Volume control circuit

A volume control circuit is provided which is not influenced by power supply fluctuations and which eliminates the audio signal component when the circuit is adjusted to minimum volume. An audio input signal is amplified by a differential amplifier and supplied to a substantially doubly balanced differential amplifier. Gain control of the audio signal is achieved by adjusting the bias voltage of the doubly balanced differential amplifier. The collector electrodes of two transistors used during minimum volume operation are connected directly to a power supply line. The controlled audio signal is supplied from the doubly balanced differential amplifier to a current mirror circuit which functions to cancel fluctuations in the power source. The connection of the collector electrodes of the two transistors in the doubly balanced differential amplifier to the power supply line functions to eliminate an audio component on the output when the volume control is set to minimum volume.

BACKGROUND OF THE INVENTION 
This invention relates to a control circuit and more particularly to a 
volume control circuit. 
A conventional volume control circuit, which may be an integrated 
semiconductor circuit, is generally shown in FIG. 1. This volume control 
circuit is used in an audio circuit stage of a television receiver. 
Referring now to FIG. 1, an audio signal at input terminal 10 is applied 
to differential amplifier 11 via coupling condenser C11. The differential 
amplifier 11 comprises first and second transistors Q11 and Q12. The base 
electrode of the first transistor Q11 is connected to the condenser C11 
and positive terminal of a fixed DC voltage source V11 through resistor 
R13. The negative terminal of voltage source V11 is connected to ground 
line L10. The emitter electrode of the first transistor Q11 is connected 
to the ground line L10 through a resistor R11. The base electrode of the 
second transistor Q12 is also connected to the positive terminal of the 
voltage source V11 through a resistor R14. The emitter electrode of the 
second transistor Q12 is connected to the ground line L10 through a 
resistor R12. The audio signal, after being amplified by the differential 
amplifier 11, is supplied from the collector electrode of each of the 
first and second transistors Q11 and Q12 to a doubly balanced differential 
amplifier 12 which controls the gain of the audio signal. The doubly 
balanced differential amplifier 12 comprises third to sixth transistors 
Q13 to Q16. The emitter electrodes of the third and fourth transistors Q13 
and Q14 are connected in common to the collector electrode of the first 
transistor Q11. The emitter electrodes of the fifth and sixth transistors 
Q15 and Q16 are connected in common to the collector electrode of the 
second transistor Q12. 
The base electrodes of the third and sixth transistors Q13 and Q16 are 
connected in common to a positive terminal of a variable DC voltage source 
V12. The negative terminal of voltage source V12 is connected to the 
ground line L10. The base electrodes of the fourth and fifth transistors 
Q14 and Q15 are connected in common to a positive terminal of a fixed DC 
voltage source V13. The negative terminal of voltage source V13 is 
connected to the ground line L10. The collector electrodes of the third 
and fifth transistors Q13 and Q15 are connected in common to a line L11 of 
a power supply source (not shown) through a resistor R15. The collector 
electrodes of the fourth and sixth transistors Q14 and Q16 are connected 
in common to the line L11 of the power supply source. The doubly balanced 
amplifier 12 delivers a controlled audio signal from the common collectors 
of the third and fifth transistors Q13 and Q15 to the base electrode of an 
emitter follower transistor Q17. The collector electrode of the transistor 
Q17 is connected to the line L11 of the power supply source. The emitter 
electrode of the transistor Q17 is connected to the ground line L10 
through a constant current source 13 and is also connected to an output 
terminal 14. The controlled audio signal is supplied from the output 
terminal 14 to a loudspeaker (not shown). 
When an audio signal is applied to the differential amplifier 11 via the 
condenser C11, a constant current having a value Io flows through the 
collector and emitter path of the second transistor Q12 in accordance with 
a value E11 of the voltage source V11. On the other hand, an audio signal 
current having a value Io+i.sub.s, where i.sub.s is an audio signal 
component superimposed on the constant current, flows through the 
collector and emitter path of the first transistor Q11 in accordance with 
the value E11 of the voltage source V11 and level of the audio input 
signal. Both the constant and audio signal currents are supplied to the 
doubly balanced differential amplifier 12 to control the gain of the audio 
signal. The gain control function of the doubly balanced differential 
amplifier 12 is achieved by adjusting a value E12 of the voltage source 
V12 in relation to a value E13 of the voltage source V13. When the value 
E12 is larger than the value E13 and the value E12 is increased until the 
third and sixth transistors Q13 and Q16 conduct and the fourth and fifth 
transistors Q14 and Q15 are cut off, the gain of the audio signal, that 
is, the volume is maximized. On the contrary, when the value E12 is 
smaller than the value E13, and the value E12 is decreased until the third 
and sixth transistors Q13 and Q16 are cut off and the fourth and fifth 
transistors Q14 and Q15 conduct, the gain of the audio signal is 
minimized. 
