Note: Descriptions are shown in the official language in which they were submitted.
11~647'~
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates generally to current
transfer amplifier, and is directed more particularly to a
current transfer amplifier which will remove the non-linear
distortion caused by a switch.
Description of the Prior Art
A prior art audio amplifier is of a so-called
constant voltage transfer type which uses various signal
voltage sources such as a phono-cartridge, tape recorder,
tuner or the like and drives the speaker thereof in the
form of a voltage, so that the switching and transfer of the
signal are all carried out in the voltage mode.
The audio amplifier of the constant voltage type,
in which the signal switching(processing) and signal transfer
are carried in the voltage mode, can not be free from the
following various problems. That is, before a signal is
transmitted or transferred to a power amplifier, the signal
passes through many non-linear elements such as a switch and
so on. In this case, however, since all the non-linear ele-
ments are deemed as a series impedance, the input signal to
the power amplifier is determined as the voltage dividing
ratio of the impedance. While, when viewed from the input
terminal of the power amplifier, a closed loop which termi-
nates at the output impedance is formed. Therefore, an
electromotive force is generated by the magnetic flux which
passes through the loop area and hence hum and flux at the
output stage of the speaker are easily picked up. Further,
since a closed loop is formed when viewed from a load
~146~74
terminal, a ground impedance is inserted as a series imped-
ance. Therefore, a return current must be returned to a
constant voltage source. It is rather difficult that, in
order to reduce the affect by the return current, the
ground potentials at a pre-amplifier stage and a power
amplifier stage are made coincident. For this reason,
there are necessary various counter measures, for example,
a bus bar for the ground is used to reduce the series
impedance or a shield wire is used for the same purpose.
Further, since in the constant voltage type a variable re-
sistor for volume adjustment is of the resistance value
dividing type, when the variable resistor for volume
adjustment is adjusted as, for example, about 1/2 in volume
i.e. volume is reduced by -6 dB, a noise becomes maximum,
namely the S/N ratio becomes worst. Also, even upon a
normal using state (the variable resistor for volume adjust-
ment is reduced further from 1/2), the S/N ratio is
deteriorated by about 10 to 20 dB. Thus, there appears such
a problem that very unnatural sense is caused in view of
hearing.
As described above, the audio amplifier of the
constant voltage type proposes various problems. In fact,
as the audio amplifier is improved in property, non-linear
distortions by the change-over switch, materials of wires
and so on become problems. At present, the property of high
accuracy or distortion ratio of, for example, about 0.003~
is required, so that the non-llnear distortion by the change-
over switch or material of wires (or relay and so on) can
not be neglected.
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OBJECTS AND SUMMARY OF THE INVENTION
Accordingly, an object of the present invention
is to provide a current transfer amplifier free from the
above-mentioned defects.
Another object of the invention is to provide a
current transfer amplifier in which a non-linear switching
element is current-driven.
In accordance with one example of the present
invention, a current transfer amplifier is provided, which
includes.
voltage signal source means for supplying a
voltage signal to be amplified;
voltage/current converting means supplied with
the voltage signal from said voltage source means for
converting the same into a current signal proportional
to the voltage slgnal;
a tone control circuit of a current amplification
type supplied with the current signal from said voltage/
current converting means;
a non-linear switching element interposed between
the output of said voltage/current converting means and
the input of said tone control circuit to transmit said
¦ current signal therethrough;
a variable resistor having a pair of terminals
¦ connected between the output of said tone control circuit
and a reference point to convert the output signal of
said tone control circuit into voltage signal.
The other objects, features and advantages of the
present invention will be apparent from the following
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~1 ~6~'7~
descriptions taken in conjunction with the attached drawings
through which the like referencesdesignate the same elements
and parts.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a block diagram showing an example of
the current transfer amplifier according to the present
invention;
Figs. 2 to 4 are respectively connection diagrams
each showing a voltage/current converter;
Figs. 5 and 6 are connection diagrams respectively
showing toné control circuits of a current amplification
type used in the invention; and
Fig. 7 is a block diagram showing another example
of the lnvention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be hereinafter described
with reference to the attached drawings.
