KR790001825Y1 - Protective circuit for pulse width modulated signal amplifier - Google Patents

Abstract

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Description

Protection circuit of pulse width modulation amplifier

1 is a connection diagram showing one example of a protection circuit of a pulse width modulated amplifier according to the present invention.

2 is an explanatory diagram of the operation.

3 is a portion of another embodiment.

It has already been proposed that a pulse width modulated amplifier is configured to width modulate a square wave signal as a modulated signal as a modulated signal such as an audio signal, and to increase the modulated signal to be supplied to a speaker (load). Such an amplifier is characterized in that a transistor used as an amplifying element responds to a width-modulated square wave signal so that switching can be used efficiently as an amplifier.

In general, a heavy amplifier uses a push-pull type using a pair of transistors. In this case, the pair of transistors performs opposite "on" and "off" operations in response to a modulated signal. Therefore, when the modulated signal is stopped for some reason, one end of the pair of output transistors is in an "on" state, and in a so-called OCL heavy-duty speaker, the speaker is connected directly to the connection point of the pair of output transistors (direct connection). Since the direct current flows through the transistor and the speaker, the transistor and the speaker may be destroyed if this state continues for a long time. In the case where the duty signal is extremely different even in the state where the duty signal is present, such that one of the transistors continues the "on" state for a long time as described above. In this case, too, the above-mentioned risks arise.

In addition, even when a pair of output transistors and a load are connected via a DC blocking capacitor, the capacitor has a low impedance when the power is turned on. There is also a concern.

The present invention solves these drawbacks, and will be described next with reference to the drawings.

In Fig. 1, reference numeral 1 denotes a generator of a modulated signal (pulse signal) having a repetition frequency of, for example, a square wave signal of 100 KHZ, (2) a modulated signal source such as voice, and (3) a pulse width modulator. At the output side, a pulse width modulated signal is obtained in response to the modulated signal. (4) is a differential amplifier and is composed of a pair of transistors 5a and 5b and a common transistor 6 inserted into a common current path thereof as usual.

The output from the modulator 3 described above is supplied to the base of the transistor 5a in either transistor diagram of the differential amplifier 4, and the output of the transistor 5b is supplied through the transistor 7 to the transistor 8b. Is supplied to the base, and the output of the transistor 5b is supplied to the transistor 8a. The transistors 9a and 9b are a pair of output transistors, which are connected to the complementary element, and the output terminal 10 is drawn out at the connection point P of both transistors. Reference numeral 11 denotes a low pass filter, which is composed of an inductance 12a and a capacitor 12b. A speaker as a load is connected to this output terminal 10. In the illustrated example, the so-called two power supply system represents a heavy aeration system. The power supply terminal 14a is a positive voltage (+ Vcc) source and the terminal 14b is a negative voltage (-Vcc) source. Therefore, when the transistor 5a is " on " due to the signal that has now been modulated, the transistor 7 is also " on ", thus the transistor 8b is " on " and the output transistor 9b. In this case, the transistor 5b is in the "off" state, and accordingly, the transistor 8a is also "off", and therefore the output transistor 9a is also "off". Next, when the transistor 5a is off, the transistor 5b is turned on, whereby the output transistors 9a and 9b are also in the opposite state as described above. The current distribution period of the transistors 9a and 9b is changed, in this case, the contacts of the transistors 9a and 9b. Shows the waveform of the voltage at the point P in FIG. 2. In this connection point P of the comrade, "on" period (time t ₁ -t _2, such t ₃ -t ₄₎ of the transistor (9a) is the terminal The potential of the terminal 14b (in this example) is at the potential of the 14a (Vcc in this example) and on the contrary, in the period when the transistor 9b is "on" (time t ₂ -t ₃ , t ₄ -t _5, etc.) In the case of a signal as shown in Fig. 2, when the signal is obtained at the connection point P, the high frequency component is removed by passing through the low pass filter 11, so that a low frequency signal as shown by the dashed line 15 is shown. Only the component is supplied to the speaker 13, which is driven.

In the present invention, a detection circuit 16 for detecting a signal obtained at the output terminal 10 is provided, and the common transistor 6 of the differential amplifier 4 is controlled in response to this output, whereby a direct current is applied to the connection point P. When the signal is obtained for a predetermined period or more, by turning off the transistor 16, the transistors 5a and 5b of the differential amplifier 4 are "off" together, thereby resulting in a pair of output transistors 9a. (9b) Even in the "off" state.

