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Power management

Simple power supply voltage splitters based on audio amplifiers
- Part 3

March 31, 2014 | Petre Petrov | 222906256

Petre Petrov in the third part of this article
considers voltage splitters based on power audio amplifiers.

Page 1 of 6

[Part 1 of this article offered an overview of voltage
splitters and their various implementations. Part 2 looked at some features of audio operational
amplifiers.]

Voltage splitters based on power audio
amplifiers
From the point of view of the power supply the audio
amplifiers are generally of two types. The first type is used mainly in
applications with a single power supply. Some can be used with a dual power
supply but that is an unusual application and sometimes it is not possible.
Examples of these types of PAAs are LM386, LM380, LM384, TBA820M and
similar.

The second type of PAAs are used mainly in applications with a
dual power supply, but some simple additional circuit can work also with a
single power supply. Examples of these types of PAAs are TDA2030, TDA2040,
TDA2050, LM1875 and similar ICs. Although there are a lot of similarities
between these PAAs we should note a lot of differences and particularities
between them.

Figure 6 present the circuit diagram of a very
simple non-adjustable voltage splitter using an LM386.

Figure 6: Non adjustable voltage or
adjustable splitter with LM386.

The circuit works in the full
power supply range of the IC, but the output current is limited by the power
dissipation of the IC, which depends on the power supply. Resistors R2 and R3
are not obligatory but have a protection role (e.g., if we omit them we should
take special consideration of the pcb layout because the inputs of LM386 can
capture a lot of noise and that noise will be amplified and will be seen at the
output of the IC).

The input resistance of the LM386 is around 50kΩ and
the inputs are with PNP transistors and can go to ground. Resistor R4 may be
necessary for the stability of the IC.

The trimmer potentiometer P1 is
used to adjust the output voltage. We can apply the offset from P1 to the
inverting or non-inverting input of the IC. Jumper J1 is for application of the
offset to the non-inverting input and J2 is for the application of the offset to
the inverting input. J1, J2, R1 and P1 are not obligatory and can be omitted in
the simplest case.

Petre Petrov in the third part of this article considers voltage splitters
based on power audio amplifiers.

[Part 1 of this article offered an overview of voltage
splitters and their various implementations. Part 2 looked at some features of audio operational
amplifiers.]

Voltage splitters based on power audio
amplifiers
From the point of view of the power supply the audio
amplifiers are generally of two types. The first type is used mainly in
applications with a single power supply. Some can be used with a dual power
supply but that is an unusual application and sometimes it is not possible.
Examples of these types of PAAs are LM386, LM380, LM384, TBA820M and
similar.

The second type of PAAs are used mainly in applications with a
dual power supply, but some simple additional circuit can work also with a
single power supply. Examples of these types of PAAs are TDA2030, TDA2040,
TDA2050, LM1875 and similar ICs. Although there are a lot of similarities
between these PAAs we should note a lot of differences and particularities
between them.

Figure 6 present the circuit diagram of a very
simple non-adjustable voltage splitter using an LM386.

Figure 6: Non adjustable voltage or
adjustable splitter with LM386.

The circuit works in the full
power supply range of the IC, but the output current is limited by the power
dissipation of the IC, which depends on the power supply. Resistors R2 and R3
are not obligatory but have a protection role (e.g., if we omit them we should
take special consideration of the pcb layout because the inputs of LM386 can
capture a lot of noise and that noise will be amplified and will be seen at the
output of the IC).

The input resistance of the LM386 is around 50kΩ and
the inputs are with PNP transistors and can go to ground. Resistor R4 may be
necessary for the stability of the IC.

The trimmer potentiometer P1 is
used to adjust the output voltage. We can apply the offset from P1 to the
inverting or non-inverting input of the IC. Jumper J1 is for application of the
offset to the non-inverting input and J2 is for the application of the offset to
the inverting input. J1, J2, R1 and P1 are not obligatory and can be omitted in
the simplest case.

Sometimes we are in need of more current, higher voltages and higher
power dissipation than those offered by the LM386. Also we may need to adjust
the output voltages from the VS. In some of these cases we may use more powerful
ICs such as the LM380 and LM384. These two ICs have practically the same
internal circuit, but the LM384 can provide more power. Also, the LM380 is
produced in 8- and 14-pin packages while the LM384 is produced only in a 14-pin
package. Both ICs can require additional heat sinks.

Figure 7 presents
the circuit which can be used as an adjustable or non-adjustable VS.

Figure 7: Adjustable or non-adjustable voltage
splitter with LM380 in a DIP8 and with LM380 or LM384 in a 14-pin
DIP.