Fig. 1 is a block diagram schematically showing an
example of the current transfer amplifier according to the
present invention. In Fig. 1, reference numerals 1 through
5 each designate input terminals which are respectively sup-
plied with output signals from signal sources though not
shown. In this case, by way of example, the input terminal
1 is supplied with the output signal from a phono-cartridge
(not shown), the input terminal 2 is supplied with the output
signal from a tuner circuit (not shown), the input terminal
` 11~6~
3 is supplied with an auxi.liary input signal, the input
terminal 4 is supplied with the output signal from a first
tape recorder (not shown), and the input terminal 5 is
supplied with the output signal from a second tape recorder
(not shown).
- The phono-output supplied to the input terminal 1
is applied to an equalizer amplifier 6 of the current ampli-
fication type, which will be described later, converted
thereby from the voltage signal to the corresponding current
signal, and then fed to a function switch 9 of a non-linear
switch means 8. The tuner output and the auxillary input
applied to the input terminals 2 and 3 are respectively
applied to flat amplifiers 10 and 11 of the current amplifi-
cation type, which will be described later, converted from
the voltage signals to the corresponding current signals,
and then fed to the function switch 9. The outputs of the
first and second tape recorders applied to the input termi-
nals 4 and 5 are respectively applied to flat amplifiers 14
and 15 of the current amplification type, converted from
the voltage signals to the corresponding current signals,
- and then fed to a tape monitor switch 18 provided in the
switch means 8 and supplied with the cutput of the function
switch 9.
The input signals fed to the input terminals 1
to 3 are selected by the ~unction switch 9, and a desired
one of -the outputs from the tape recorders is selected by
turning the tape monitor switch 18 ON, and they are then
processed. The processed signals through the switches 9
and 18 are desirably changed over by respectively making
ON a tape copy switch 19 and a mode switch 20 provided at
` 11~6~74~
the next stage of the switch 18. The signal passed through
the mode switch 20 is applied to a tone control circuit 21
which consists of tone control switches 22 and 23 provided
at the input and output sides thereof, a tone control ampli-
fier 21a of the current amplification type with a feedback
circuit 21b, which is provided between the switches 22 and
23 and will be described later, and a signal transmission
path 26 direc-tly connecting the switches 22 and 23.
When the tone control is carried out, the current
signal is applied to the amplifier 21a of the current ampli-
fication type to vary its gain before and after, for example,
1 KHz. While, no tone control is carried out, the switches
22 and 23 are so changed over that the tone control amplifier
21a is short-circuited (or tone-cancelled) by the signal
transmission path 26.
As set forth above, the respective signals, which
are converted in the amplifiers 6, 10, 11, 14 and 15 from the
voltage signals to the current signals, are supplied to the
switch means 8, signal-processed in the current mode and
then sequentially supplied to the tone control circuit 21.
The tone control clrcuit 21 is also a current
amplifier and -the output therefrom is applied to a variable
resistor 27 provided at the output side thereof. Although
this variable resistor 27 is similar to a variable resistor
of the normal resistance division type in view of construc-
tion, its operation is different from that of the normal
variable resistor since the output from the tone control
circuit 21 is a current signal. That is, the variable resis-
tor of the normal voltage division type delivers the
potential generated between its movable piece and the
~46~7~
ground as an input signal (voltage signal) for the next
stage. However, in the example of the invention shown in
Fig. 1, the output terminal of a movable piece 27b of the
variable resistor 27 is connected commonly to one end
(signal input terminal) of its resistor member 27a and the
movable piece 27b is slided on the resistor member 27a to
vary its resistance value to thereby convert the current
signal to a voltage signal which will be applied to an
ordinary power amplifier 28 with a feedback circuit 28a
provided at the next stage of the variable resistor 27.
