Ulllted States Patent
`[19]
`[11] Patent Number:
`5,834,977
`
`Maehara et a1.
`[45 J Date of Patent:
`Nov. 10, 1998
`
`U8005834977A
`
`[54] AMPLIFYING CIRCUIT WITH POWER
`SUPPLY SWITCHING CIRCUIT
`I
`t
`E" M h
`Ki
`S t h'
`ryu;
`nven ors:
`lJll
`ae ara,
`a os 1
`Sugimoto, Gunma, both of Japan
`
`[75]
`
`[73] Assignee: Sanyo Electric C0., Ltd., Osaka, Japan
`
`[21] Appl No . 740 759
`'
`"
`’
`Filed:
`Oct. 30, 1996
`
`[22]
`
`[30]
`
`Foreign Application Priority Data
`
`Japan ------------------------------------ 7984055
`[JP]
`00t- 31: 1995
`[51]
`Int. C1.6 ........................................................ H03F 3/20
`
`[52] U'S' CL --------------------------------------------- 330/297; 330/273
`[58] Field of Search ..................................... 330/202, 267,
`330/273, 297
`
`[56]
`
`.
`References CIIEd
`
`us. PATENT DOCUMENTS
`
`4,433,307
`4,598,255
`
`330/297
`2/1984 Yokoyama
`/1986 Hong ....................................... 330/297
`
`~
`9/1995 Kanaya 6‘ ‘11-
`574504337
`Primary ExamineriJames B. Mullins
`Attorney, Agent, or Firm—Loeb & Loeb LLP
`[57]
`ABSTRACT
`An amplifying circuit according to the present invention has
`an amplifying unit for amplifying an input signal to produce
`an amplified signal, a battery for generating a constant
`voltage (a first voltage), a step-up converter for always
`generating an increased voltage (or a second voltage) by
`increasing the constant voltage, and a selection changing
`circuit for supplying the increased voltage to the amplifying
`unit as an electric source voltage when a level of the
`amplified signal
`is higher than the constant voltage and
`supplying the constant voltage to the amplifying unit as the
`Gleam? source "Oltage When a 16V“ 0f the amplified Signal
`is lower than the constant voltage. Therefore, because the
`electric source voltage supplied to the amplifying unit is
`changed according to the level of the amplified signal, a loss
`occurring in an electric power consumed in the amplifying
`unit can be reduced. Also, because the increased voltage is
`always generated by the step-up converter as compared with
`a
`rior art in which an
`increased volta e is not alwa s
`geIfierated, even though It/he increased sigfal is a high fri—
`quency signal,
`the electric source voltage can reliably
`change with the level of the amplified signal, so that any
`distortion of the amplified signal can be prevented.
`
`9 Claims, 6 Drawing Sheets
`
`
`
`INTEL 1318
`
`INTEL 1318
`
`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 1 0f 6
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`5,834,977
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`US. Patent
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`5,834,977
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`

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`US. Patent
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`Nov. 10, 1998
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`Sheet 3 0f 6
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`5,834,977
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`US. Patent
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`Nov. 10, 1998
`
`Sheet 4 0f 6
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`5,834,977
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`FIG.4
`
`INCREASED
`
`VOLTAGE
`
`Vref
`
`Vb
`
`FIG.5
`
`
`
`N 0
`
`IO
`
`
`
`FIG. 3 (THIS EMBODIMENT)
`
`I
`
`. 0
`
`IO
`
`OUTPUT ELECTRIC POWER (W)
`
`
`
`CONSUMEDELECTRICPOWER(W)
`
`
`
`
`
`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 5 of6
`
`5,834,977
`
`FIG.6
`
`
`
`

`

`US. Patent
`
`Nov. 10, 1998
`
`Sheet 6 0f 6
`
`5,834,977
`
`A.._A
`
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`_.0mohfieomo
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`

`5,834,977
`
`1
`AMPLIFYING CIRCUIT WITH POWER
`SUPPLY SWITCHING CIRCUIT
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates generally to an amplifying
`circuit, and more particularly to an amplifying circuit used
`for an on-vehicle type audio amplifier in which the ampli-
`fying efficiency is improved.
