JPS6348970Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention relates to a protection device for amplifiers, particularly push-pull amplifiers.

Generally, in a PWM type amplifier using a push-pull amplifier, the output transistor is pulse-driven with a square wave up to almost the full power supply voltage, so if a power supply failure or bias fluctuation occurs, the output transistor will receive a current that exceeds its allowable value. It was also fragile. For this reason, there have conventionally been protection devices that detect the current flowing through the output transistor and cut off the input signal and power supply when the current value exceeds a set value. Although such a protection device can protect the output transistor, since the input signal and power supply are cut off, the amplifier becomes inoperable. Therefore, when an amplifier equipped with this protection device is used as an emergency broadcasting device, there is a problem that evacuation guidance broadcasting cannot be made in an emergency.

The object of this invention is to provide a protection device that can prevent the output transistor of a push-pull amplifier from being destroyed and can also broadcast evacuation guidance in an emergency.

Hereinafter, this invention will be described in detail based on an illustrated embodiment. In this embodiment, this invention is applied to a PWM modulation type amplifier, and the audio signal from the audio signal source 2 is modulated into a PWM modulation signal by a modulator 4, and then passed through a drive circuit 5 to a pair of NPN transistors. 6 and the base of PNP transistor 8.

The collector of the NPN transistor 6 is connected to the contact 14a of the changeover switch 14. The contact 14b of this changeover switch 14 is connected to +E of the positive power supply 15.
The other contact 14c is connected to the ground potential. The collector of the PNP transistor 8 is connected to the contact 16a of the changeover switch 16, the contact 16b of this changeover switch 16 is connected to the -E potential of the negative power supply 18, and the other contact 16c is connected to the ground potential point. . The emitters of the NPN and PNP transistors 6 and 8 are connected to the input side of the demodulator 10 via resistors 20 and 22.

The output side of the demodulator 10 is connected to a load 12 such as a speaker.
connected to one end of the The other end of the load 12 is connected to a contact 24a of a changeover switch 24. A contact 24b of this changeover switch 24 is connected to the ground potential, and the other contact 24c is connected to the positive power supply 1.
5, and the other contact 24
d is connected to the −E/2 potential of the negative power supply 18. These changeover switches 14, 16, 24 are controlled by latch circuits 36, 38 which will be described later, and when the latch output is "0", the contacts 14a, 1
6a, 24a are connected to contacts 14b, 16b, 24b, respectively.

The output side of the demodulator 10 is connected to an average value detection circuit 26 formed of, for example, an integrating circuit, and the average value is supplied to comparators 28 and 30. Comparator 28
outputs "1" when the average value becomes greater than the positive reference voltage provided by the positive reference power supply 32.
occurs. Comparator 30 also produces an output "1" when the average value becomes less than the negative reference voltage provided by negative reference power supply 34. Comparator 2
The output "1" of 8 is latched by the latch circuit 36,
The output "1" of comparator 30 is latched into latch circuit 38.

When the output of the latch circuit 38 becomes "1", the contact 14a of the changeover switch 14 is switched from the contact 14b to the contact 14c, and the changeover switch 24 is switched from the contact 14b to the contact 14c.
The contact 24a is switched from the contact 24b to the contact 24d. Further, when the output of the latch circuit 36 becomes "1", the contact 16a of the changeover switch 16 is switched from the contact 16b to the contact 16c, and the contact 24a of the changeover switch 24 is switched from the contact 24b to the contact 24c.

The amplifier protection device configured as described above operates as follows. It is now assumed that latch circuits 36 and 38 are each reset and their outputs are both "0". As a result, the contacts 14a, 16a, 24
a are connected to contacts 14b, 16b, and 24b, respectively. In this state, a voltage of +E is applied between the collector and emitter of the NPN transistor 6, and a voltage of -E is applied between the collector and emitter of the PNP transistor 8, thereby push-pull amplifying the PWM modulation signal. At this time, if there is no failure in the positive power supply 15 and the negative power supply and there is no fluctuation in the bias of the drive circuit 5, the output (audio signal) of the demodulator 10 swings equally in both positive and negative directions, so the output of the average value detection circuit 26 is 0, the outputs of the comparators 28, 30 and the outputs of the latch circuits 36, 38 are respectively "0", and the changeover switches 14, 16, 24 are not switched.

