Regulated DC power supply

A power supply circuit for supplying stabilized positive and negative DC voltages of opposite polarity but equal magnitude requires only a single voltage source that supplies a first DC voltage, and a polarity converting circuit connected to the voltage source provides a second DC voltage equal in magnitude but opposite in polarity to the first DC voltage. First and second voltage stabilizing circuits are connected to the voltage source and the polarity converting circuit, respectively, to provide between respective output terminals and a common ground line the equal level but opposite polarity voltages. Voltage stabilization is provided in the stabilizing circuits by the use of Zener diode circuits to provide biasing to control the collector-emitter paths of respective transistors, and polarity conversion is accomplished by a switching transistor controlled by a pulse-width modulation signal generator having its input derived from a level comparison circuit.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a circuit for supplying positive 
and negative voltages and, more particularly, to a power supply circuit to 
supply positive and negative power-source voltages derived from a single 
positive or negative voltage source. 
2. Description of the Prior Art 
In audio equipment, such as systems for reproducing stereophonic sound 
signals recorded on records, discs, or tapes, positive and negative power 
source voltages are often used with the intention of increasing the 
dynamic range of the audio output signals, for suppressing noise in the 
audio output channels, and for reducing the absolute values of the power 
source voltages in the system. These positive and negative power source 
voltages are generally required to have substantially the same absolute 
values and, therefore, the power source employed in the audio apparatus 
using positive and negative power source voltages must be capable of 
producing and supplying positive and negative voltages having the same 
values. 
There has been proposed a power supply circuit that supplies positive and 
negative power-source voltages derived from two batteries, or sets of 
series-connected batteries, having a positive terminal grounded and a 
negative terminal grounded, respectively. In such power supply circuit as 
previously proposed, however, there are inherent problems and 
disadvantages involved, for example, multiple batteries are required and 
this involves a relatively large space requirement in the apparatus and 
also increases the weight of the entire system. 
There has also been proposed a power supply circuit that uses a single DC 
voltage source to provide the required positive and negative voltages of 
the same value and employs a DC-DC convertor, operating with a DC voltage 
supplied from a single DC voltage source. Such power supply circuit also 
has disadvantages in that the DC-DC convertor must be constructed using: a 
transformer, a switching circuit connected to the primary of the 
transformer, a control circuit to control the operation of the switching 
circuit, and two detecting circuits connected to the secondary of the 
transformer, in order to produce the positive and negative DC voltages of 
the same absolute value. Thus, it is seen that the configuration of the 
entire power supply circuit becomes complicated and must be constructed on 
a relatively large scale. Additionally, further problems are involved 
because it is difficult to increase the efficiency in converting positive 
and negative DC voltages supplied from a single DC voltage source, and 
also it is easy to overload the switching device in the switching circuit 
portion of this known power supply circuit. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a power 
supply circuit for supplying positive and negative power source voltages 
of substantially equal level that avoids the above-mentioned problems and 
disadvantages inherent in the prior art. 
Another object of this invention is to provide a power supply circuit that 
can supply stabilized positive and negative power source voltages having 
substantially the same absolute value, which has a relative simple 
configuration, and which uses a unitary DC voltage source. 
A further object of this invention is to provide a power supply circuit 
that can supply stabilized positive and negative power source voltages 
having a relative simple configuration and requiring only a single DC 
voltage source, and which can prevent the stabilized positive and negative 
power source voltages from being adversely affected by sudden variations 
occurring in the DC voltage from the single DC voltage source. 
A still further object of the present invention is to provide a power 
supply circuit that can supply stabilized positive and negative power 
source voltages using only a single DC voltage source, whereby a power 
source unit can be miniaturized in both size and weight. 
In accordance with an aspect of the present invention, there is provided a 
power supply circuit comprising a single voltage source providing a DC 
voltage and a first stabilizer circuit connected between the voltage 
source and a power-source, voltage-supply output terminal, a polarity 
converting circuit connected to the single voltage source for producing at 
another output terminal a second DC voltage having substantially the same 
absolute value as that of the first DC voltage but with opposite polarity, 
and a second stabilizing circuit connected between the output of the 
polarity converting circuit and the other output terminal, so that 
positive and negative power source voltages of the same absolute level are 
provided using only a single voltage source. 