`2. Description of the Prior Art
`An amplifying circuit having a large output power has
`been recently required to use the amplifying circuit for an
`on-vehicIe type audio amplifier, and an amplifying circuit
`having an output power as large as possible has been
`required on condition that the amplifying circuit be operated
`by using a battery as a sole electric source.
`A first conventional amplifying circuit applied for an
`on-vehicle type audio amplifier system is described. As
`shown in FIG. 1, the first conventional amplifying circuit is
`composed of an on-vehicle type battery 31, a DC/DC
`converter 32, an amplifier 33 and a speaker SP.
`In a simple on-vehicle type audio amplifier system, the
`battery 31 functions as an electric source. Avoltage of the
`battery 31 normally ranges from 9 to 16 V, and a resistance
`of the speaker SI’ functioning as a load is normally 4 £2.
`Therefore, assuming that the voltage generated by the bat-
`tery 31 is directly used as an electric source of the amplifier
`33 without using the DC/DC converter 32, an output power
`obtained in the amplifier 33 is almost 20 W at the most.
`Therefore,
`in the first conventional amplifying circuit
`shown in FIG. 1, a constant voltage supplied from the
`battery 31 is increased by using the DC/DC converter 32,
`and the increased voltage is supplied to the amplifier 33 as
`a voltage of an electric source. In this case, because the
`electric source voltage supplied to the amplifier 33 becomes
`higher than the constant voltage generated by the battery 31,
`an output power of the amplifier 33 is larger than that in a
`case that the constant voltage of the battery 31 is directly
`supplied to the amplifier 33.
`However, though the constant voltage of the battery 31 is
`always increased by the DC/DC converter 32 and the
`increased voltage is used as a voltage of an electric source,
`a converting efficiency in the DC/DC converter 32 is low
`and no more than almost 80%.
`
`Also, because a hiin voltage is always supplied to the
`amplifier 33 even though no signal or a signal having a very
`low level
`is output from the amplifier 33, a loss of a
`consumed electric power with respect to an output power is
`enlarged, and there is a drawback that a total coefficient in
`the amplifier 33 is undesirably lowered.
`To avoid this drawback, a second conventional amplify-
`ing circuit shown in FIG. 2 is proposed by inventors of the
`present invention. That is, the second conventional ampli-
`fying circuit is composed of a battery 41, a voltage increas-
`ing chopper 42, an amplifier 43, a bypass circuit 44, a
`comparator 45 and the speaker SP.
`When the battery 41 is connected with the amplifier 43, a
`constant voltage of 12 V is supplied to the amplifier 43. In
`detail, because a switching circuit SW41 is set to an “off”
`condition, the voltage increasing chopper 42 is not operated,
`and the constant voltage +Vb of the battery 41 is supplied to
`the amplifier 43 through the bypass circuit 44. Therefore, an
`input signal AS input to the amplifier 43 is amplified by the
`amplifier 43, and an amplified signal ZS generated in the
`amplifier 43 is output from the amplifier 43 to the speaker
`SP.
`
`10
`
`15
`
`40
`
`45
`
`60
`
`65
`
`2
`In this case, a difference Va-ZS between an electric
`potential Va at a point A and an electric potential ZS of the
`amplified signal ZS is always detected by the comparator 45.
`In cases where a difference Vcc-ZS between a supplied
`voltage Vcc equal to the electric potential Va at the point A
`and the electric potential ZS of the amplified signal ZS is
`equal to or larger than a prescribed voltage of 5 V because
`a level of the amplified signal ZS is comparatively low, the
`operation of the switching circuit SW41 is stopped by the
`comparator 45 to set the switching circuit SW41 to the “011”
`condition, so that the constant voltage Vb of the battery 41
`is supplied to the amplifier 43 through the bypass circuit 44,
`and the amplifier 43 is operated by using the constant
`voltage Vb of the battery 41 as an electric source voltage.
