Semiconductor integrated circuit for electric microphone

Details Semiconductor integrated circuit for electric microphone Semiconductor integrated circuit for electric microphone

1999-05-04

2001-04-17

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Amplitude control

C327S103000, C327S309000

Reexamination Certificate

active

06218883

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor integrated circuit comprising a voltage conversion circuit and a bias circuit for obtaining a voltage change from a change in the capacitance of a capacitor used in, for example, an electric microphone.
2. Description of Related Art
FIG. 7
is a circuit diagram of a voltage conversion circuit according to related art for obtaining a voltage change resulting from a change in the capacitance of a capacitor where the voltage conversion circuit is part of an amplifier circuit. As shown in
FIG. 7
, this amplifier circuit
100
comprises a voltage conversion circuit
101
, coupling capacitor
102
and amplifier
103
. An electric microphone
105
(microphone below) in which the capacitance of a capacitor is changed by sound waves is connected between ground GND and the input terminal IN of the amplifier circuit
100
.
The capacitor of the microphone
105
is precharged, and the capacitance of the capacitor changes with the sound waves picked up by the microphone
105
thus causing the output voltage of the microphone
105
to vary according to the capacitance change. Note that the output voltage from the microphone
105
is applied to the input terminal IN of the amplifier circuit
100
.
The voltage conversion circuit
101
voltage converts the voltage Vin input to the input terminal IN, and passes the converted voltage through coupling capacitor
102
to the amplifier
103
. The amplifier
103
then outputs the amplified voltage from output terminal OUT. The voltage conversion circuit
101
uses a depletion type n-channel FET
111
and resistor
112
to voltage convert the voltage Vin input to the input terminal IN. The node between the gate and source of FET
111
is biased by the bias circuit of diodes
113
and
114
, and the voltage Vgs between the gate and source of FET
111
fluctuates around 0 V. The drain current Id of the FET
111
is proportional to the square of pinchoff voltage Vp.
The relationship between pinchoff voltage Vp and the drain current Idss when the gate-source voltage Vgs is 0 V can be obtained from the following equation (a):
Idss=&bgr;×Vp
2
  (a)
where &bgr; is a coefficient determined by the gate size of the FET
111
.
If the change in voltage Vin resulting from a change in the capacitance of the microphone
105
capacitor is &Dgr;Vin, then the change &Dgr;Id in the drain current Id of FET
111
caused by &Dgr;Vin when Vgs=0 can be obtained from the following equation (b).
&Dgr;
Id=−
2×
Idss×&Dgr;Vin/Vp
  (b)
The following equation (c) can therefore be derived from the above equations (a) and (b).
&Dgr;
Id=−
2×&Dgr;
Vin×&bgr;×Vp
  (c)
Thus, if the resistance of resistor
112
is R, the change &Dgr;Vr in voltage drop Vr due to resistor
112
when there is a &Dgr;Id change in the drain current Id can be obtained from the following equation (d).
&Dgr;
Vr=&Dgr;Id×R=−
2×&Dgr;
Vin×&bgr;×Vp×R
  (d)
If R=Vp/(−2×Idss), then we know from the above equations (b) and (d) that &Dgr;Vr=&Dgr;Vin.
With respect to the dc characteristics of the voltage conversion circuit
101
, if Vx is the potential at point X, Vx will be the supply voltage Vdd minus the voltage drop of the resistor
112
, and can be expressed as shown in equation (e) when current Idss flows to resistor
112
.
Vx=Vdd−R×Idss=Vdd−R×&bgr;×Vp
2
  (e)
When the amplifier circuit
100
is an IC device, however, variations during the manufacturing process produce variations in the pinchoff voltage Vp of FET
111
. We know from equation (d) that the change &Dgr;Vr in the voltage drop Vr varies in proportion to the pinchoff voltage Vp, and, as a result, from equation (e) that the potential Vx at point X varies.
Variations during the manufacturing process also produce variations in the absolute value of the resistance R of resistor
112
, and we know from equation (d) that the change &Dgr;Vr in the voltage drop Vr varies in proportion to this resistance R. Furthermore, resistance R and coefficient &bgr; also have a temperature characteristic, which produces variation in potential Vx at point X.
A problem with the related art described above is therefore that a stable voltage gain and output voltage range cannot be obtained in the output voltage of the amplifier circuit
100
.
In addition, the output voltage Vout from the output terminal OUT is easily saturated, and a large amplification factor cannot be achieved in the amplifier
103
, due to variations in potential Vx at point X. It is therefore necessary for a coupling capacitor
102
to cut the dc component of the output voltage from the voltage conversion circuit
101
, and then amplify by means of the amplifier
103
. However, the output voltage from the voltage conversion circuit
101
cannot be dc amplified by the amplifier
103
, and a high capacitance coupling capacitor
102
is required, making it difficult to integrate the amplifier circuit
100
.
SUMMARY OF THE INVENTION
The present invention is therefore directed to a semiconductor integrated circuit comprising an electric microphone amplifier circuit for resolving the above problems, and more specifically to a semiconductor integrated circuit comprising an amplifier circuit that can be integrated into a semiconductor integrated circuit by using paired FETs formed in the same manufacturing process to limit the center of the dc characteristic of a voltage conversion circuit output voltage to ½ the supply voltage, and suppress variations in the output voltage due to various factors.
To achieve the above object, a semiconductor integrated circuit having an amplifier circuit for amplifying a voltage change accompanying a change in microphone capacitor capacitance comprises: a voltage conversion circuit for voltage converting this voltage change; an amplifier for amplifying a voltage converted by the voltage conversion circuit; and a reference bias circuit for producing and outputting a reference bias voltage to the amplifier. The voltage conversion circuit applies voltage conversion so that the midpoint of the voltage change is a value approximately ½ the dc voltage supplied to the amplifier.
In the dc characteristic of a voltage conversion circuit thus comprised, the output voltage of the voltage conversion circuit is ½ the dc supply voltage. The output signal from the voltage conversion circuit can therefore be amplified by the amplifier by direct coupling, and it is not necessary to provide a coupling capacitor. The amplifier and voltage conversion circuit can therefore be provided in the same chip, enabling easy integration and facilitating size reductions.
The voltage conversion circuit further preferably comprises a first FET for converting a voltage change to a drain current change, and a second FET for converting the drain current change of the first FET to a voltage. In this case, the first and second FETs are depletion type FETs formed in the same process.
By thus using a specific type of FET, variations in FET output voltage can be eliminated by eliminating variations in the manufacturing processes and temperature characteristics of the FETS. It should be noted that the gate length and gate width of the first and second FETs are preferably the same because variations in the manufacturing processes and temperature characteristics of the FETs can thus be more reliably eliminated, and variations in FET output voltage can be thereby eliminated.
The gate and source of the second FET are further preferably connected to the drain of the first FET. The same benefits can be achieved in this case.
Yet further preferably, bias circuits of the same configuration are connected between the gate and source of the first FET and the second FET. By thus using bias circuits of the same configuration, variations in the manufacturing processes and tem

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