JPS60141005A - Output muting circuit - Google Patents

Abstract

PURPOSE:To prevent generation of signal waveform distortion and attenuation of level by preventing generation of the conductive state of a switching element because of a leakage current of a switching element drive circuit. CONSTITUTION:In the normal operating state (non-muting state) where a control input terminal 38 is kept to a low level, even if a leakage current is produced to a transistor (TR)22, since a leakage absorption path exists due to a high resistance value resistor 48, no effect is given to a TR18. That is, at non-muting the TR18 is kept to a high impedance state. Furthermore, since an emitter of a TR46 is grounded via the high resistor 48 and only a voltage VF at the muting state and a voltage corresponding to a negative value -VF of the signal at the non-muting state is applied between the collector and emitter of the TR46, no leakage current giving effect on the dielectric strength is produced. Furthermore, even if the leakage absorption circuit 44 like this is placed, an excellent output muting is obtained at application/interruption of the power supply and the muting operation is not deteriorated because of the leakage absorption.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an output muting circuit, and more particularly to an improvement in an output muting circuit that prevents the generation of transient sound output when power is turned on and off.

In order to prevent the generation of transient sounds that occur when the power is turned on and off, various audio amplifiers have output muting that cuts off the signal output for a certain period of time when the power is turned on and off to prevent the output of transient sounds. Measures are being taken.

FIG. 1 shows a conventional output muting circuit.

The amplifier 2 is an output amplifier installed at the final stage of various audio amplifiers, and a signal 4 to be extracted from a pre-stage amplifier (not shown) is applied to its input section. An output terminal 12 is formed at the terminal 6 of the amplifier 2 via a capacitor 8 and a resistor 10, and an output muting circuit 14 is installed between the output point and a reference potential point. That is, between the output point and the reference potential point,
A transistor 18 as a switching element is connected together with the resistor 6, and a control signal of a switching element drive circuit 20 is applied to the base of the transistor 18. This switching element drive circuit 20 includes a transistor 22.24 and a resistor 26.28.30.32.3.
It consists of 4. A drive voltage (C) is applied to the power supply terminal 36, and a muting control input (not shown) is applied to the control input terminal 38.

- In the case of a power-supply audio amplifier, when the power is turned on or off, even if the audio signal is muted in the amplifier or the previous stage signal processing circuit, there will be a loss of power due to the operating point fluctuation of the final stage amplifier. There is a frequency popping sound, but
This is prevented by performing muting control for a certain period by the output muting circuit 14. The output muting circuit 14 utilizes the saturation resistance of the transistor 18 to maintain the control input terminal 38 at a high level during power on/off transitions to turn on the transistors 24 and 22 and the transistor 18, thereby reducing the output point. The load resistance is controlled from a predetermined value to zero, and the output terminal 12 is attenuated by the resistor 10.
This prevents a transient stage from occurring.

Figure 2 shows its operating waveforms, where A is the output waveform of the amplifier 2 from power-on to power-off, B is the signal output generated at the output terminal 12 when the output muting circuit 14 is not installed, and C is the output muting circuit. This is the signal output generated at the output terminal 12 when the timing circuit 14 is installed. In these 8, B and C, VDC indicates a change in the output operating point, S indicates a signal component to be extracted, tl indicates a muting period when the power is turned on, and t2 indicates a muting period when the heavy source is cut off. When the output muting circuit 14 is not installed, the low frequency transient sound signals sNl and SN2 generated at the output terminal 12 are as shown in B, but when the output muting circuit 14 is installed, as shown in C. As such, its occurrence can be prevented.

However, such an output muting circuit 14
When an audio amplifier is constructed from a semiconductor integrated circuit, it is necessary to synchronize the operations of the signal muting circuit inside the integrated circuit and the output muting circuit 14 in order to obtain good muting characteristics. switching element drive circuit 20 of switching circuit 14
It is preferable that the device including the control circuit and the control circuit for controlling these components be constructed on a single chip integrated circuit.

In addition, in this output muting circuit 14, (1) If even a slight leakage current occurs in the transistor 22, the transistor 18 becomes conductive or semi-conductive (especially in the forward direction), which causes distortion of the signal waveform and level. This may cause attenuation.

In particular, leakage current is likely to occur when the external temperature is high or when used at a high power supply voltage Vcc.

(2) In a normal operating state, the collector potential of the transistor 18 is based on the contact point (GND) level, so at the negative peak of the output signal, it becomes a low voltage level below the GND level. Therefore, 1-transistor 1B
The Hess potential of the transistor 22 is higher than its collector potential by the forward drop voltage vF of the PN junction diode, but it is detected to a negative peak value -vF, and the voltage between the collector and emitter of the transistor 22 changes depending on the level of the output signal. A voltage higher than the power supply voltage VCC is applied, and leakage current is likely to occur.

(3) In order to prevent this, if a high resistance 40 is added, the collector will be clamped at GND-VF by the diode between the collector bases 1 and 18. Figure 3 shows the waveform in this case,
S is a signal waveform, and VOC is raised in the positive direction at the center level of the signal waveform. Also, depending on the load resistance, the negative side may be clipped, causing distortion.

SUMMARY OF THE INVENTION An object of the present invention is to provide an output muting circuit that prevents conduction of a switching element due to leakage current of a switching element drive circuit, and prevents signal waveform distortion and level attenuation.