`In contrast, in cases where a level of the amplified signal
`ZS is heightened the level of the amplified signal ZS
`approaches the electric potential Va at the point A. When the
`difference Vcc-ZS between the supplied voltage Vcc and the
`electric potential ZS becomes smaller than the prescribed
`voltage of 5 V, the operation of the switching circuit SW41
`is started by the comparator 45 to set the switching circuit
`SW41 to an “on” condition. Therefore, the operation of the
`voltage increasing chopper 42 is started, the constant voltage
`Vb of the battery 41 is increased by the voltage increasing
`chopper 42, the supplied voltage Vcc equal to the electric
`potential Va at the point A is increased, and a high voltage
`generated by the voltage increasing chopper 42 is supplied
`to the amplifier 43.
`Thereafter, when the diiference Vcc-ZS becomes equal to
`or larger than the prescribed voltage of 5 V because the level
`of the amplified signal ZS is lowered, the operation of the
`switching circuit SW41 is stopped by the comparator 45, so
`that the constant voltage Vb of the battery 41 is supplied to
`the amplifier 43 through the bypass circuit 44, and the
`amplifier 43 is operated by using the constant voltage Vb of
`the battery 41.
`Accordingly, assuming that a set of the comparator 45 and
`the voltage increasing chopper 42 is not arranged, when the
`level of the amplified signal ZS is heightened to make the
`difference Va-ZS between the electric potential Va at the
`point A and the electric potential ZS small, the amplified
`signal ZS is clipped and distorted to make the level of the
`amplified signal ZS higher than the constant voltage Vb.
`However, in the conventional amplifying circuit shown in
`FIG. 2, when the difference Va-ZS is lowered to a value
`lower than the prescribed voltage of 5 V, because the
`operation of the voltage increasing chopper 42 is started
`under the control of the comparator 45 to increase the
`electric potential Va at the point A and an electric source
`voltage for the amplifier 43 is heightened, the distortion of
`the output of the amplifier 43 can be prevented.
`Also, in the first conventional amplifying circuit shown in
`FIG. 1, the DC/DC converter 32 functioning as a voltage
`increasing circuit at a low efliciency is always operated to
`supply the increased voltage to the amplifier 33. Therefore,
`an efficiency (or a ratio of an output electric power to a
`consumed electric power) in the first conventional amplify-
`ing circuit shown in FIG. 1 is lowered. However,
`in the
`second conventional amplifying circuit shown in FIG. 2,
`though the voltage increasing chopper 42 is operated at a
`low efficiency in the same manner, because the voltage
`increasing chopper 42 is not operated when the level of the
`amplified signal ZS is comparatively low but is operated
`when the amplified signal ZS having a high level is output
`from the amplifier 43, the efficiency in the second conven-
`tional amplifying circuit shown in FIG. 2 can be improved
`as compared with that in the first conventional amplifying
`circuit shown in FIG. 1.
`
`

`

`5,834,977
`
`3
`the second conventional amplifying circuit
`Therefore,
`shown in FIG. 2 is tried to be used as parts of an on-vehicle
`hybrid IC. However,
`the increase of the electric source
`voltage supplied to the amplifier 43 is not always performed
`by the voltage increasing chopper 42. That is, the voltage
`increasing chopper 42 is not operated until the difference
`Va-ZS becomes lower than the prescribed voltage of 5 V but
`the operation of the voltage increasing chopper 42 is started
`to increase the electric source voltage supplied to the ampli-
`fier 43 when the difference Va-ZS becomes lower than the
`prescribed voltage of 5 V. Therefore,
`in cases where the
`amplified signal ZS is a high frequency signal having a
`frequency close to 20 kHz, the level of the amplified signal
`ZS is steeply changed, so that the operation of the voltage
`increasing chopper 42 cannot follow the steep change of the
`level of the amplified signal ZS. As a result, the level of the
`amplified signal ZS reaches the electric source voltage Vcc,
`so that there is a drawback that the output of the amplifier 43
`is undesirably distorted.