In case the power supply line L11 of the power supply source includes a 
ripple component, a ripple component is superimposed on the controlled 
audio signal at the collector electrodes of the third and fifth 
transistors Q13 and Q15. In other words, an undesirable audio noise 
component is added to the controlled audio signal. Also, since the above 
described volume control circuit is formed on an integrated circuit, the 
ground line L10 is grounded via a common lead line 15. This common lead 
line 15 is connected to the common emitter resistors R11 and R12 of the 
first and second transistors Q11 and Q12. Since the lead line 15 has a 
very small resistance, a very small voltage corresponding at the level of 
the audio input signal to the input terminal 10 occurrs across the lead 
line 15. The voltage across the lead line 15 varies in accordance with the 
level of the audio input signal. Accordingly, the potential of the ground 
line L10 continuously fluctuates and causes current fluctuation in the 
constant current flowing through the collector and emitter path of the 
second transistor Q12. As a result, even if the volume control is adjusted 
to the minimum, the output signal delivered from the output terminal 14 
includes an audio signal component. Further, if the voltage source V11 has 
an internal resistor, the second transistor Q12 is biased by the voltage 
across the internal resistor which corresponds to the level of the audio 
input signal, in addition to the value E11 of the voltage source V11. As a 
result, an audio component occurs even if the volume control is adjusted 
to the minimum. 
SUMMARY OF THE INVENTION 
It is accordingly an object of this invention to provide a volume control 
circuit which is not influenced by the fluctuation of a power supply 
source. 
It is an additional object of this invention to provide a volume control 
circuit which eliminates the audio signal component when the volume 
control is adjusted to the minimum. 
It is a further object of this invention to provide a volume control 
circuit which can be formed as a semiconductor integrated circuit. 
According to the present invention, a volume control circuit includes a 
pair of reference potential lines, an input terminal and an output 
terminal. The input terminal is connected to a differential amplifier 
comprising first and second transistors. The differential amplifier is 
constituted so that the emitter electrode of each of the transistors is 
coupled to one of the reference potential lines and the base electrode of 
the first transistor is coupled to the input terminal. The audio input 
signal, after being amplified by the differential amplifier, is supplied 
from the collector electrode of each of the transistors to a substantially 
doubly balanced differential amplifier, which comprises third to sixth 
transistors. The emitter electrodes of the third and fourth transistors of 
the doubly balanced differential amplifier are connected in common to the 
collector electrode of the first transistor. The emitter electrodes of the 
fifth and sixth transistors are connected in common to the collector 
electrode of the second transistor. The collector electrodes of the fourth 
and fifth transistors are connected in common to the other reference 
potential line. In order to control the volume, a variable reference 
voltage is applied to the common base electrodes of the third and sixth 
transistors and a fixed reference voltage is applied to the base 
electrodes of the fourth and fifth transistors. A controlled audio signal 
is delivered from the collector electrode of each of the third and sixth 
transistors to a current mirror circuit, which comprises seventh and 
eighth transistors. The emitter electrode of the seventh transistor of the 
current mirror circuit is connected to the collector electrode of the 
third transistor and the emitter electrode of the eighth transistor is 
connected to the collector electrode of the sixth transistor; both emitter 
electrodes also are coupled to the other reference potential line. The 
base electrodes of the seventh and eighth transistors are connected to 
each other. The collector electrode of the eighth transistor is connected 
in common to its base electrode; and the collector electrode of the 
seventh transistor is coupled to the output terminal. The controlled audio 
signal from the output terminal is provided to a loudspeaker. 