The first half of the circuit is a VS with a LM380 in an
8-pin package; the second half of the circuit is a VS with an LM380 or LM384 in
a 14-pin package. Both parts of the circuit are independent.

The jumpers
J1 to J4 can be used for adjustment of the output voltages. In all cases the
input voltage of IC1 will be amplified around 50 times and will appear between
the output of the IC and GND1. That gain should be taken into consideration if
we wish to adjust the output voltage with a trimmer potentiometer.

In
some cases we may wish to have a stable output voltage between the output of the
VS and the input ground terminal. In these cases we should apply a reference
voltage at the appropriate input of the PAA.

Figure 8 presents simple
adjustable voltage splitter with an LM386.

Figure 8: An adjustable voltage splitter with
an LM386 and with a reference input voltage from an LM385-1.2.

The reference input voltage of around 1.2V is obtained from an LM385-1.2. In
that case the internal gain of the LM386 is 20; the minimal gain of that IC is
around nine. The adjustment of the output voltage is made with the trimmer
potentiometer P1. The jumpers J1 and J2 are used to apply the reference voltage
to the inverting or non-inverting input of the PAA. The resistors R3, R4 and R5
are not obligatory, but improve the parameters of the VS.

The IC TBA820M also provides several options to create adjustable and
non-adjustable VSs. That IC has the possibility to externally adjust the voltage
gain and the frequency compensation. Figure 9 presents the circuit of a simple
VS around the TBA820M.

Figure 9: A VS circuit using the
TBA820M.

The input pin 3 of the IC can go to the ground. That
input is protected with an internal resistor of around 2kΩ. The AC gain of the
IC should be set to the minimum in order to reduce the noise at the output of
the IC. The frequency compensation is adjusted with C4. If needed, the output
voltage is adjusted with the trimmer potentiometer P1. Only one of the jumpers
J1 and J2 should be closed, depending on the required mode of operation.

If we are in need of more current and power in single polarity
applications we can use a VS based on single power audio amplifiers such as the
TDA2002 (10W) and TDA2003 (10W) and dual PAAs such as the TDA2004R (6W+6W),
TDA2005R/M/S (11W+11W), TDA2009/A (12W+12W) and similar.

The DC test
circuit of these ICs can be used as simple DC non-adjustable VSs. Here we will
examine one of these cases. Figure 10 presents the circuit of a VS based around
a TDA2003. The circuit also works with the older and less popular now
TDA2002.

Figure 10: A VS with the TDA2003 or older and
less popular now TDA2002.

For a power supply of 14.4V the output
voltage of the IC is between 6.1V and 7.7V. The inputs of the IC are connected
to single internal NPN transistor. The negative feedback between the output of
the circuit and the emitter of the input transistor is accomplished by R2 and
R3. The voltage gain of the IC should be set to minimum in order to have minimum
output noise. The output voltage is adjusted with the trimmer potentiometer
P1.

PAAs such as the TDA2030/40/50, LM1875, LM1876 (dual PAA), LM1877
(dual PAA) can be considered as some sort of power operational amplifiers with
differential inputs, but nevertheless there are some significant differences
among these PAAs. E.g., the input stage of the TDA2030/40/50 uses input PNP
transistors while the input stage of the LM1876 and LM1877 is with NPN
transistors.

Care should be taken about the maximum differential input
voltages of the ICs and if needed we should add external protection with
resistors, diodes and capacitors. For example, the maximum input voltage of some
ICs can be very low, e.g., only ±0.7V.

Figure 11 presents the circuit of
a VS using a TDA2030, TDA2040 or TDA2050.

Figure 11: A VS with a TDA2030, TDA2040 or
TDA2050.

The IC works as a DC amplifier with gain set by R3 and
R2. The gain should be above the minimal value and appropriate external
frequency compensation should be used. The problem with the DC configuration is
that the IC amplifies its DC offset.

The internal offset depends on the
temperature. In most cases that is not important but in all cases it should be
taken into consideration. In Figure 11 the output voltages are adjusted in large
scale with the trimmer potentiometer P1 and that is one of the advantages of
that VS. The gain of the IC is calculated with the formula below:

Av = 1
+ R3/R2.

In order to improve the stability of that and similar VSs we may
need to put a small high-current inductor in parallel with the resistor in
series with the output (in this case the resistor is R4*).

Sometimes we are in need of two or more DC power supplies that use the
ground line of the input voltage. In these cases we can use several single
amplifiers or dual, triple or quadruple PAAs or OAs. Figure 12 shows the circuit
diagram of an adjustable power supply that has three output power supplies - V1,
V2 and V3 - that share a common ground GND.