`Also, because a switching operation of the switching '
`circuit SW41 is repeated at a high speed, in cases where an
`AM radio is, for example, operated in the neighborhood of
`the amplifying circuit, there is another drawback that an
`adverse influence of a switching noise is exerted on the AM
`radio.
`
`10
`
`15
`
`SUMMARY OF THE INVENTION
`
`A first object of the present invention is to provide, with
`due consideration to the drawbacks of such a conventional
`
`amplifying circuit, an amplifying circuit in which an electric
`source voltage following the change of a level of an ampli-
`fied signal output from an amplifier is reliably supplied to
`the amplifier to prevent the distortion of an output of the
`amplifier while improving an efficiency of an electric power
`in the amplifier even though the level of the amplified signal
`is steeply changed.
`A second object of the present invention is to provide an
`amplifying circuit
`in which any adverse influence of a
`switching noise is not exerted on an AM radio even though
`the AM radio is placed in the neighborhood of the ampli-
`fying circuit.
`in an amplifying circuit
`To achieve the first object,
`according to the present invention, an increased voltage is
`generated by a voltage increasing circuit by always increas-
`ing a constant voltage. In cases where a level of an amplified
`signal output from an amplifying unit is equal to or higher
`than the constant voltage, the increased voltage is selected
`by a selection changing circuit and is supplied to the
`amplifying unit as an electric source voltage. Also, in cases
`where the level of the amplified signal is lower than the
`constant voltage,
`the constant voltage is selected by the
`selection changing circuit and is supplied to the amplifying
`unit as an electric source voltage. Therefore, assuming that
`the selection changing circuit is not operated even though
`the level of the amplified signal becomes equal to or higher
`than the constant voltage, an output of the amplifying unit is
`distorted because the level of the amplifying signal exceeds
`the constant voltage. However, because the increased volt-
`age is always generated by the voltage increasing circuit, the
`increased voltage can be supplied to the amplifying unit as
`an electric source voltage by the selection changing circuit,
`so that the distortion of the output of the amplifying unit can
`be suppressed.
`is always
`Also,
`though the voltage increasing circuit
`operated, any load is not connected to the voltage increasing
`circuit when the increased voltage generated in the voltage
`
`40
`
`45
`
`60
`
`65
`
`4
`increasing circuit is not selected by the selection changing
`circuit, a consumed electric power in the voltage increasing
`circuit is almost zero, an efficient in the amplifying circuit is
`not
`lowered by the operation of the voltage increasing
`circuit, so that the efficiency (or a ratio of an output electric
`power to a consumed electric power) can be improved as
`compared with that in a conventional AB-elass amplifying
`circuit.
`
`Also, the increased voltage is generated by always oper-
`ating the voltage increasing circuit, the increased voltage
`generated in the voltage increasing circuit is quickly selected
`by the selection changing circuit when the increased voltage
`is required, and the increased voltage is supplied to the
`amplifying unit as an electric source voltage. Therefore,
`even though the level of the amplifying signal is changed at
`a high frequency, the supply of the increased voltage to the
`amplifying unit can sufficiently follow the change of the
`level of the amplified signal. Accordingly, the distortion of
`the output of the amplified signal which occurs in the
`conventional amplifying circuit in cases where the level of
`the amplifying signal is changed at a high frequency can be
`suppressed.
`Here, the amplifying unit in the amplifying circuit accord-
`ing to the present invention is formed of a high efficient type
`amplifier comprising a signal amplifying unit for generating
`the amplified signal by amplifying an input signal, and a
`supplementary electric source unit for generating a change—
`able voltage following the change of the level of the ampli-
`fied signal by using an electric source voltage and setting the
`changeable voltage to an electric source voltage of the signal
`amplifying unit.