In the above described volume control circuit, the controlled audio signal 
is supplied through the current mirror circuit. Therefore, it is not 
influenced by the fluctuation in the other reference potential line. 
Furthermore, since the controlled audio signal from the common collector 
electrodes of the fourth and fifth transistors is bypassed to the other 
reference line, an audio component is not generated when the volume 
control is adjusted to the minimum. 
The objects and advantages of the present invention will become apparent to 
persons skilled in the art from a study of the following description of 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 2, an audio signal at input terminal 20 is applied to 
differential amplifier 21 via coupling condenser C21. The differential 
amplifier 21 comprises first and second transistors Q21 and Q22. The base 
electrode of the first transistor Q21 is connected to the condenser C21 
and to a positive terminal of a fixed DC voltage source V21 through 
resistor R21. The negative terminal of a source V21 is connected to ground 
line L20. The emitter electrode of the first transistor Q21 is connected 
to the ground line L20 through a constant current source 22. The base 
electrode of the second transistor Q22 is also connected to the positive 
terminal of the voltage source V21 through a resistor R22. The emitter 
electrode of the second transistor Q22 is connected to the ground line L20 
through a constant current source 23 and to the emitter electrode of the 
first transistor Q21 through a resistor R23. 
The audio signal, after being amplified by the differential amplifier 21, 
is supplied from the collector electrode of each of the first and second 
transistors Q21 and Q22 to a substantially doubly balanced amplifier 24 
which controls the gain of the audio signal. The doubly balanced 
differential amplifier 24 comprises third to sixth transistors Q23 to Q26. 
The emitter electrodes of the third and fourth transistor Q23 and Q24 are 
connected in common to the collector electrode of the first transistor 
Q21. The emitter electrodes of the fifth and sixth transistors Q25 and Q26 
are connected in common to the collector electrode of the second 
transistor Q22. The base electrodes of the third and sixth transistors Q23 
and Q26 are connected in common to a positive terminal of a variable DC 
voltage source V22, the negative terminal of which is connected to the 
ground line L20. The base electrodes of the fourth and fifth transistors 
Q24 and Q25 are connected in common to a positive terminal of a fixed DC 
voltage source V23, the negative terminal of which is connected to the 
ground line L20. 
A controlled audio signal is supplied from the collector electrode of each 
of the third and sixth transistors Q23 and Q26 to a current mirror circuit 
25. The collector electrodes of the fourth and fifth transistors are 
connected in common to line L21 of a power supply source Vcc. Accordingly, 
the controlled audio signal is bypassed to the line L21. The current 
mirror circuit 25 comprises seventh and eighth transistors Q27 and Q28. 
The emitter electrode of the seventh transistor Q27 is connected to the 
collector electrode of the third transistor Q23 and to the line L21 of the 
power source supply through a resistor R24. The emitter electrode of the 
eighth transistor Q28 is connected to the collector electrode of the sixth 
transistor Q26 and to the line L21 of the power source supply through a 
resistor R25. The base electrode of the eighth transistor Q28 is connected 
to the base electrode of the seventh transistor Q27. The collector 
electrode of the eighth transistor Q28 is connected to its base electrode 
and to the ground line L20 through a constant current source 26. 
The controlled audio signal is supplied from the collector electrode of the 
seventh transistor Q27 to the base electrode of an emitter follower 
transistor Q29 in load circuit 27. The base electrode of the transistor 
Q29 is connected to the ground line L20 through a resistor R26. The 
emitter electrode of the transistor Q29 is connected to the ground line 
L20 through a constant current source 28 and to an output terminal 29. The 
controlled audio signal is provided from the output terminal 29 to a 
loudspeaker. 
The operation of the above described volume control circuit is as follows. 
When an audio signal having a value Vs is applied to the differential 
applifier 21 via the input terminal 20 and the condenser C21, the value of 
the audio signal current flowing through resistor R23 is expressed as 
follows: 
EQU i.sub.s =Vs/Ra; (1) 
where Ra is the resistance of resistor R23. 
Accordingly, if the current of the constant current source 22 is equal to 
that of the constant current source 23 and is expressed as I.sub.01, a 
value of the audio signal current at the collector electrode of the first 
transistor Q21 is (I.sub.01 +i.sub.s) and the value of the audio signal 
current at the collector electrode of the second transistor Q22 is 
(I.sub.01 -i.sub.s). 
