Amplifiers – Combined with automatic amplifier disabling switch means
Reexamination Certificate
2000-04-17
2001-04-03
Pascal, Robert (Department: 2817)
Amplifiers
Combined with automatic amplifier disabling switch means
C330S293000, C330S260000
Reexamination Certificate
active
06211730
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pre-amplifier circuits in audio equipment.
2. Description of the Prior Art
Audio equipment having functions such as a recording function for recording on a recording medium a voice signal having a low signal level which is inputted from a microphone and a voice signal having a high signal level which is inputted from an external voice input terminal and a monitor function for reproducing and outputting the inputted voice signals has been known.
FIG. 3
illustrates a conventional pre-amplifier circuit provided in such audio equipment.
A pre-amplifier circuit comprises a voice input termina
101
, a first switch
102
, a buffer
103
an amplifier
104
, a second switch
105
, and a slide switch
106
for controlling both the switches
102
and
105
. A voice signal having a low signal level from a microphone or a voice signal having a high signal level from an external voice input terminal is inputted to the voice input terminal
101
.
When the voice signal having a high signal level from the external voice input terminal is inputted to the voice input terminal
101
, the slide switch
106
is set at such a position that contacts
106
b
and
106
c
are connected to each other (a large signal-side position), and the first switch
102
and the second switch
105
are respectively connected to contacts
102
a
and
105
a,
as shown in FIG.
3
. Consequently, the voice signal having a high signal level which has been inputted to the voice input terminal
101
is outputted through the buffer
103
.
When the voice signal having a low signal level from the microphone is inputted to the voice input terminal
101
, the slide switch
106
is set at such a position where the contact
106
b
and a contact
10
a
are connected to each other (a small signal-side position), and the first switch
102
and the second switch
105
are respectively connected to contacts
102
b
and
105
b.
Consequently, the voice signal having a low signal level which has been inputted to the voice input terminal
101
is amplified by the amplifier
104
and is outputted.
In the conventional pre-amplifier circuit, two signal paths, that is, a signal path including the buffer
103
and a signal path including the amplifier
104
are required. Further, in a case where the slide switch
106
is switched to the small signal-side position, when the voice signal having a high signal level is inputted, the amplifier
104
enters a hard gripped state. Accordingly, a power line greatly swings, so that noise is outputted.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a pre-amplifier circuit capable of copying with input voice signals having different signal levels by only one signal path.
Another object of the present invention is to provide a pre-amplifier circuit capable of copying with input voice signals having different signal levels by only one signal path as well as reducing
A first pre-amplifier circuit according to the present invention is characterized by comprising an operation amplifier having a noninverting input terminal connected to a voice input terminal;
a negative feedback circuit having a first resistor which is connected between an output terminal of the operation amplifier and an inverting input terminal of the operation amplifier, and a second resistor and a switching element which are connected in series between the inverting input terminal of the operation amplifier and a ground; an operating switch for switching a small signal input mode and a large signal input mode; and an on-off control circuit for bringing the switching element into an off state to operate the operation amplifier as a buffer circuit when the operating switch is switched to the large signal input mode, while bringing the switching element into anon state to operate the operation amplifier as an amplifier circuit when the operating switch is switched to the small signal input mode.
It is preferable that there is provided a noise generation preventing circuit for bringing the switching element into an off state when the signal level of an output signal of the operation amplifier is not less than a predetermined level.
Used as the noise generation preventing circuit is one comprising a voltage divider circuit for dividing an output voltage of the operation amplifier, a rectifier circuit for rectifying an output of the voltage divider circuit, and a control element for bringing the switching element into an off state when the output voltage of the rectifier circuit is not less than a predetermined value.
A second pre-amplifier circuit according to the present invention is characterized by comprising an operation amplifier having a noninverting input terminal connected to a voice input terminal; a negative feedback circuit having a first resistor which is connected between an output terminal of the operation amplifier and an inverting input terminal of the operation amplifier, and a second resistor and a switching element which are connected in series between the inverting input terminal of the operation amplifier and a ground; a driving circuit for always bringing the switching element into an on state to operate the operation amplifier as an amplifier circuit; and an on-off control circuit for bringing the switching element into an off state to operate the operation amplifier as a buffer circuit when the signal level of an output signal of the operation amplifier is not less than a predetermined level.
Used as the on-off control circuit is one comprising a voltage divider circuit for dividing an output voltage of the operation amplifier, a rectifier circuit for rectifying an output of the voltage divider circuit, and a control element for bringing the switching element into an off state when the output voltage of the rectifier circuit is not less than a predetermined value.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 3582675 (1971-06-01), Jordan, Jr.
patent: 3693030 (1972-09-01), Walters
patent: 4119801 (1978-10-01), Jacobson
patent: 5014017 (1991-05-01), Ishiguro et al.
patent: 5241283 (1993-08-01), Sutterlin
Armstrong Westerman Hattori McLeland & Naughton LLP
Nguyen Patricia T.
Pascal Robert
Sanyo Electric Co,. Ltd.
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