`Because the changeable voltage is supplied to the signal
`amplifying unit as the electric source voltage according to
`the level of the amplified signal in the high efficient type
`amplifier, the electric source voltage having a high level is
`supplied to the signal amplifying unit when the amplified
`signal is a high level, and the electric source voltage having
`a low level is supplied to the signal amplifying unit when the
`amplified signal is a low level. Therefore, though a loss of
`the consumed electric power in a general amplifier occurs in
`cases where a voltage of a high level is supplied to the
`general amplifier as an electric source voltage when a signal
`of a low level is input to the general amplifier, the loss of the
`consumed electric power can be suppressed in the high
`efficient type amplifier. Accordingly,
`the efficiency in the
`high efficient type amplifier can be heightened as compared
`with that in the normal amplifier in which a constant voltage
`is supplied as an electric source voltage.
`Also,
`the selection changing circuit of the amplifying
`circuit according to the present invention further comprises
`a switching device arranged between the voltage increasing
`circuit and the amplifying circuit. A reference voltage lower
`than the constant voltage is compared with the amplified
`signal in the selection changing circuit. Thereafter, in cases
`where the level of the amplified signal is equal to or higher
`than the reference voltage, the switching device is set to an
`“on” condition, and the increased voltage generated in the
`voltage increasing circuit is supplied to the amplifying unit
`as an electric source voltage. In contrast, in cases where the
`level of the amplified signal is lower than the reference
`voltage, the switching device is set to an “off” condition, and
`the constant voltage is supplied to the amplifying unit
`through a bypass circuit as an electric source voltage.
`Assuming that the constant voltage is merely compared
`with the level of the amplified signal
`in the selection
`changing circuit, in cases where the level of the amplified
`
`

`

`5,834,977
`
`10
`
`15
`
`5
`signal is steeply heightened, the amplifying circuit cannot
`follow the steep change of the level of the amplified signal,
`the level of the amplified signal reaches the constant voltage
`when the level of the amplified signal is about to exceed the
`constant voltage, and the amplified signals is distorted.
`However, in the present invention, the reference voltage
`lower than the constant voltage is compared with the level
`of the amplified signal in the selection changing circuit, and
`the selection of the constant voltage of the bypass circuit or
`the increased voltage generated in the voltage increasing
`circuit is performed in the selection changing circuit accord-
`ing to a high or low relationship between the reference
`voltage and the level of the amplified signal. Therefore, even
`though the level of the amplified signal is steeply changed,
`the increased voltage generated in the voltage increasing
`circuit is selected when the level of the amplified signal
`exceeds the reference voltage before the level of the ampli-
`fied signal reaches the constant voltage, and the increased
`voltage is supplied to the amplifying unit as an electric
`source voltage. Accordingly, the distortion of the amplified '
`signal can be prevented.
`the amplifying circuit
`To achieve the second object,
`according to the present
`invention further comprises a
`microcomputer for stopping the operation of the voltage
`increasing circuit and controlling signals of the amplifying ’
`circuit in a lump in cases where an AM radio is operated in
`the neighborhood of the voltage increasing circuit having an
`amplifier or the switching device. In cases where the voltage
`increasing circuit has the switching device, a problem that a
`switching noise is mixed with a radio signal received by the
`AM radio as a noise can be suppressed.
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a circuit view of a first conventional amplifying
`circuit applied for an on-vehicle type audio amplifier sys-
`tem.
`
`FIG. 2 is a circuit View of a second conventional ampli-
`fying circuit applied for the on-vehicle type audio amplifier
`system.
`FIG. 3 is a circuit view of an amplifying circuit according
`to a first embodiment of the present invention.
`FIG. 4 is a view showing an operation of the amplifying
`circuit according to the first embodiment of the present
`invention.
`
`40
`
`45
`
`FIG. 5 is a graphic view showing an effect of the ampli-
`fying circuit according to the first embodiment of the present
`invention.