The audio signal current from each of the first and second transistors Q21 
and Q22 is supplied to the doubly balanced amplifier 24 to control the 
gain of the audio signal. The control function of the doubly balanced 
differential amplifier 24 is achieved by adjusting the value E22 of the 
voltage source V22 in relation to the value E23 of the voltage source V23. 
When the value E22 is larger than the value E23, and the value E22 is 
increased until the third and sixth transistors Q23 and Q26 conduct and 
the fourth and fifth transistors Q24 and Q25 are cut off, the audio signal 
current from the collector electrode of each of the first and second 
transistors Q21 and Q22 flows only through the collector and emitter path 
of each of the third and sixth transistors Q23 and Q26, respectively. 
Accordingly, the gain of the audio signal, that is, the volume, is 
maximized. On the contrary, when the value E22 is smaller than the value 
E23, and the value E22 is decreased until the third and sixth transistors 
Q23 and Q26 are cut off and the fourth and fifth transistors Q24 and Q25 
conduct, the audio signal current from the collector electrode of each of 
the first and second transistors Q21 and Q22 flows only though the 
collector and emitter paths of each of the fourth and fifth transistors 
Q24 and Q25, respectively. Accordingly, the gain of the audio signal is 
minimized. 
The maximum gain of the audio signal in the volume control circuit is 
calculated below. Since the seventh and eight transistors Q27 and Q28 
constitute the current mirror circuit 25, the value of the controlled 
audio signal current flowing through the resistor R24 is equal to that of 
the controlled audio signal current flowing though the resistor R25, where 
the resistance of the resistor R24 is equal to that of the resistor R25. 
That is, 
EQU I.sub.02 +I.sub.01 -i.sub.s =I.sub.c +I.sub.01 +i.sub.s (2) 
where I.sub.02 is the current of the constant current source 26 and I.sub.c 
is the controlled audio signal supplied from the collector electrode of 
the seventh transistor Q27. 
From the expression (2), I.sub.c can be calculated as follows: 
EQU I.sub.c =I.sub.02 -2i.sub.c (3) 
Accordingly, the output voltage Vo of the controlled audio signal at output 
terminal 29 is expressed as follows: 
EQU Vo=(I.sub.02 -2i.sub.s)(R.sub.b), (4) 
where R.sub.b is the resistance of the resistor R26. 
The AC component of the output voltage Vo is as follows: 
EQU Vo=(2i.sub.s)(R.sub.b) (5) 
As a result, the maximum gain G.sub.max of the audio signal is expressed as 
follows: 
##EQU1## 
In the above circuit, the volume control is not influenced by fluctuations 
in the power source. Even if the power source in line L21 includes a 
fluctuation component such as a ripple, the bias voltage across the base 
and emitter electrodes of each of the seventh and eighth transistors Q27 
and Q28 is unvaried because the seventh and eight transistors Q27 and Q28 
constitutes the current mirror circuit 25. Accordingly, the ripple 
component in line L21 is cancelled by the seventh and eighth transistors 
Q27 and Q28. The controlled audio signal from the collector electrode of 
the seventh transistor Q27 does not include the fluctuation element or 
ripple of the power source. The volume control circuit also does not 
generate an audio component when the volume control is adjusted to the 
minimum. Since both the collector electrodes of the fourth and fifth 
transistors Q24 and Q25 are connected to the line L21, the controlled 
audio signal current flowing through the collector and emitter path of 
each of the fourth and fifth transistors Q24 and Q25 is constantly 
bypassed to the line L21. Accordingly, when the volume is minimum, the 
controlled audio signal is not provided from the output terminal 29. 
Furthermore, even if the volume control circuit is integrated into a 
semiconductor, and the ground line L20 is grounded through a common lead 
line 30 outside the semiconductor, the current flowing from the constant 
current sources 22 and 23 into the lead line 30 is unvaried and is 
independent of the audio input signal. Accordingly, the potential in the 
ground line L20 does not fluctuate. As a result, the volume control 
circuit does not generate an audio component when the volume is minimum. 
It will be apparent to those skilled in the art that various modifications 
and variations could be made in the volume control circuit of the 
invention without departing from the scope or spirit of the invention.