This invention provides a switching element for stopping signal output of an output amplifier, a switching element drive circuit for driving this switching element, and a switching element installed between this switching element drive circuit and the switching element. The present invention is characterized by comprising a leakage absorption circuit that absorbs leakage current generated in the switching element drive circuit.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings.

FIG. 4 shows an embodiment of the output muting circuit of the present invention, and the same parts as those of the output muting circuit shown in FIG. 1 are given the same reference numerals.

In FIG. 4, in the output muting circuit 14, a leak absorption circuit 44 is installed between the base terminal 42 of the transistor 18 as a switching element and the output section of the switching element drive circuit 20. That is, a transistor 46 whose base is grounded is installed as an impedance conversion element between the base terminal 42 and the switching element drive circuit 20, and the output part of the switching element drive circuit 20 and the reference potential point (GND) are connected. A high resistance 48 is installed between the transistor 46 and the reference potential point, and a diode 50 is installed between the base of the transistor 46 and the reference potential point.

This diode 50 is for preventing the transistor 46 from being saturated in the switching state, and therefore may be omitted or may be provided with a constant voltage to prevent saturation, depending on the case.

According to such a configuration, the control input terminal 38 is maintained at a low level in a normal operating state (non-muting state B).
Even if a leakage current occurs in the transistor 22,
Since there is a leakage absorption path by the high resistance 48, the transistor 18 is not affected at all. That is, during non-muting, the transistor 18 maintains a high impedance state.

Furthermore, since the emitter of the transistor 46 is grounded via the high resistance 48, the voltage between the collector and emitter of the transistor 46 is VF- in the muting state, and negative peak value -vF of the signal in the non-muting state. Since only the corresponding voltage is applied, leakage current due to withstand voltage does not occur.

Moreover, when viewed from the base of the transistor 18, the collector of the transistor 46 is connected and there is no supply path for the base current, so there is no possibility of clamping at the time of a large signal.

Further, even if such a leak absorption circuit 44 is installed, a good output muting operation can be obtained when the power is turned on and off, and the muting operation due to leak absorption is not impaired.

When the audio amplifier is configured with a semiconductor integrated circuit, it is possible to synchronize each operation of the muting circuit inside the integrated circuit and the output muting circuit 14, and the switching element drive circuit 20 of the output muting circuit 14 However, when such a control circuit is integrated, a relative leakage current is likely to occur in discrete transistors whose breakdown voltage can be freely selected, and conventional circuits have no countermeasures against this. However, this embodiment can solve this problem.

Even if a leakage current occurs in the transistor 22, it has no effect on the transistor 18, so that distortion of the signal waveform or attenuation of the level does not occur. In particular, even when the external temperature is high, the influence of leakage current can be avoided.

Also, to perform a similar muting operation externally,
As shown in FIG. 5, a control current may be applied to the output point of the switching element drive circuit 20, ie, the emitter of the transistor 46, from another control input terminal 56 formed through a diode 52 and a resistor 54.

FIG. 6 shows another embodiment of the output muting circuit of the present invention. The output muting circuit of the present invention can be applied not only to integrated circuits but also to ordinary individual components.

In this embodiment, the leak absorption circuit 44 also serves as the switching element drive circuit 20, and the control input terminal 38 corresponds to that of the switching element drive circuit 20 of the previous embodiment. In this case, a control input terminal 38 is formed at the collector of the transistor 46 via the current limiting resistor 26, and a control input is directly applied to the control input terminal 38 to drive the transistor 18. That is, by applying a non-muting signal level, for example &f power supply voltage Vcc, as a control input to the control input terminal 38, which is sufficient to turn the transistor 46.18 into the power/node-off state, the leakage absorption circuit 44 absorbs leakage. Functions equivalent to those of the switching element drive circuit 20 can be obtained, and the switching element drive circuit 20 can be omitted.

According to such a configuration, the circuit configuration is simplified by omitting the switch-notching element drive circuit 20, and highly reliable operation is achieved without being affected by leakage current caused by the withstand voltage and temperature of the transistor 18. can be realized.

As explained above, according to the present invention, output muting can be performed satisfactorily when power is turned on and off, and occurrence of conduction state of the switch notching element due to leakage current of the switch notching element drive circuit can be prevented. It is possible to prevent the occurrence of signal waveform distortion and level attenuation.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing a conventional output muting circuit.
2 and 3 are explanatory diagrams showing the operating waveforms, FIG. 4 is a circuit diagram showing an embodiment of the output muting circuit of the present invention, and FIGS. 5 and 6 are diagrams showing the output muting circuit of the present invention. FIG. 3 is a circuit diagram showing another example of the circuit. 14... Output muting circuit, 18... Transistor as a switching element, 20... Switching element drive circuit, 44... Leak absorption circuit, 46...
...Transistor, 48...High resistance. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] (11) A switching element that stops the signal output of the output amplifier, a drive circuit that drives this switching element, and a current path between this drive circuit and the switching element. An output muting circuit comprising: a leakage absorption circuit installed to absorb leakage current generated in the drive circuit. (2) The leakage absorption circuit includes an emitter and a switching element at the output section of the drive circuit. Claim 1, characterized in that the drive circuit comprises a transistor whose collector is connected to the control input section of the drive circuit and whose base is grounded, and a leakage absorbing resistor installed at the output section of the drive circuit. Output muting circuit. (3) The leak absorption circuit is configured as a drive circuit for the switching element by applying a control input to the emitter side of the transistor via a control current setting resistor. The output muting circuit according to item 1.