One embodiment of the present invention further comprises a choke coil 
having one end connected between the single voltage source and the input 
of the first voltage stabilizer circuit and a capacitor connected across 
the series circuit of the choke and voltage source. 
By constructing the power supply circuit according to the present 
invention, and in the case where the voltage source comprises a unitary 
battery, the number of batteries used can be reduced so that the 
power-source unit can be miniaturized and, further, the use of a DC-DC 
convertor is not necessary. 
Because the positive and negative power-source voltages having 
substantially the same absolute value are obtained using only a single 
voltage source, and because these power-source voltages are stabilized 
against variations in the DC voltage supplied from the single voltage 
source by operation of the first and second stabilizer circuits, the power 
supply circuit according to the present invention is particularly suitable 
for employment in audio apparatus utilized in automobiles, in which the 
automobile battery is the single voltage source. The power supply circuit 
according to the present invention is also suitable for use in forming a 
power source of an operational amplifier that needs positive and negative 
power source voltages which are highly symmetrical. 
The above and other objects, features, and advantages of the present 
invention will become apparent from the detailed description of an 
illustrative embodiment taken in conjunction with the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the power supply circuit according to the present 
invention having a pair of loads connected thereto is shown in FIG. 1, in 
which a single battery, such as the conventional twelve-volt, lead-acid 
cell typically provided in an automobile, is used as the DC voltage 
source. Battery 1 has its negative terminal connected to an earth or 
common line 2 at relative ground potential, that is, battery 1 is 
connected in a negative-ground circuit. Choke coil 3 and capacitor 4 are 
connected in series between the positive and negative terminals of battery 
1 to aid in absorbing or suppressing level variations occurring suddenly 
in the positive voltage output of battery 1. Between the connecting point 
P of choke coil 3 and capacitor 4 and power-source voltage supply output 
terminal 7 is connected the collector-emitter path of NPN transistor 6, 
which forms part of voltage stabilizer circuit 5. In addition to NPN 
transistor 6 having its collector lead connected to connection point P and 
its emitter lead connected to power-source, voltage-supply, output 
terminal 7, voltage stabilizer circuit 5 includes Zener diode 8 connected 
between the base lead of NPN transistor 6 and the earth or ground line 2, 
and a diode biasing circuit consisting of resistors 9 and 10 and capacitor 
11 for supplying predetermined current to Zener diode 8 to cause it to 
produce a constant voltage. The constant voltage thus produced by Zener 
diode 8 is applied to the base circuit of NPN transistor 6, so that the 
voltage at the emitter lead of NPN transistor 6 is thereby stabilized. 
Also connected to connecting point P between choke coil 3 and capacitor 4 
is an input terminal 12a of polarity converting circuit 12. This circuit 
12 produces a negative voltage relative to ground potential that has 
substantially the same absolute value as that of a positive voltage 
present at connection point P. A common input terminal 12b of the polarity 
converting circuit 12 is connected to earth or ground line 2, and the 
output terminal 12c is connected through the collector-emitter path of PNP 
transistor 14 to power-source, voltage-supply, output terminal 15. Thus, 
polarity converting circuit 12 acts to convert the positive voltage 
relative to ground fed in at input 12a to a negative voltage relative to 
ground of the same magnitude fed out at output terminal 12c. A second 
voltage stabilizer circuit 13 is formed by PNP transistor 14 having its 
collector lead connected to output terminal 12c of polarity converting 
circuit 12 and its emitter lead connected to power source voltage-supply, 
output terminal 15 and includes the same components as stabilizer 5. More 
specifically, Zener diode 16 is connected between the base lead of PNP 
transistor 14 and the earth or ground line 2, note that the polarity of 
the diode is reversed from that of stabilizer 5, and a diode biasing 
circuit including resistors 17 and 18 and capacitor 19 supplies a 
predetermined current to Zener diode 16 to cause the diode to produce a 
constant voltage at its anode. The constant voltage provided at the anode 
of Zener diode 16 is applied to the base circuit of PNP transistor 14, 
whereby the voltage obtained at the emitter lead thereof is stabilized. 