In other words, the resistor 27 whose resistance value itself
is varied, is provided at the input side of the power ampli-
fier 28 to control its input level. The voltage signal
applied to the power amplifier 28 is amplified thereby as a
large voltage signal and then delivered to an output terminal
29. If a speaker (not shown), by way of example, is connect-
ed to the output terminal 29, the output from the power
amplifier 28 is sounded.
Now, examples of the voltage/current converter
useable in the invention will be described in detail with
respect to Figs. 2 to 4.
Fig. 2 shows an example of the voltage/current
converter which is used as the flat amplifier 10 of the cur-
rent amplification type shown in Fig. 1. In Fig. 2, the
elements and parts corresponding to those of Fig. 1 are
marked wi-th the same references.
In the example of Fig. 2, at the output side of
an operational amplifier lOa, provided is a bridge circuit
30 which consists of transistors 31 and 32 different in the
conductivi-ty type and resistors 33 and 34. The bases of
1146~
the transistors 31 and 32 are connected together to the
output terminal of the operational amplifier lOa, and the
transistors 31 and 32 are operated alternately in response
to the voltage signal applied to the input terminal 2.
The emitters of the transistors 31 and 32 are connected
commonly to provide a common connection point 35. This
common connection point 35 is connected through a resistor
lOb', which will form a part of a feedback circuit lOb shown
in Fig. 1, to the inverted input terminal of the operational
amplifier lOa and also through a feedback impedance lOb",
which will form another part of the feedback circuit lOb, to
the ground. This feedback impedance lOb" may be a mere re-
sistor or an impedance circuit made of other parts. That is,
these resistor lOb' and feedback impedance lOb" form the
feedback circuit lOb of a so-called feedback ratio ~. Now,
if the voltage produced between the common connection point
35 and the ground is taken as V' ! this voltage V' is divided
by the resistor lOb' and the feedback impedance lOb", and
the voltage across the feedback impedance lOb" is applied
to the inverted input terminal of the operational amplifier
lOa as the feedback signal.
The collector of the transistor 31 is connected
through a constant current source lOc' to a positive power
supply terminal +B and also to one end of a resistor 33
directly, while the collector of the transistor 32 is con-
nected through a constant current source lOc" to a negative
power supply terminal -B and also to one end of a resistor
34 directly which has the same resistance value as that of
the resistor 33. The other ends of the resistors 33 and 34
are connected together to the ground through a load circuit
36.
g
6~
Now, lf it is assumed that the voltage at the
input terminal 2 is taken as V, the resistance value of
the resistor lOb' as R and the impedance of the feedback
impedance lOb" as Z, the voltage V' appeared between the
common.connection point 35 and the ground becomes as ex-
pressed by the following formula (1).
V' = R z Z V -~--------- ~1)
.
Now, if the feedback current flowing through the
resistor lOb' and feedback impedance lOb" is taken as IN,
the voltage V' can be expressed as follows:
V~ = (R + Z? IN ............ (2)
From the formulae (1) and (2), the following
formula (3) is derived.
IN 1 ....................... (3)
V Z
While, if the current amplification factors (hfe)
of the transistors 31 and 32 are equal with each other, a
current I flowing through the constant currcnt sources lOc'
and lOc" is expressed as follows:
I = Il + Il = I2 + I2
where Il and I2 represent the currents respectively flowing
through the transistors 31 and 32, and Il' and I2' represent
the currents respectively flowing through the resistors 33
and 34.
Further, the feedback current IN and a load current
Io are respectively expressed as follows:
IN = Il - I2 ' (5)
Io = Il' - I2' ............. (6)
Since Il - I2 = I2' - Il' is established from the
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~146~
formula (4), the following formula (7) is obtained from
the above formulae(5) and (6).
IN = -Io .................. (7)
If this formula (7) is substituted into the
formula (3), the following formula (8) is obtained.
Z ' - (8~
The current signal expressed by the formula (8)
i.e. load current Io has no relation to the power supply
voltages +B and depends upon only the impedance value Z
of the feedback impedance lOb" and the input voltage V.