`
`FIG. 6 is a circuit view of a high efficiency amplifier
`according to a second embodiment of the present invention.
`FIG. 7 is a view showing an operation of the high
`efficiency amplifier according to the second embodiment of
`the present invention.
`FIG. 8 is a circuit view of an amplifying circuit according
`to a third embodiment of the present invention.
`DESCRIPTION OF THE PREFERRED
`EMBODIMENTS
`
`Preferred embodiments of an amplifying circuit according
`to the present invention are described with reference to
`drawings.
`(First Embodiment)
`As shown in FIG. 3, an on-vehicle type audio amplifying
`circuit according to a first embodiment of the present
`invention comprises a battery 11, a step-up converter 12, a
`bypass circuit 13, a switching circuit 14, an amplifier unit 15
`and a speaker SP.
`
`60
`
`65
`
`6
`The battery 11 is an example of an electric source, and a
`constant voltage +Vb of 12 V is generated as an example of
`a first voltage.
`The step-up converter 12 is an example of the voltage
`increasing circuit, and the step-up converter 12 comprises a
`coil L11, a diode D11, a condenser C11, an oscillator 12A
`and a switching transistor SW11. An ON/OFF operation is
`repeatedly performed by the switching transistor SW11
`under the control of the oscillator 12A, the constant voltage
`+Vb generated in the battery 11 is increased according to a
`duty ratio of the ON/OFF operation, and an increased
`voltage of 18 V is always generated as an example of a
`second voltage.
`The bypass circuit 13 is formed of a diode D12 connected
`with the step-up converter 12 in parallel. The constant
`voltage Vb is supplied to the amplifying unit 15 through the
`bypass circuit 13.
`The switching circuit 14 is an example of the selection
`changing circuit. The switching circuit 14 comprises a
`comparator 14A and a switching transistor SW12. Alevel of
`an amplifying signal ZS and an output voltage of the
`amplifying unit 15 equal to an electric potential Va at a point
`A are always detected by the switching circuit 14. When the
`electric potential Va becomes lower than a reference voltage
`Vref lower than the constant voltage +Vb, the switching
`transistor SW12 is set to an “off” condition to select the
`constant voltage +Vb of the bypass circuit 13. In contrast,
`when the electric potential Va becomes equal to or higher
`than the reference voltage Vref,
`the switching transistor
`SW12 is set to an “on” condition to select the increased
`
`voltage of the step-up converter 12.
`The amplifying unit 15 is an example of the amplifying
`unit. An input signal AS input to the amplifying unit 15 is
`amplified to generate the amplified signal ZS, and the
`amplified signal ZS is output to the speaker SP having a load
`of 4 £2. The amplifying unit 15 comprises a pair of amplifiers
`15A and 15B connected with each other in a balanced
`
`transformerless (BTL).
`In the above configuration, an operation of the amplifying
`circuit is described.
`When an operation of the amplifying circuit is started, the
`constant voltage +Vb of 12 V is supplied from the battery 11
`to the amplifying unit 15. Thereafter, the step-up converter
`12 is operated by operating the oscillator 12A and perform-
`ing an on/oif operation in the switching transistor SW11, and
`the increased voltage of about 18 V is generated from the
`constant voltage +Vb.
`At this time, the switching transistor SW12 is set to the
`“off” condition, the constant voltage +Vb of 12 V is supplied
`from the battery 11 to the amplifying unit 15 through the
`bypass circuit 13, the input signal AS is amplified by using
`the constant voltage +Vb, and the amplified signal ZS is
`generated in the amplifying unit 15 and is transmitted to the
`speaker SP.
`During the operation of the amplifying circuit, the electric
`potential Va at the point A is always compared with the
`reference voltage Vref by the comparator 14A, and the
`switching transistor SW12 is set to the “on” or “off” con-
`dition according to a high or low relationship of the electric
`potential Va and the reference voltage Vref. In this case, the
`reference voltage Vref is set to 10 V.