Further, capacitor 20 having a large capacitance value, relative to 
capacitors 9 and 19, is connected between the collector lead of PNP 
transistor 14 and the common or earth line 2 to absorb sudden variations 
arising in the negative voltage supplied from output terminal 12c of 
polarity converting circuit 12. 
The two output voltages from the power supply circuit of FIG. 1 are 
available across three output terminals, specifically, common terminal 21 
is connected in the earth or ground line 2, and the positive voltage from 
the positive terminal of battery 1 is available through the voltage 
stabilizer circuit 5 across power-source, voltage-supply, output terminal 
7 and common terminal 21. Similarly, the negative voltage obtained from 
the output terminal 12c of polarity converting circuit 12, having the same 
absolute value as the positive voltage from battery 1, is obtained through 
the voltage stabilizer circuit 13 across common terminal 21 and 
power-source, voltage-supply output terminal 15. 
In the circuit of FIG. 1, when the positive voltage supplied from battery 1 
suddenly varies in level, or when there is an apparent voltage level 
variation caused by external noise pulses that combine with the positive 
voltage from the battery, such level variations are absorbed by a 
combination of the choke coil 3 and capacitor 4, so that the level 
variations are effectively eliminated from the positive voltage present at 
connecting point P. Even if the positive voltage at connecting point P 
does vary in level, this voltage is subjected to further stabilization by 
voltage stabilizer circuit 5, so as to produce the stabilized power source 
voltage between positive output terminal 7 and common terminal 21. 
Similarly, if the negative voltage produced at the output terminal 12c of 
polarity converting circuit 12 varies in level, voltage sources are 
stabilized by capacitor 20 and this negative voltage is further stabilized 
by voltage stabilizing circuit 13, so that the required stabilized 
negative power source voltage is present across common terminal 21 and 
negative power-source, voltage-supply, output terminal 15. 
Polarity converting circuit 12 of FIG. 1 is shown in more detail in FIG. 2, 
in which input terminal 12a is connected to the emitter lead of switching 
transistor 22 and capacitor 23 aids in absorbing level variations in the 
positive voltage supplied to the emitter lead and is connected between the 
emitter lead of switching transistor 22 and the common or center lead 
terminal 12b. Coil 24 operates as an energy charging device and is 
connected between the collector lead of switching transistor 22 and common 
input terminal 12b, which is connected to the ground or common line 2. A 
detector circuit is formed of diode 25 and capacitor 26 and is connected 
between the collector lead of switching transistor 22 and the common line 
at terminal 12b, with the connection point between the anode of diode 25 
and one end of capacitor 26, that is, the output of the detecting circuit 
portion, being connected to output terminal 12c of the polarity converting 
circuit 12. The output of a pulse-width modulation signal generator 27 is 
connected to drive the base circuit of switching transistor 22, and the 
control terminal (input) of pulse-width modulation signal generator 27 is 
connected to the output terminal of level comparison circuit 28. A 
reference input terminal of level comparison circuit 28 is connected to 
the common line at terminal 12b and the comparison input terminal of level 
comparison circuit 28 is connected through resistor 29 to input terminal 
12a and through resistor 30 to output terminal 12c. Resistors 29 and 30 
form a voltage divider and are selected to have the same resistance 
values. 