That is, the derived current signal is the complementary
- current to that flowing through the feedback circuit lOb
with the feedback ratio ~, so that the conversion accuracy
of the voltage to current becomes very good.
Also, since the output stage of the converter
shown in Fig. 2 is the bridge circuit which employs the
constant current sources, when viewed from the power supply
side, no current variation appears in the output stage.
Accordingly, the converter is not affected any by the fluc-
tuation of the power supply voltage and hence can be used
as the equalizer amplifier 6 of the current amplification
type and the flat amplifiers 10 to 15 shown in Fig. 1 with
good effects.
Fig. 3 shows another example of the voltage/
current converter which is also useable in this invention.
In Fig. 3, the elements and parts corresponding to those of
Fig. 2 are marked with the same references and their des-
cription will be omitted for the sake of brevity.
The difference point between the examples of
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~146~
Figs. 2 and 3 resides in that, in the example of Fig. 3,
ln place of the resistors 33 and 34 forming the parts of
the bridge circuit 30 of Fig. 2, transistors 33' and 34',
each of which is of the base grounded type, are used to
form a part of a bridge circuit 30'. In detail, the tran-
sistors 33' and 34' have the emitters respectively connected
to the collectors of the transistors 31 and 32, the collec-
tors connected together to the load circuit 36, and the
bases respectively connected through bias sources 37 and
38 to the power supply terminals +B and -B, and a constant
current source 39 is inserted between the bases of the
transistors 33' and 34'.
In the example shown in Fig. 3, since the tran-
sistors 33' and 34' are both of the base grounded configuration,
the current gains thereof are both 1 and hence the variations
of the currents flowing through, for example, the collectors
of the transistors 31 and 32 become substantially equal.
By constructing the converter as described above
in connection with Fig. 3, this converter performs the
effect substantially same as that of the first example and
also such an effect to deliver the output effectively.
Fig. 4 shows a further example of the voltage/
current converter which is also useable in the invention.
In Fig. 4, the elements and parts corresponding to those of
Fig. 2 are marked with the same references and their detailed
explanation will be omitted for the sake of simplicity.
The difference point between the examples of Figs.
2 and 4 resides in that, in this example, in place of the
resistors 33 and 34 forming the parts of the bridge circuit
30 shown in Fig. 2, constant voltage elements such as Zener
~146~
diodes 33" and 34" are resr,ectively used to form the cor-
responding parts of a bridge circuit 30". That is, in the
example of Fig. 4, the cathode of the Zener diode 33" and
the anode of the Zener diode 34" are respectively connected
to the collectors of the transistors 31 and 32, and the
anode of the Zener diode 33" and the cathode of the Zener
diode 34" are connected together to the load circuit 36.
By constructing the converter as shown in Fig. 4,
this converter can perform the effect substantially same as
that of Fig. 2 and further produce such an output which
depends on the withstanding voltages of the Zener diodes
33" and 34" and is larger than that of the example shown in
Fig. 2 by suitably selecting the withstanding voltages of
the Zener diodes 33" and 34".
The above examples of the voltage/current converter
can be all also used as the equalizer amplifier 6 shown in
Fig. 1.
Now, the fundamental construction of the current
amplification type tone control circuit, which can be used
in the invention will be described with reference to Fig. 5.
In Fig. 5, 41 designates an amplifier which com-
prises a first impedance circuit 42 inserted between its
first input terminal, for example, inverted input terminal
and its output terminal and a second impedance circuit 43
inserted between its second input terminal, for example,
non-inverted input terminal and an output terminal 45. The
common connection point between the inverted input terminal
of the amplifier 41 and the first impedance circuit 42 is
connected to an input terminal 40 and the output terminal
45 is connected in series to a load 44.
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~1~647~
If it is assumed that the differential input
impedance of the amplifier 41 is sufficiently large and
the amplification factor thereof is taken as A, the follow-
ing formulae (9) and (10) are respectively established.
