The use of audio amplifiers for voltage splitting
Introduction
Sometimes 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 specialized 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 analog 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.
The use of the power audio amplifier, audio operational amplifier (AOA) and similar ICs and audio modules as VSs have a lot advantages:
- PAAs have been produced over many years from many manufacturers. -They are available from many distributors.
-They are well known and low priced. - PAAs are easily tested, and replacement in case of need is easy.
- PAAs have low noise in the audio frequency range and outside that range.
-They do not produce a lot of output noise, radio waves or electromagnetic interferences (EMI). - Many of the PAAs have internal thermal protection, over-current protection and sometimes protection from reactive loads and over-voltage protection.
- Many PAAs can be easily mounted on additional heat sinks if needed.
As an illustration only, before we consider the VS based on an amplifier, we will examine several useful VSs based on diodes and transistors.
Voltage splitters based on diodes and Zener diodes
We may need a few milliamps from two or more low-voltage power supplies or reference voltages derived from common DC power supplies. Also the power management of the circuit may not need to be very strict. In some of these cases we can use VSs based on diodes, Zener diodes and shunt regulators.
Modern industry offers a large variety of Zener diodes with power dissipation between 0.3 and 1.3 W with a reference voltage tolerance of ±2% or better. These Zener diodes can be used for implementing some sort of VSs. We show three examples in Figure 2.
Figure 2: Voltage splitters with diodes and Zener Diodes. a) Voltage splitter with diodes; b) Voltage splitter with Zener diodes; and c) Voltage splitter with two shunt regulators (TL431).
Figure 2a shows simple VS with diodes. We can connect in series any appropriate numbers of diodes or light emitting diodes (LEDs) and they will work as shunt or parallel regulators. In that case we have two diodes D1 and D2 producing the positive output voltage +V1 and one diode producing the negative output voltage -V3. The output ground GNDout can be in any point between the diodes.
Figure 2b shows a simple VS with Zener diodes. We can connect in series any appropriate numbers of Zener diodes and they will work as shunt or parallel regulators. In that case we have two diodes D1 and D2 producing the positive output voltage +V1 and +V2 and two diodes producing the negative output voltages -V3 and – V4. The output ground GNDout can be in any point between the Zener diodes. In that case GNDout is between D2 and D3. The Zener diodes can be from the same or different type.
We may use shunt regulators as TL431 instead of diodes and Zener diodes. The advantage of that solution is that we may adjust the output voltages by selection of resistors or with trimmer potentiometers or with other elements.
Figure 2c shows a simple VS with a TL431 adjustable shunt regulator. In that case we have two TL431 or LM341 producing the positive output voltage +V1 and the negative output voltage -V3. The voltage V1 is adjusted with trimmer potentiometer P1 and the negative output voltage -V3 is adjusted with P2.
We can connect any appropriate number of shunt regulators in series as is shown in Figures 2a and 2b. In fact these regulators can be considered as adjustable Zener diodes.
Voltage splitters based on bipolar junctions transistors (BJTs)
The VS from Figure 2 does not have push-pull output stages and can waste a lot of power without load. We can avoid this drawback with VSs based on BJTs. They are especially good when we are in need of high output voltages, high currents, high power, or when we do not need very good regulation of the output voltages.
As examples we will discuss shortly several simple applications below. These circuits have push-pull output stages and simple regulation of the output voltages. They are similar to the circuit of transistorized audio amplifiers working as DC amplifiers.
Figure 3 shows two examples of simple VSs around two transistors.
Figure 3: Simple voltage splitters with transistors.
Figure 3a uses the transistors T1 and T2 for buffering the voltage from the voltage divider build around resistors R1 to R4 and diodes D1 and D2. The diodes D1 and D2 are for temperature compensation; they are not obligatory. If used, D1 should be in thermal contact with T1 and D2 should be in thermal contact with T2. If D1 and D2 are not used R2 and R3 should be increased accordingly.
The resistors R5, R6 and R7 provide simple local feedback slightly improving and protecting the circuit. R5 is much larger than R6 and R7. The values of the components in the circuit are calculated similarly to the components in the circuits of an emitter follower.
Figure 3b uses three transistors and more effective regulation of the output voltage with negative feedback. The resistor R1 and the trimmer-potentiometer P1 provide negative feedback that stabilizes the output voltages. The output voltages +V1 and -V2 are set by P1, R1 and R2. D1 and D2 are for temperature compensation. Resistors R4 and R5 provide local negative feedback and some protection of the output transistors T2 and T3.
Sometimes we are in need of adjustment and more effective regulation of the output voltages produced by VSs. In that case we may use the classical differential amplifier with transistors to solve these problems. Figure 4 shows a VS built around five transistors, T1 to T5.
Figure 4: Voltage splitter based on differential amplifier with transistors.
T1 and T2 work as a differential amplifier. T3 is the amplifier and driver of the output transistors T4 and T5. The resistor R6 provides negative feedback that stabilizes the output voltage. R7* and C2* are not obligatory. C1 is obligatory because it provides the frequency compensation of the circuit.
The output voltages +V1 and -V2 are set by R1, R2 and P1. The diodes D1, D2 and D3 are for biasing of the output transistors and for temperature compensation. The trimmer potentiometer P2 is for adjustment of the quiescent current of the output transistors, e.g., from 1 to 10 mA depending on the load. Resistors R4 and R5 provide local negative feedback and some protection of the output transistors T4 and T5.
Voltage splitter with operational amplifier and BJTs
If the load is changing or is not symmetrical, the VS from Figure 3 and Figure 4 may not give good results. To solve the problem sometimes we may use a VS based on a single op amp such as a TL071, OPA134, NE5534/A, or LM741 with additional complementary BJTs such as PN2222A+PN2907A, BD135+BD136, etc. Figure 5 shows one example of such a VS.
Figure 5: Simple voltage splitter with operational amplifier and BJTs.
The output voltage is adjusted with P1. The VS works as a DC amplifier with a gain Av equal to Av = 1+ R4/R5. R5 can be omitted if not needed and the gain will be unity or the circuit will work as a follower and current buffer.
The output current of the VS is limited to 50 to 200n mA depending on the transistors T1 and T2 and the output capabilities of the OA. The capacitor C4* is used only if we are in need of external frequency compensation for the OA. Most PAAs include everything from Figure 5 to build adjustable and non adjustable VSs. That is why we will continue the article with them.
Coming up in Part 2: Notes about some features of audio power amplifiers.
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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