Figure 12: Adjustable power supply which has
three output power supplies V1, V2 and V3 which shares a common ground
GND.

In fact this is a VS from the second type (Figure 1b). That circuit is built around the dual low-power
audio amplifier TDA2822M. The TDA2822M features PNP input transistors and an
internally fixed AC gain of around 100 (36 to 41 dB), but here the IC is used as
a dual DC amplifier. At a power supply of 6V the quiescent output voltage at
each output is typically 2.7V. We may adjust the output voltages V2 and V3 with
the trimmer potentiometers P1 and P2. The Jumpers J1 to J4 select the mode of
operation of the VS.

Voltage splitters based on audio operational
amplifiers
Many modern electronic systems require power supply
currents well below 50mA. And we are in need of short-circuit protection of
these voltages. Audio operational amplifiers (AOAs) can be a good choice for VSs
in that case. Modern industry offers a large variety of single, dual and
quadruple OAs applicable in these VSs. Table 2 gives some operational amplifiers
appropriate for VSs.

Table 2: Some high current, high-voltage or
high-power operational amplifiers appropriate for voltage splitters. * - The
gain is set externally in wide range. ** - approximate value, please see the
data sheet.

Figure 13 presents the circuit diagram of a VS based
around dual paralleled OAs.

Figure 13: A VS based around dual paralleled op
amps.

We may use practically any dual op amp with a current
limiting function that is stable at unity gain and stable with a high capacitive
load. The IC is selected according to the required output voltages, current and
power dissipation. The output voltage is adjusted with the trimmer potentiometer
P1. The equalization resistors R3 and R4 are obligatory and depend mainly on the
stability of the OAs when driving capacitive loads.

If the current
provided by two paralleled OAs is not enough, we may use three or four
paralleled OAs under the condition that the power dissipation of the package of
the IC is not exceeded. Figure 14 gives the circuit diagram of a VS based around
quadruple paralleled OAs.

Figure 14: A VS based around quadruple
paralleled OAs.

The dual switch S1A+S1B is for selecting two
options: In position 1 we have two independent voltage splitters and the output
voltages are adjusted with P1 and P2. In position 2 we have a single voltage
splitter with four paralleled OAs and the output voltage is adjusted with P1. In
the VS we may use a lot of ICs starting with low-cost OAs like the LM324 and
TL084 or using the more expensive OPA4132, OPA4134, etc.

Voltage
splitters based on DC amplifiers and motor drivers
Modern industry offers ICs
specially designed to work as power operational amplifiers, e.g., the L165. The
L165 is similar to the TDA2030, TDA2040 and TDA2050, and to some degree the
LM1875. It is a special type of operational amplifier designed to work in power
supplies and servo applications compensated for gain above 10 (20dB). Figure 15
gives the circuit diagram of a VS based around an L165.

Figure 15: A VS based around an L165 power
operational amplifier.

The output voltage is adjusted with the
trimmer potentiometer P1. The input of the IC is protected with C1, R3, D1 and
D2. The gain of the IC is adjusted with R5 and R4 and should be above
10.

Voltage splitter with amplifiers and voltage
regulators
We may use PAA and voltage regulators in the same VS and
obtain additional regulated output voltages around the virtual ground of the
output. Figure 16 gives the simplified circuit diagram of a VS built around PAA
IC3.

Figure 16: Simplified circuit diagram of a VS
built around a PAA, an LM317 and an LM337.

The output voltages
V1 and -V2 depend on the PAA. The additional output voltages V3 and -V4 are
produced by an LM317T and LM337T. Output voltage V3 is adjusted with P1 and -V4
is adjusted with P2. The circuit around IC3 is not given with all
details.

Voltage splitters based on power and high-voltage
operational amplifiers
Modern industry offers OAs with power supply
voltages above ±22V and with guaranteed output current above ±15mA. Also there
are OAs with significant power dissipation capability. These OAs are sometimes
called high-voltage OAs (HVOAs), high-current OAs (HCOAs) or high-power OAs
(HPOAs). Some of these OAs are appropriate for VSs and we should mention them
because they extend the field of the applications of the VSs.

Table 2 gives some examples from the series OPAxxx produced
by Burr-Brown/Texas Instruments. The circuits with classical OAs and PAAs with
differential inputs mentioned above in the article are applicable to these OAs.
In that case, a slight redesign of the presented circuit of the VS is needed and
we will not discuss them again. We will mention that many of theses HVOAs and
HPOAs can be used in voltage splitters from 36V and 48V batteries and DC power
supplies.

Coming up in Part 4: Reducing the noise from voltage
splitters.

About the author

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

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