Electrical audio signal processing systems and devices – Having microphone
Reexamination Certificate
2000-07-05
2003-02-04
Harvey, Minsun Oh (Department: 2644)
Electrical audio signal processing systems and devices
Having microphone
C381S111000, C381S112000, C381S113000, C327S552000, C327S559000, C330S303000, C330S288000
Reexamination Certificate
active
06516069
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microphone unit which is formed in a semiconductor chip and comprises a pressure sensitive element such as an electret capacitor, and also to a microphone filter for removing direct-current (DC) components and low-frequency components unnecessary for a sound signal from an output signal from the microphone unit.
2. Description of the Background Art
A conventional microphone unit and microphone filter are shown in FIG.
4
.
FIG. 4
illustrates a microphone unit MU
2
comprising an electret capacitor EC. Upon receipt of sound pressure, the electret capacitor EC varies its capacitance and generates an input signal Vin between its both electrodes. A ground potential GND is applied to one end of the electret capacitor EC. Then, an impedance converter comprised of diodes D
1
, D
2
, a resistance R
1
, and N-channel MOS transistors T
1
, T
2
is connected across the electret capacitor EC. More specifically, the anode of the diode D
1
is connected to one end of the electret capacitor EC and the cathode thereof to the other end of the electret capacitor EC. The diode D
2
is connected across the electret capacitor EC with its anode and cathode connected in the reverse fashion for those of the diode D
1
. The resistance R
1
is connected in parallel across the electret capacitor EC. The source of the transistor T
1
is connected to one end of the electret capacitor EC and the gate thereof to the other end of the electret capacitor EC. The drain of the transistor T
1
is connected to the source of the transistor T
2
. A power supply potential Vdd is applied to the drain of the transistor T
2
and a predetermined potential Vref
2
to the gate of the transistor T
2
. Further, the ground potential GND is applied to the back gates of the transistors T
1
and T
2
.
When no input signal Vin is applied, the gate-source voltage of the transistor T
1
is maintained at 0 V by the diodes D
1
, D
2
and the resistance R
1
. Upon application of the input signal Vin, variations occur in the gate-source voltage of the transistor T
1
. This effects a change in the drain-source current. In the transistor T
1
being of a depletion type, the current flows between the drain and source even if the gate-source voltage is 0 V. The variations in the drain-source current of the transistor T
1
causes variations in the drain-source current of the transistor T
2
, thereby changing the gate-source voltage of the transistor T
2
. This potential change at the source of the transistor T
2
becomes an output signal Vout
2
.
As shown in
FIG. 4
, a microphone filter FT
2
is configured as a CR circuit composed of a capacitor C
1
and a resistance R
4
. The capacitor C
1
receives at its one end the output signal Vout
2
from the microphone unit MU
2
and is connected at its other end to one end of the resistance R
4
. Further, a predetermined potential Vref
1
is applied to the other end of the resistance R
4
.
The microphone filter FT
2
removes DC components and low-frequency components included in the output signal Vout
2
by outputting a voltage dropped at the resistance R
4
. Since serving as a sound signal, the output signal Vout
2
should have an audio-frequency region in the range of approximately 100 Hz to 20 kHz. Thus, DC and low-frequency components unnecessary for the sound signal are removed from the output signal Vout
2
.
The output from the microphone filter FT
2
is fed into an amplifier. Illustrated in
FIG. 4
is an amplifier including a voltage follower and an inverting amplifier. More specifically, the output from the microphone filter FT
2
is fed into a positive input of an operational amplifier OP
1
. The operational amplifier OP
1
receives its own output at its negative input, serving as a voltage follower. The output from the operational amplifier OP
1
is then fed into a negative input of an operational amplifier OP
2
through a resistance R
2
. Also, the operational amplifier OP
2
receives its own output Vout
3
at its negative input through a resistance R
3
, serving as an inverting amplifier. Here, a predetermined potential Vref
1
is applied to the positive input of the operational amplifier OP
2
.
The microphone filter FT
2
removes DC and low-frequency components from the output signal Vout
2
at a cut-off frequency, f=1/(2&pgr;CR), where C is the capacitance of the capacitor C
1
and R is the resistance of the resistance R
4
. In order to remove low-frequency signals approximately below 100 Hz and DC components from the output signal Vout
2
, the product of the capacitance C and the resistance R, i.e., time constant, must be large; for example, such a combination as the capacitance of 1 &mgr;F and the resistance of 1.6 k&OHgr; or the capacitance of 100 pF and the resistance of 16 M&OHgr; becomes necessary. Forming such high capacitance and resistance in combination in a single semiconductor chip increases chip area, preventing downsizing and cost reduction of semiconductor chips. For this reason, the conventional microphone filter FT
2
cannot fit in a semiconductor chip in which the microphone unit MU
2
is formed, and other discrete parts such as a capacitor and a resistance are required to form the filter.
Even with the use of discrete parts such as a capacitor and a resistance, it is difficult to achieve downsizing and cost reduction because of a high cost of such parts, an increase in processing steps, and the impossibility of housing the microphone filter in the semiconductor chip in which the microphone unit is formed. Incidentally, the amplifier is not fitted into the same semiconductor chip as the microphone unit is formed.
SUMMARY OF THE INVENTION
A first aspect of the present invention is directed to a microphone filter comprising: a capacitor having one end, and the other end to which an output from a microphone is fed; a first transistor having a first current electrode which is connected to the one end of the capacitor, a second current electrode to which a first fixed potential is applied, and a control electrode; a second transistor having a first current electrode, a second current electrode which is connected to the second current electrode of the first transistor, and a control electrode which is connected to the control electrode of the first transistor; and a constant current source connected to the first current electrode and the control electrode of the second transistor.
A second aspect of the present invention is directed to a microphone unit comprising: a microphone formed in a semiconductor chip; and a microphone filter of the first aspect, which is formed in the semiconductor chip, wherein an output from the microphone is fed into the other end of the capacitor.
According to a third aspect of the present invention, the microphone unit of the second aspect further comprises: an amplifier formed in the semiconductor chip and having an input which is connected to the first current electrode of the first transistor of the microphone filter.
A fourth aspect of the present invention is directed to a microphone unit comprising: a microphone formed in a semiconductor chip; and an amplifier formed in the semiconductor chip and having an input into which an output from the microphone is fed.
The microphone filter of the first aspect can utilize, as its resistance, a differential resistance which is produced by a channel length modulation effect or an Early effect of the voltage-current characteristics between the first and second current electrodes of the first transistor. This achieves a large-time-constant microphone filter containing resistance and capacitance as its constituents. Since the first and second transistors and the constant current source form a current mirror circuit, the microphone filter is resistant to variations in the voltage-current characteristics of the first transistor due to temperature changes, and can be formed in a semiconductor chip without a significant increase in chip area.
In accordance with the second aspect, the microphone filter of th
Araki Toru
Takeuchi Takanobu
Grier Laura A.
Harvey Minsun Oh
Mitsubishi Denki & Kabushiki Kaisha
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