JPS645371Y2 - - Google Patents

Description

[Detailed description of the invention] This invention relates to an analog signal transmission control circuit, and particularly relates to a signal transmission control circuit using an analog switch suitable for use in muting and signal switching of audio equipment, and furthermore, in squelch gate circuits of communication equipment. .

When it is necessary to control on/off of an analog signal in an electronic circuit device, an analog switch circuit called an analog gate circuit or a mute circuit is used. Figure 1 shows a typical example of such a circuit, in which an analog signal applied to the input terminal IN passes through a signal attenuator 1 whose attenuation operation is controlled by an external control signal, and is amplified to a predetermined degree. The signal is guided to an amplifier 2 having an amplifier 2 and outputted therefrom. The attenuator 1 includes a series resistance element R ₁ inserted in series in a signal line, and a variable impedance element Q ₁ provided between the output terminal of this resistance and a reference potential, for example, ground, and connected in parallel to the signal line. It becomes more.
This impedance element _Q1 is a bipolar transistor, and the control signal is connected to the base of the resistor.
This control signal is applied via R ₂ and controls the transistor Q ₁ to be turned on or off, thereby changing its impedance from large (or small) to small (or large).

The output of the attenuator 1 is a DC blocking capacitor.
It is applied to the base of the bipolar transistor Q ₂ which is the active element of the amplifier 2 via C ₁ . _R3 , _R4
is the base bias resistance of this transistor, R ₅ and R ₆ are collector resistance and emitter resistance, respectively, and capacitor C ₂ is a bypass capacitor.

In such a configuration, when the external control signal is at a low level, transistor _Q1 is in an off state and exhibits a high impedance. Therefore, the amount of attenuation due to this high impedance and the series resistor _R1 is small, and the signal IN is applied to the amplifier 2 with almost no loss. On the other hand, if the control signal becomes high level and transistor _Q1 is turned on, its impedance becomes extremely small, the amount of attenuation increases, and the signal
IN is significantly damped and the objective is achieved.

Here, when transistor Q ₁ is off and the signal is passed through and output, a loss will occur between the input terminal IN and the output terminal OUT of the amplifier due to the series resistor R ₁ and the input impedance of amplifier 2. . This loss increases as the series resistance increases and as the input impedance of the amplifier 2 decreases, so the amplification degree of the amplifier 2 is determined to compensate for this loss. However, in order to maximize the amount of attenuation of the damper 1 when the transistor Q ₁ is on, the value of the series resistor R ₁ must be increased because the on-resistance of the transistor Q ₁ can be suppressed to a substantially constant value. As a result, the transistor
Loss increases when Q ₁ is off. Yotsute amplifier 2
The degree of amplification must be increased accordingly. If such a high gain amplifier is used, the power supply voltage must be increased in terms of dynamic range, distortion characteristics, etc., which is inconvenient. Furthermore,
A problem also arises in that the attenuated extremely small signal when transistor Q ₁ is on is applied to the input of amplifier 1 and leaks to the output OUT due to its high gain.

Further, a DC bias is applied to the input terminal of the amplifier 2, and a charging/discharging current flows to the capacitor _C1 due to the on/off operation of the transistor _Q1 , and spike-like noise is generated at the output. In order to eliminate this switching noise, the attenuator side terminal of capacitor C ₁ can be clamped to ground potential in a DC manner, and for this purpose there is a method of inserting a resistor in parallel with transistor Q ₁ . However, this insertion resistance and the series resistance _R1 cause signal loss, which is undesirable.

An object of the present invention is to provide an analog signal transmission control circuit that ensures sufficient attenuation, has little loss, and does not generate switching noise.

The transmission control circuit of the present invention includes a signal attenuator and an amplifier (including a buffer amplifier) for amplifying the output of the attenuator, and this amplifier is constructed of a voltage-controlled active element exhibiting high input impedance. , the output of the attenuator is directly connected and applied to the control input of this active element, and a clamp element is connected to the input end of the attenuator to clamp the DC potential of this input end to a constant potential. It is a feature.

The present invention will be explained below with reference to the drawings.

FIG. 2 is a circuit diagram showing an embodiment of the present invention, and parts equivalent to those in FIG. 1 are designated by the same reference numerals. In this example, a self-biased N-channel junction field effect transistor is used as the active element Q ₂ of the amplifier 2, and the gate electrode of this transistor Q ₂ is directly connected to the output of the attenuator 1 in a direct current manner. . Therefore, capacitor C ₁ and bias resistors R ₃ and R ₄ shown in FIG. 1 are not required.
A clamping resistor _R7 is connected between the input terminal IN of attenuator 1 and the ground, so that its input terminal is always clamped at zero volts DC, and as a result, the gate input of transistor _Q2 is always at zero volts DC. It will be fixed at The other configurations are the same as those shown in FIG. 1, and their explanation will be omitted.

Here, the input impedance of the field effect transistor _Q2 is generally an extremely high impedance of tens of MΩ to several hundreds of MΩ, so no current flows to the gate, and therefore it operates as a voltage-controlled active element. . Therefore, even if the value of the series resistor R ₁ is selected to be extremely large in order to increase the amount of attenuation of the attenuator 1, the loss caused by this will be negligibly small.
In other words, since the loss during signal passing when transistor Q ₁ is off is small, the amplification degree of amplifier 2 can be made smaller than that shown in FIG. It is also possible to configure a buffer amplifier with a so-called source follower circuit of "1". Since the series resistance R ₁ can be large, the amount of attenuation when the transistor Q ₁ is on can be extremely large.

Furthermore, the gate input of amplifier 2 is always fixed at ground potential by resistor _R7 , and has no time constant due to a capacitor, etc., so the transistor
This means that switching noise due to _Q1 on/off will not occur.

In the above description, a junction field effect transistor is used as the active element, but an insulated gate field effect transistor such as a MOS may also be used, or a combination of a field effect transistor and a bipolar transistor may be used.

According to the present invention, with an extremely simple configuration, it is possible to increase the amount of attenuation when the signal is disconnected, reduce the loss when transmitting the signal, and eliminate switching noise when the signal is disconnected. Various applications such as mute circuits, squelch circuits, and signal switching circuits are expected.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of a conventional analog signal transmission control circuit, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Explanation of symbols of main parts, 1... Attenuator, 2...
Amplifier, Q ₁ ...Switching transistor, Q ₂
...Amplification transistor, R ₁ ...Series resistance, R ₇
...Resistance for clamping.

Claims (1)

[Scope of utility model registration request] Signal attenuation means having an attenuation resistance element inserted in series in a signal line and a variable impedance element provided between the output end of this resistance element and a reference potential point and whose impedance is controlled by a control signal; and this signal attenuation means. an analog signal transmission control circuit including an amplifying means that inputs the output of the signal attenuating means, the amplifying means including a clamping means for clamping a DC potential at an input terminal of the signal attenuating means to a reference potential, the amplifying means receiving the output of the signal attenuating means. 1. An analog signal transmission control circuit comprising a self-biased field effect transistor exhibiting a voltage-controlled high input impedance and having a gate of a control electrode to which an output is directly connected in a direct current manner.