1 il (VO - V')A -------- (g)
V" - VO = Z2 i2 ................ ------- (10)
-Where V" is the output voltage of the amplifier 41; V' is
the input voltage to the inverted input terminal of the
amplifier 41; Zl and Z2 are respectively the impedances of
the impedance circuits 42 and 43; VO is the voltage pro-
duced across the load 44; and il and i2 are respectively
the currents flowing through the impedance circuits 42 and
43.
From the right side of the formula (9), the fol-
lowing formula (11) is derived.
O + Zl il)/(l + A) .... ........(11)
Further, when the above formula (9) is substitutedinto the formula (10), the following formula (12) is obtained.
Z i = (V - V')A - VO = (A - l)Vo
Also, when the formula (11) is substituted into
that (12), the following formula (13) is derived.
Z i = (A - l)Vo - A(A-Vo + Zl 1 /
= _ O _ 1 1 .............. .....~- (13)
1 + A 1 + A
If A is made ~ in the formula (13), the following
formula (14) is ohtained.
2 i2 -Zl il - - -~ (14)
From the above formula (14), the current transfer
function of the circuit shown in Fig. 5 is expressed as
follows:
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~1 46~
i2 Z
Z2 ' ~ - - (15)
~hat is, it could be understood that the circuit
of Fig. 5 operates as a current amplifier.
At this time, the voltage VO appeari~g across the
load 27, which corresponds to the variable resistor 27 in
the example of Fig. 1, becomes as follows:
VO = i2-ZL .............. ---------(16)
where ZL is the impedance value of the load 44.
E'rom the above formula (16), it would be under-
stood that, according to the circuit of Fig. 5, even though
non-linear elements such as the impedance circuits 42 and
43, diode and so on are used at the side of the load 44, the
clrcuit is free from the non-linear distortion and so on.
Fig. 6 shows an example in which the circuit shown
in Fig. 5 is applied to the tone control circuit 21 shown
in Fig. 1. In Fig. 6, in order to facilitate the explana-
tion, the respective circuit elements are marked with their
resistance values and capacitance values, and reference
letters L and H respectively represent low and high frequency
band compensating portions.
In the example of Fig. 6, if the impedance values
Zl and Z2 are calculated in correspondence with the example
of Fig. 5, they are expressed as follows:
~ cl ~C3 3 3
2 (R2 + r2 // ~ 1 + R4 + r4) (18)
At this time, the curL~nt transfer function is
expressed as follows:
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1~464~
~ -- --------- (19)
ll 2
Upon the low frequency band compensation, a movable
piece 46 of a variable resistor of the low frequency compen-
sating portion L is moved to the right to thereby make the
impedance value Z2 small. In other words, the current
transfer becomes good and hence the curren-t increases to
boost the low frequency characteristic. While, when the
movable piece 46 is moved to the left, the impedance value
Z2 increases. Thus, the current transfer becomes bad, hence
the current decreases and accordingly the low frequency char-
acteristic is cut.
While, upon the high frequency band compensation,
the movable piece 47 is moved to the right to make the
impedance Z2 small, or the current transfer becomes good,
the current increases and hence the high frequency band char-
acteristic is boosted. When the movable piece 47 is moved
to the left, the impedance Z2 becomes large or the current
transfer becomes poor. Thus, the current decreases, and
accordingly the high frequency characteristic is cut.
Therefore, it will be understood that the circuit
of Fig. 6 can easily represent desired total characteristics
in which the low and high frequency bands for the tone con-
trol are compensated and becomes the tone control circuit of
the current amplification type.
Fig. 7 shows another example of the current trans-
fer amplifier according to the invention, in which the
elements and parts corresponding to those of Fig. 1 are
marked with the same references and their detailed descriptlon
will be omitted.