Polarity converting circuit 12 of FIG. 2 is utilized in a situation where 
the positive voltage provided at connection point P is supplied to input 
terminal 12a and common terminal 12b is connected to the earth or common 
line 2, which is to be connected to ground. In such condition a pulse 
width modulated output signal from pulse-width modulation signal generator 
27 is fed to drive the base circuit of switching transistor 22 to cause it 
to switch in response thereto. When switching transistor 22 is conductive, 
a current flows from the connecting point P through the emitter-collector 
path of switching transistor 22 and through coil 24 to common terminal 
12b, and when switching transistor 22 is non-conductive such current is 
prevented from flowing. As a result, positive and negative pulses arise 
across coil 24 and are detected by diode 25 to produce a negative voltage 
at the anode thereof. This negative voltage at the anode of diode 25 is 
smoothed by capacitor 26 and, thus, a smooth negative voltage is obtained 
at output terminal 12c. This negative voltage at output terminal 12c and 
the positive voltage supplied at input terminal 12a from connecting point 
P are supplied to the comparing input terminal of level comparison circuit 
29 through resistors 30 and 29, respectively. That is, the negative 
voltage obtained at output terminal 12c and the positive voltage at 
connecting point P are combined at the comparing input terminal of level 
comparison circuit 28, in which the combined voltage supplied thereto is 
compared with the voltage at the reference input terminal 12b, that is, a 
zero voltage level, to produce a resultant output voltage corresponding to 
the difference therebetween. This resultant output is supplied to the 
control terminal of pulse-width modulation signal generator 27, and the 
duty cycle of the pulse width modulated signal fed to the base lead of 
switching transistor 22 is controlled by the output of level comparison 
circuit 28, so that the ratio of the time period in which switching 
transistor 22 is conductive to the time period in which switching 
transistor 22 is nonconductive is controlled. The result of such control 
is that the level of the negative voltage obtained at output terminal 12c 
is correspondingly controlled. In such control, the normal point for 
control is based upon the condition in which the combined voltaged fed to 
the comparing input terminal of level comparison circuit 28 coincides with 
the voltage at the reference input terminal of level comparison circuit 28 
and, therefore, control is obtained in such a manner that the combined 
voltage fed to the comparing terminal of level comparison circuit 28 is 
zero. That is, the absolute value of the negative voltage obtained at 
output terminal 12c coincides exactly with the absolute value of the 
positive voltage supplied at input terminal 12a from connecting point P 
and, thus, a negative voltage of the same absolute value as the positive 
voltage is accurately produced at output terminal 12c. 
In the manner described above, the power supply circuit according to the 
present invention accurately supplies positive and negative power-source 
voltages at power-source, voltage-supply, output terminals 7 and 21 and 15 
and 21 having the same absolute value from a single power source. 
Accordingly, in the case where a pair of equal loads 31 and 31' are 
connected between positive, power-source, voltage-supply, output terminal 
7 and common terminal 21 and between negative, power-source 
voltage-supply, output terminal 15 and common terminal 21, respectively, 
as shown in FIG. 1, which would be the case where the inventive power 
supply circuit is utilized to form a power source unit in a high-fidelity 
stereo audio system, a pair of currents of the same value will flow in 
opposite directions through the earth or common line 2. This means that 
substantially no total current will flow through the earth or common line 
2. Accordingly, although the earth line 2 has a certain inherent 
resistance no voltage drop is caused by such resistance and, consequently, 
no noise arises from earth line 2. 
Although the embodiment shown in FIG. 1 is provided with a single battery 1 
supplying the positive voltage as the DC voltage source, it is understood 
that a battery supplying a negative voltage could be employed in its 
place. In such negative battery embodiment, the voltage stabilizer 
circuits 5 and 13 would be interchanged, and polarity converting circuit 
12 would be arranged to produce a positive voltage having the same 
absolute value as that of the negative voltage that would be obtained at 
connecting point P. The modified power supply circuit would then be 
operative to supply negative and positive power source voltages from the 
power-source, voltage-supply, output terminals 7 and 15, respectively. 
Additionally, it should be understood that choke coil 3 can be omitted from 
the embodiment of FIG. 1. Also, in the case where such voltage stabilizing 
circuits as shown with reference numeral 5 and 13 in FIG. 1 are used, 
there is the further advantage that the output impedance at the 
power-source, voltage-supply output terminal is reduced. 
Although illustrative embodiments of the present invention have been 
described in detail above with reference to the accompanying drawings, it 
is to be understood that the invention is not limited to those precise 
embodiments, and that various changes and modifications can be effected 
therein by one skilled in the art without departing from the scope or 
spirit of the invention, as defined by the appended claims.