`In detail, in cases where the level of the amplified signal
`ZS is comparatively low and the electric potential Va is
`lower than the reference voltage Vref of 10 V, an output of
`the comparator 14A is set to a low level “L”, the switching
`transistor SW12 is set to the “off” condition according to the
`output of the comparator 14A, and the constant voltage +Vb
`
`

`

`5,834,977
`
`7
`is supplied to the amplifying unit 15 through the bypass
`circuit 12. During the supply of the constant voltage +Vb to
`the amplifying unit 15 through the bypass circuit 12, the
`step-up converter 12 is operated, and the increased voltage
`of 18 V is generated. However, because the switching
`transistor SW12 is set to the “off” condition, any load is not
`connected with the step-up converter 12, so that an electric
`power consumed in the step-up converter 12 is almost zero.
`Therefore, even though the increased voltage is always
`generated in the step-up converter 12, there is no probability
`that an efficiency in the entire amplifying circuit is lowered.
`Here the efficiency in the entire amplifying circuit denotes a
`ratio of an electric power output from the speaker SP to an
`electric power consumed in the amplifying circuit.
`Thereafter, when the level of the amplified signal ZS
`becomes high and the electric potential Va becomes equal to
`or higher than the reference voltage Vref of 10 V, an output
`of the comparator 14A is set to a high level “H”,
`the
`switching transistor SW12 is set
`to the “on” condition
`according to the output of the comparator 14A, and the '
`increased voltage of about 18 V obtained in the step-up
`converter 12 is supplied to the amplifying unit 15.
`Thereafter, when the level of the amplified signal ZS
`becomes again low and the electric potential Va becomes
`lower than the reference voltage Vref,
`the output of the .
`comparator 14A is again set to “L”, the switching transistor
`SW12 is again set to the “off” condition according to the
`output of the comparator 14A, and the electric source
`voltage +Vcc supplied to the amplifying unit 15 is again set
`to the constant voltage +Vb supplied through the bypass
`circuit 12.
`
`10
`
`15
`
`Therefore, even though the level of the input signal AS is
`changed at a high frequency, as shown in FIG. 4, because the
`electric source voltage +Vcc changing with the level of the
`amplified signal ZS is supplied to the amplifying unit 15, the
`amplified signal ZS of which the level is changed at the high
`frequency can be stably obtained.
`Accordingly, assuming that the constant voltage +Vb of
`about 12 V is always supplied to the amplifying unit 15
`regardless of the change of the level of the input signal AS
`(or the amplified signal ZS),
`the amplified signal ZS is
`undesirably clipped when the level of the amplified signal
`ZS reaches or exceeds the constant voltage +Vb. However,
`in this embodiment, when the electric potential Va exceeds
`the reference voltage Vref of 10 V, because the switching
`transistor SW12 is set to the “on” condition by the com-
`parator 14A to select the increased voltage always generated
`by the stcp-up converter 12, the increased voltage is supplied
`to the amplifying unit 15, so that
`the distortion of the
`amplified signal ZS can be prevented.
`Also, even though the level of the input signal AS (or the
`amplified signal)
`is,
`for example, steeply heightened,
`because the increased voltage is rapidly selected by the
`comparator 14A when the electric potential Va exceeds the
`reference voltage Vref lower than the constant voltage +Vb,
`the increased voltage can be reliably supplied to the ampli-
`fying unit 15 before the level of the amplified signal ZS
`reaches the constant voltage +Vb. Accordingly, even though
`the level of the input signal AS (or the amplified signal) is
`steeply changed, the distortion of the amplified signal ZS
`can be reliably prevented.