In this state, if the positive power supply 15 fails,
When the output of the NPN transistor 6 becomes 0, the output of the detection circuit 26 shifts in the negative direction, and exceeds the negative reference voltage, the outputs of the comparator 30 and the latch circuit 38 each become "1", and the changeover switch 14 The contact 14a of the switch 24 switches from the contact 14b to the contact 14c, and the contact 24a of the changeover switch 24 switches from the contact 14b to the contact 14c.
b to contact 24d. As a result, NPN
The collector of the transistor 6 has a reference potential, and the other end of the load 12 has a potential of -E/2. Since a potential of -E is applied to the collector of the PNP transistor 8, a voltage of +E/2 is applied between the collector of the NPN transistor 6 and the other end of the load 12, and a voltage of +E/2 is applied from the collector of the PNP transistor 8 to the other end of the load 12. A voltage of -E/2 is applied between the terminals 12 and 12 and the other end.

When the negative power supply 18 fails, the output of the detection circuit 26 shifts in the positive direction and exceeds the positive reference voltage, and the outputs of the comparator 28 and latch circuit 36 become "1".
, the contact 16a of the changeover switch 16 changes from the contact 16b to the contact 16c, and the contact 24a of the changeover switch 24 changes from the contact 24b to the contact 24c.
Switch to . As a result, the collector of PNP transistor 8 is at the reference potential, and the other end of the load is +E/
The potential becomes 2. Since +E potential is supplied to the collector of NPN transistor 6, NPN
A voltage of +E/2 is applied between the collector of the transistor 6 and the other end of the load 12, and a voltage of -E/2 is applied between the collector of the PNP transistor 8 and the other end of the load 12. Ru. The same operation occurs even when the bias of the drive circuit 5 fluctuates.

This protection device grounds the collector of one transistor and raises the potential at the other end of the load 12 when there is a power supply failure or bias fluctuation.
E/2 or -E/2, and connect +E/2 between the collector of NPN transistor 6 and the other end of load 12.
Since the voltage of -E/2 is applied between the collector of the PNP transistor 8 and the other end of the load 12, it can be tolerated even if there is a power supply failure or bias fluctuation. A current exceeding the value is not applied, and the transistor 6,
8 from being destroyed, and can also make emergency broadcasts, although the output is lower than in normal conditions.

In the above embodiment, this invention was applied to a PWM type amplifier, but it can also be applied to an ordinary push-pull amplifier. Furthermore, the detection circuit 26 detects the average value of the output of the load 12, but as shown in FIG. You may.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a protection device of one embodiment of an amplifier according to this invention, and FIG. 2 is a partial block diagram of another embodiment of the same. 6...NPN transistor, 8...PNP transistor, 10...demodulator, 12...load, 14...
... first changeover switch, 16 ... second changeover switch, 24 ... third changeover switch, 26 ... detection circuit, 28 ... second comparator, 30 ... first comparator.

Claims (1)

[Scope of utility model registration request] A pair of base electrodes each connected to a signal source.
PNP and NPN transistors, a load whose one end is connected to the emitters of both transistors via a resistor, whose contact is connected to the collector of the NPN transistor, and whose positive potential contact is connected to a first positive potential. a first changeover switch whose reference potential contact is connected to the reference potential point;
A second changeover switch has its contact connected to the collector of the PNP transistor, its negative potential contact connected to the first negative potential, and its reference potential contact connected to the reference potential point, and a contact connected to the other end of the load. , connect its reference potential contact to the reference potential point, connect its positive potential contact to a second positive potential whose value is smaller than the first positive potential, and connect its negative potential contact to a value larger than the first negative potential. a third changeover switch connected to the second negative potential; a circuit for detecting the output of the load or the average value of the voltage between the two resistors; 1st time when I was little
a first comparator for switching a contact of a changeover switch from its positive potential contact to its reference potential contact, and a first comparator for switching a contact of a third changeover switch from its reference potential contact to its negative potential contact; and the detection circuit. When the output of the switch is greater than a predetermined positive reference signal, the contact of the second changeover switch is switched from its negative potential contact to its reference potential contact, and the contact of the third changeover switch is switched from its reference potential contact to its reference potential contact. A protection device for an amplifier consisting of a second comparator that switches to a positive potential contact.