over many years from a lot of manufacturers. They are well known, the internal circuit is published, and they can be easily tested. In case of damage these ICs can be easily replaced.

Each of the circuits in this article is operational, but has some particularities so the circuits should be evaluated before the decision of the applicability to targeted equipment is taken. The presented circuits are simple and do not need complicated redesign or adjustment to work properly.

Three types of voltage splitters
Generally speaking there are three types of VSs. The block diagrams of these three types are illustrated in figure 1.

Figure 1: General block diagram of the three main types of VSs. a) voltage splitter with two outputs; b) voltage splitter with four outputs; and c) voltage splitter with three virtual grounds.

Figure 1a shows the most frequent version of the VS. The input voltage between +Vin and GNDin is divided into two parts, not necessarily equal. These parts can be fixed or adjustable to some degree. Usually there are some minimal differences between the input and the output voltages which depend on the VS.

The two output voltages are between +V1 and GNDout and between -V2 and GNDout. In this type of VS there is no isolation between the input and the output voltages and there is no direct connection between the input ground GNDin and the output ground GNDout (sometimes called virtual ground).

Figure 1b shows the block diagram of the second type of VS. The input voltage between +Vin and GNDin is divided into four parts, not necessarily equal. These parts can be fixed or adjustable to some degree. In that case there is a direct connection between the input ground GNDin and the output ground GNDout. That application can be called a multi-output linear regulator.

But care should be taken because in the case of the VSs each of the outputs V1, V2 and V3 can be driven with push-pull stages not by a single-output buffer. And that is not the case of the linear regulator where usually we have a single transistor at each output (the output is not a push-pull stage).

Figure 1c shows the third version of the VS. The input voltage between +Vin and GNDin is divided into four parts, not necessarily equal. In fact this application has three voltage splitters, each of them dividing its input voltage into two parts. Each output ground—GND1, GND2 and GND3—is driven with push-pull stages by the VS.

We should pay attention to the measurement of the output voltages of the VS. In that case V1 and -V2 are measured to GND1, V3 and -V4 are measured to GND2, and V5 and -V6 are measured to GND3.

Here we will use the mainly the VSs based on the block circuit from figure 1a and more rarely the VSs based on figure 1b.

Advantages of analogue voltage splitters based on audio amplifiers
Modern industry offers a large variety of switching DC/DC converters and they can be used as a kind of VS. But these devices may not always be available over long terms, can be unaffordable, and can produce a lot of electromagnetic noise, or have other drawbacks.

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