JPS6348971Y2 - - 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, conventional protection devices have been designed to detect the current flowing through the output transistor, and when the current value exceeds a set value, cut off the input signal and power supply. Although such a protection device can protect the output transistor, the amplifier is rendered inactive since the input signal and power supply are cut off. 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, a description will be given based on an illustrated embodiment of this invention. In this embodiment, this invention is applied to a PWM type amplifier, and as shown in the figure, the audio signal from the audio signal source 2 is changed into a PWM modulated signal by a PWM modulator 4, and the NPN signal is transmitted through a drive circuit 6. It is supplied to the bases of transistor 8 and PNP transistor 10.

The collector of the NPN transistor 8 is connected to the contact 12a of the changeover switch 12. Further, the contact 12b of the changeover switch 12 is connected to the +E potential point, and the contact 12c is connected to the ground potential point. The collector of the PNP transistor 10 is connected to the contact 14a of the changeover switch 14. Further, the contact 14b of the changeover switch 14 is connected to the -E potential point, and the contact 14c is connected to the ground potential point. The emitters of both transistors 8 and 10 are connected via resistors 16 and 18, and the connection point 20
is directly connected to the contact 22b of the changeover switch 22, and is also connected to the contact 22c via the capacitor 24.
It is connected to the. In addition, this changeover switch 22
The contact 22a is connected to the input side of the demodulator 26. The contacts 12a, 14a, 22a of the changeover switches 12, 14, 22 are normally connected to the contact 12.
b, 14b, and 22b, and when the outputs of latch circuits 28 and 30, which will be described later, become "1", they are connected to contacts 12c, 14c, and 22c.

A load 3 such as a speaker is connected to the output side of the demodulator 26.
One end of 2 is connected and the other end is grounded. Therefore, the contact 1 of the changeover switch 12, 14, 22
2a, 14a, 22a are contacts 12b, 14b, 2
2b, a voltage of +E is applied between the collector and emitter of the NPN transistor 8, and the collector and emitter of the PNP transistor 10 are connected to each other.
A voltage of -E is applied between the emitters, and the PWM modulated signal is push-pull amplified by both transistors 8 and 10, demodulated into an audio signal by a demodulator 26, and supplied to a load 32.

The output of the load 32 is supplied to an average value detection circuit 34 made up of, for example, an integrating circuit. When both transistors 8 and 10 are operating normally, the demodulated signal swings equally in both positive and negative directions, so the average value output is "0". This average value output is the comparator 3
6,38. Comparator 36 is reference power source 4
The comparator 38 generates an output "1" when the average value output becomes larger than the positive reference voltage given by the reference power supply 42. Output "1" when it becomes small
occurs. Therefore, when both transistors 8 and 10 operate normally and the output of average value detection circuit 34 is "0", both comparators 36 and 38 do not generate an output "1".

However, if the +E potential disappears for some reason, the output of the transistor 8 becomes 0, and the average value output of the average value detection circuit 34 shifts in the negative direction. When this average value output becomes smaller than the negative reference voltage, the comparator 38 generates an output "1", and this output "1" is held in the latch circuit 28. By this latch output, the contact 12a of the changeover switch 12 is switched to the contact 12c, and the contact 22a of the changeover switch 22 is switched to the contact 22c.
Switched to side c. Therefore, a capacitor 2 is connected between the collector and emitter of the PNP transistor 10.
4. When a voltage of -E is applied through the demodulator 26 and the load 32 and a negative PWM modulation signal is supplied to the base, the PNP transistor 10 amplifies this modulation signal and the capacitor 24 is charged. . And the positive voltage is applied to the base of NPN transistor 8.
When PWM modulation signal is supplied, capacitor 2
4 as a power source to amplify this modulated signal.

Further, when the potential of -E disappears, the average value output of the average value detection circuit 34 shifts in the positive direction, and the comparator 3
6 produces an output "1", and the latch output of the latch circuit 30 connects the contact 14a of the changeover switch 14 to the contact 14c, and the changeover switch 22
Contact 22a is connected to contact 22c and operates in the same manner as described above. By appropriately selecting the capacitance of the capacitor 24, a voltage of +E/2 is applied between the collector and emitter of the NPN transistor 8, and a voltage of −E/2 is applied between the collector and emitter of the PNP transistor 10.
2 voltages can be applied respectively. In the above explanation, the case where -E or +E disappeared was explained, but if the supply voltage decreases or the modulator 4,
The same operation occurs even in the case of a failure of the drive circuit 6.

According to this protection device, in a push-pull amplifier with two positive and negative power supplies, if one power supply fails or the modulator or drive circuit fails,
Since only one power supply supplies both transistors 8 and 10 with positive and negative voltages that are smaller than in normal cases, overcurrent does not flow to the transistors, preventing them from being destroyed. Amplification operation can be maintained and evacuation guidance broadcasts can be made in emergencies. Furthermore, since it has a simple configuration in which the power supply and the capacitor are switched in accordance with the level shift of the average value output of the load, it is easy to manufacture.

In the above embodiment, this invention was applied to a PWM type amplifier, but this invention can also be applied to a push-pull amplifier that amplifies ordinary low frequency signals.

[Brief explanation of drawings]

The figure is a block diagram of an amplifier protection device according to this invention. 2...Audio signal source, 8...NPN transistor,
10... PNP transistor, 12... First changeover switch, 14... Second changeover switch, 20
...Emitter coupling point, 22...Third changeover switch, 24...Capacitor, 32...Load, 34...
...Average value detection circuit, 36...First comparator, 38
...Second comparator.

Claims (1)

[Scope of utility model registration request] A signal source is connected to the bases of a pair of NPN and PNP transistors, and when the energizing signal is not supplied, the contact contacts the first contact, and when the energizing signal is supplied, the contact contacts the second contact. Among the first to third changeover switches that contact the contacts, the contact of the first changeover switch is connected to the collector of the NPN transistor, the first contact is connected to the positive potential point, and the second contact is connected to the reference potential point. Connect the contact of the second changeover switch to the collector of the above PNP transistor, the first contact to the negative potential point, the second contact to the reference potential point, and one end to the reference potential point. A contact of a third changeover switch is connected to the other end of the load, a first contact is connected to the emitter connection point of both of the transistors, and a second contact is connected to the connection point via a capacitor. The load is provided with a circuit that detects the average value of its output, and when this average value exceeds the positive reference signal, the second and third
a first comparator for supplying the energizing signal to the changeover switch; a second comparator for supplying the energization signal to the first and third changeover switches when the average value exceeds a negative reference signal; An amplifier protection device equipped with a comparator.