Impedance converter for a condenser microphone

Details Impedance converter for a condenser microphone Impedance converter for a condenser microphone

1999-04-26

2002-09-17

Mei, Xu (Department: 2644)

Electrical audio signal processing systems and devices

Circuitry combined with specific type microphone or loudspeaker

With electrostatic microphone

C381S120000

Reexamination Certificate

active

06453048

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an impedance converter used for a condenser microphone, and more specifically, to an impedance converter for a condenser microphone, in which vacuum tubes such as a triode, an amplifier tube and so on are used for converting an impedance.
BACKGROUND OF THE INVENTION
In an impedance converter used for a condenser microphone, a voice signal input at a high impedance is output from a condenser microphone to an amplifier at a low impedance. Thereby, in a condenser microphone with a small value in a an effective electrostatic capacity, a frequency response at a low frequency band is obtained therefrom. Further, in such a kind of impedance converters, a vacuum tube is mainly used as amplification means for maintaining a suitable voice generated from a condenser microphone.
A conventional impedance converter as described above is shown in a circuit of
FIG. 10
, which was used for various condenser microphones. The circuit of
FIG. 10
includes input terminals
141
,
142
, a bias circuit
110
provided therein with a gird resistance
111
connected in parallel with each other, a bias resistance
112
and a condenser
113
, a vacuum tube for amplification (hereinafter called as an amplifier tube)
120
, a load resistance
130
, a power terminal
143
, an output terminal
144
, and a ground terminal
145
.
A condenser microphone portion
100
includes a voice signal generating circuit
101
for generating a voice signal, and a condenser
102
which comprises a vibration plate (not shown) in which vibration is caused by sound applied thereto from outside thereof and a fixed electrode (not shown), and connected with input terminals
141
and
142
of an impedance converter. In the voice signal generating circuit
101
, a voice voltage is generated, based on an electrostatic capacity corresponding to a change of distance between the vibration plate and the fixed electrode thereby a voice signal is produced. The voice signal produced by the voice signal generating circuit
101
is output to the impedance converter through the input terminal
141
and
142
. The condenser
102
comprised of a vibration plate and an electrode has an electrostatic capacity of about 5 to 100 [pF].
The condenser microphone portion
100
is connected with the impedance converter through the input terminals
141
and
142
, of which the input terminal
141
is connected with the amplifier tube
120
, and of which the input terminal
142
is connected with the ground terminal
145
which is an earth.
In the amplifier tube
120
, a vacuum tube for amplification is used for impedance conversion. That is, the amplifier tube
120
comprises a cathode connected with the power terminal
143
, a grid connected with the input terminal
141
, and a plate connected with the output terminal
144
. In such a kind of the amplifier tube
120
, a voice signal applied at a high impedance to the grid side is output to the plate side at a low impedance.
A power current Ip flows from the power terminal
143
to the cathode of the amplifier tube
120
. When the plate side is an anode, the current flows inward from the grid side. When the plate side is cathode, the current flows from the plate side. Thus, the voice signal input into the impedance converter through the input terminal
141
from a condenser microphone is amplified by the amplifier tube
120
to be output from the plate side. That is, in the amplifier tube
120
, when a voice signal is applied, the plate side is at ground so that it is operated as a cathode follower.
The bias circuit
112
is disposed between the grid and the plate in the amplifier tube
120
, and comprises a grid resistance
111
and a bias resistance
112
connected in parallel therewith each other. The bias resistance
112
of the bias circuit
110
is connected in series with a load resistance
130
, and connected with a grid/ground terminal
145
through the load resistance
130
.
The flow of the current Ip to the amplifier tube
120
is lowered in voltage by the bias circuit. In the amplifier tube
120
, the plate side is loaded through the bias resistance
112
, and the grid side is loaded through the grid resistance
111
. An AC component of the power current Ip from the plate side of the amplifier tube
120
is bypassed by the condenser
113
.
In the impedance converter with the triode as described above, a voice signal applied to the amplifier tube
120
from the condenser microphone portion
100
is not applied to the grid at the input impedance enough to a bias voltage, since the applied voice signal is affected from the grid resistance
111
. As a result, a sufficient signal can not be taken out of the plate side.
When a voice signal is output from the plate of the amplifier tube
120
at a low impedance, a mutual conductance between the cathode and the grid of the amplifier tube
120
has to be increased for the purpose of solving the aforementioned problem. Because the output impedance is 1/mutual conductance and depends on the value of the mutual conductance. Therefore, a vacuum tube with a high mutual conductance value has to be selected as the amplifier tube
120
. It is known that when the grid and the plate are spaced at a short distance, the mutual conductance value of the vacuum tube is increased. However, when the spacing between the grid and the plate is at a short distance, free electrons are liberated from the grid so that it tends to cause a flow of much leaky current, and makes the amplifier tube to be operated unstably. Firstly, this problem needs to be solved.
On the other hand, when the resistance value of the grid resistance
111
is increased, the voice signal can be applied to the grid of the amplifier tube
120
at a higher impedance than that of the previous one. However, the grid resistance
111
causes a change in bias voltage of the amplifier tube, since voltages on both ends of the grid resistance are increased. Therefore, it results in making the amplifier tube to be operated unstably.
Further, in the amplifier tube
120
used as a cathode follower, there is a problem in that residual electrons in a heater for heating a cathode are electrostatically coupled and output as a ham noise together with a voice signal.
SUMMARY OF THE INVENTION
In view of the above-described problems, an object of the present invention is to provide an impedance converter for a condenser microphone, in which an input voice signal from a condenser microphone portion through an input terminal can be impressed at a sufficient impedance to a grid of an amplifier tube with a high mutual conductance and a sufficient voice signal can be taken out of a plate without a ham noise affected from a cathode.
In order to achieve the aforementioned object, there is provided an impedance converter for a condenser microphone having an input terminal into which a voice voltage signal is input from outside, an amplifier tube which comprises a cathode, a grid and a plate for amplifying said voice voltage to output it to outside therefrom, the amplifier tube being plate-grounded, a power terminal for supplying power to a cathode of said amplifier tube, a bias circuit for applying a bias voltage to a grid of said amplifier tube, a condenser connected with a plate of said amplifier tube, a load resistance connected in series with said condenser, an output terminal for outputting a voice signal amplified by said amplifier tube, and a ground terminal connected in series with said load resistance, characterized in that said bias circuit includes a first diode for loading a flow of a current from a grid of said amplifier tube at a bias voltage, a second diode connected in inversely parallel with said first diode for effecting a rectifying action so that a flow of a current from said first diode flows to the grid, and a bias resistance disposed between said amplifier tube and said load resistance and connected in parallel with said second diode and said condenser to load a flow of current from a plate of said amplifier tube at a bias voltage.
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