In this example, in place of the switch means 8
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~1 ~6 ~J~.
shown in Flg. 1, employed is a switch means 8' which includes
s S11 to S13; S21 to S23; S31 to S33; S to S ; and
S51 to S53, and these switches are desirably opened and
closed to present a desired function mode. In the example
of Fig. 7, the switches Sl1 to S53 are arranged in a matrix
pattern at the output sides of the respective constant cur-
rent sources 6c, lOc, llc, 14c and 15c. In this case, as
set forth above, for example, the signal for driving the
speaker (not shown) is derived through the switch Sll to the
output terminal 29, and the first and second tape recorder
outputs are respectively derived through the switches S52
and S53 and then through inverted type feedback amplifiers
48 and 49 to output terminals 50 and 51. Since the respective
signals, which are transferred through the circuit, are cur-
rents, they are not affected by the non-linear distortions
of the respective switches Sll to S53. When the switches
S11 to S53 are opened and closed desirably, the respective
signals may be mixed.
As described above, according to the present
invention the change-over and transfer of the internal
signals are carried out in the form of the constant current
system, so that the current flowed into the load resistor
i.e. variable resistor 27 is not affected any by the serially
inserted impedance. Therefore, the non-linear distortion
caused by the switches, grounded impedance and so on can be
neglected, and a solid wire, which is much expensive as com-
pared with a shield wire, is sufficient for wiring.
Further, with the constant current transfer system,
if only the current is transferred accurately, the signal
can be reproduced at the variable resistor 27 serving as the
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~ 4;
load in the accurate voltage mode. Therefore, it is needless
to consider the grounded impedance through which the return
current flows as in the constant voltage transfer system.
In other words, as set forth above, since the ground poten-
tial at the pre-amplifier portion is generally different
from that at the power amplifier portion, the counter measure
for reducing the affection by the return current is required
in the constant voltage transfer system. In the constant
current transfer system, however, the grounded impedance
itself through which the return current flows is not neces-
sary to be considered as set forth above, so that it is of
no need to take the influence of the return current into
account and hence the signal can be transferred accurately.
In the examples of the invention, at the input
terminal of the power amplifier 28 which serves as the
circuit to finally convert the current signal into the volt-
age signal, provided is the variable resistor 27 with a
relatively small resistance value, such as 10 KQ, so that
the examples are advantageous in view of noise. Further,
as the sound volume by the variable resistor 27 is lowered,
its resistance value is decreased and the noise is reduced
(the thermal noise is in proportion to the resistance value).
Therefore, a natural sound volume in view of acoustic sense
can be obtained.
Further, when the sound volume is lowered to about
1/2 in the constant voltage transfer system as described
above, the resis-tance value of the resistor becomes maximum.
Accordingly, the S/N ratio is deteriorated in the normal
using state. According to the constant current transfer
system of the present invention, however, the resistance
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- Pl46~74
value of the variable resistor 27 is reduced in correspond-
ence with the lowering of the sound volume and the thermal
noise is also decreased. Therefore, according to the
invention, the S/N ratio is no-t deteriorated especially in
the ordinary using state where the sound volume is lowered,
and hence natural sound is produced in view of acoustic
sense.
In addition, according to the invention, the
change-over and transfer of the signals are carried out in
the current mode, so that the signals can be mixed freely
by opening and closing a plurality of the switches suitably.
The above examples are the cases that the phono-
cartridge, tuner or the like is used as the respective signal
source before the current conversion, but the present
invention can be applied to a case where the other signal
sources are used with the same effect.
Further, it is of no need that the respective
switches used as the switch means 8 are limited to the above-
mentioned ones, but the other switches of the function mode
may be used.
In the above examples, the inverted type constant
current amplifier 21a of the tone control circuit 21 may be
a same phase type constant current amplifier.
Further, in the above example the same phase type
amplifier is used as the pre-amplifier 28, but in fact the
amplifier located at the rear stage of the variable resistor
27 may be of the same phase type or inverted type, and this
amplifier is not limited to the power amplifier but may be
other type of amplifiers.
It will be apparent that many modifications and
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~146~74~
variations could be effected by one skilled in the art
without departing from the spirits or scope of the novel
concepts of the present invention so that the spirits or
scope of the invention should be determined by the appended
claims only.
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