`Also, though the step-up converter 12 is always operated,
`because the switching transistor SW12 is set to the “off”
`condition by the comparator 14A to supply the constant
`voltage +Vb to the amplifying unit 15 when the level of the
`amplified signal ZS is low, any load is not connected with
`the step-up converter 12. Accordingly, an electric power
`
`40
`
`45
`
`60
`
`65
`
`8
`consumed in the step-up converter 12 becomes almost zero,
`an efficiency (a ratio of an electric power output from the
`speaker SP to an electric power consumed in the amplifying
`circuit) in the entire amplifying circuit is not lowered even
`though the increased voltage is always generated in the
`step-up converter 12. Therefore, as shown in FIG. 5, the
`efficiency can be improved as compared with that in an
`amplifying circuit shown in FIG. 1. That is, a relationship
`between a consumed electric power and an output electric
`power for the amplifying circuit according to this embodi-
`ment and the amplifying circuit shown in FIG, 1 is shown in
`FIG. 5. As is realized in this relationship, a consumed
`electric power in the amplifying circuit according to this
`embodiment is lower than that in the amplifying circuit
`shown in FIG. 1 in an output electric power range from 1 to
`10 W usually used.
`In addition, because the increased voltage of about 18 V
`is generated by always operating the step-up converter 12
`and the increased voltage is supplied to the amplifying unit
`15 by selecting the increased voltage in the switching circuit
`14 when the increased voltage is required, the electric source
`voltage Vcc sufficiently following the change of the level of
`the amplified signal ZS can be always supplied to the
`amplifying unit 15. Therefore, though an output distortion
`occurs in the amplified signal ZS obtained in the conven-
`tional amplifying circuit shown in FIG. 2 because the
`operation of the step-up converter 44 cannot follow the
`change of the level of the amplifying signal ZS in cases
`where the amplifying signal ZS is a high frequency signal,
`any distortion of the amplified signal ZS can be prevented in
`this embodiment.
`
`In this embodiment, a case that the amplifying circuit is
`applied for an on-vehicle type audio amplifier system is
`described. The reason that the amplifying circuit is applied
`for the on-vehicle type audio amplifier system is as follows.
`In the on—vehicle type audio amplifier system, an electric
`source is limited to a battery, and a maximum output power
`of the battery is limited. In this case, because the amplifying
`circuit is necessary to obtain a large output power exceeding
`the maximum output power of the battery,
`the elIect
`obtained in this embodiment is great in cases where the
`amplifying circuit is applied for the on-vehicle type audio
`amplifier system. However, this embodiment is not limited
`to the on-vehicle type audio amplifier system. That is, it is
`applicable that the amplifying circuit be applied for a system
`in which a constant voltage generated in a single electric
`source is increased to obtain a large output power.
`Also, in this embodiment, the two amplifiers 15A and 15R
`are connected with each other in the balanced transformer-
`
`less (BTL). However, this embodiment is not limited to the
`balanced transformerless. For example, it is applicable that
`a single large output power type amplifier be used as the
`amplifying unit 15.
`(Second Embodiment)
`Hereinafter, an amplifying circuit according to a second
`embodiment of the present
`invention is described. The
`description which is the same as in the first embodiment is
`omitted in the second embodiment.
`The difference in the second embodiment from the first
`embodiment is that a high efliciency amplifier is used as the
`amplifying unit 15, and the other constitutional elements in
`the amplifying circuit in the second embodiment are the
`same as those in the first embodiment. Therefore, the con-
`figuration and operation of the high efficiency amplifier are
`described in detail.
`
`As shown in FIG. 6, the high efficiency amplifier com-
`prises an amplifier 1 which is an example of the signal
`amplifying unit, and a supplementary electric source unit 2.
`
`

`

`5,834,977
`
`9
`In the amplifier 1, an input signal AS is amplified by using
`an electric source voltage +Vc described later, and an
`amplified signal ZS is output. Therefore, the amplifier 1
`comprises a voltage amplifying unit (not shown) for ampli-
`fying a voltage of the input signal AS, and a current
`amplifying unit (not shown) for amplifying an output current
`of the voltage amplifying unit.
`The supplementary electric