As the detection circuit 16, a pair of transistors 17a and 17b are connected to a differential amplifier configuration, and both emitters are connected to the power supply terminal 14b, and the collectors of the transistors 17a and 17b on both sides are respectively provided. It is connected to the power supply terminal 14a through the resistors 18a and 18b, and the respective collectors are connected to the bases of the transistors 19a and 19b, and the emitters of the transistors 19b are connected to the base of the transistors 19a. It can be configured by connecting the collectors of both transistors 19a and 19b to each other and connecting them to the power supply terminal 14b via the resistor 20. The output of the detection circuit 16, that is, the collector of the transistors 19a and 19b is supplied to the base of the transistor 21, and the collector is connected to the base of the transistor 5 of the differential amplifier 4 to output. The transistor 6 can be controlled by the signal obtained at the terminal 10. In addition, a time constant circuit 23 composed of a resistor 22a and a capacitor 22b is connected to the input side of the transistor 17b, and the DC signal obtained at the terminal 20 is a fixed period of time, that is, as the time constant circuit 23. The detection circuit 16 is operated when continuing for more than a predetermined period.

Therefore, in the state where the direct current is not obtained at the terminal 10 and only the AC signal is obtained and the speaker 13 is normally driven, the direct current component of the output terminal 10 becomes 0, so that the detection circuit 16 Any of the transistors 17a and 17b is in an "off" state. In this state, the base of the transistor 21 is in the "off" state by the voltage from the terminal 14b, so that the transistor 6 is in the "on" state and the amplifier is operated normally.

Next, when a direct current voltage is obtained at the connection point p, and this is continued for a certain time (time determined by the time constant circuit 23), the transistor 17b is turned "on" and the transistor 19b is also " On "state. Therefore, since the positive DC voltage is applied to the base of the transistor 21 through the transistor 19b, it is turned "on" and therefore the transistor 6 is "off". When a negative DC voltage is obtained at the connection point p, the transistor 17a is turned "on", and with this, the transistor 19a is turned "on" and the operation state as described above becomes. In other words, when positive and negative DC voltages are obtained at the connection point p, the transistors 6 are properly controlled, so that both the output transistors 9a and 9b are " off " This also protects the output transistors at the same time.

In FIG. 1, the waveform of the current I flowing through the output terminal 10 at the connection point p can be represented by the curve 22 in FIG. As is clear from this curve 22, the period T ₁ is the distribution period of the positive current, and T ₂ is the distribution period of the negative current. However, within this period T ₁ , the period of "on" of the transistor 9a does not become "on" during the entire period of the period T ₁ , as indicated by T _a . The period of the current flow period T ₁ -T _a = T _c is a current period for the discharge of energy charged to the inductance 1 2a and the speaker 13. In order to derive this, in this example, The diode 24a is inserted between the terminal 14b and the connection point p. Therefore, the electric current of the above-mentioned current distribution period _Tc flows through this diode 24a, and does not flow to the transistor 9a.

Similarly, the current in the period T _d other than the “on” period T _b of the transistor 9b in the period T ₂ is configured to be drawn out through the diode 24b. By configuring in this way, the transistors 9a and 9b can be operated in a switching manner, without any deterioration in efficiency.

In the above example, although the detection signal is obtained by the load 13, the modulated signal itself may be detected to control the transistor 6 by this.

FIG. 3 shows an example thereof. As the detection circuit 16, the capacitor 29 is connected between the collector of the transistor 25 and the emitter, and the collector is connected to the base of the transistor 27 through the diode 26. As shown in FIG. In this case, the collector is connected to the base of the transistor 28 in a subordinate manner, and the collector is connected to the base of the transistor 21 shown in FIG. The modulated signal is applied to the base of the transistor 25, but the capacitor 29 is charged through the resistor 30 in the " off " period, but the transistor " on " 25) become "on" and its collector. The capacitor 29 is discharged through the emitter. When the modulated signal disappears, the voltage across the capacitor 29 increases, which is the order of the diode 26 and the transistor 27, respectively. The transistor 27 is "on" when the sum of the dropping voltages V _D and V _{BE in} the (iv) direction is greater than (V _D + V _BE ), and the transistor 28 is "off" and thus (21) is "on". ", The transistor 6 is turned off. This also protects the output transistors 9a and 9b and the speaker 13.

In addition, the transistor 21 may be turned "on" for a certain period after the power is turned on.

Claims (1)

As shown in the drawings and described above in the text, a pair of transistors having opposite "on" and "off" operations are inserted into each other by a pulse width modulated signal and their common current paths. A differential amplifier consisting of a common transistor, a pair of output transistors connected to a complimentary to which the respective outputs of the pair of transistors are supplied, and a load derived from the connection point of the pair of transistors should be connected. An output terminal and a detection circuit for detecting a DC signal or a modulated signal obtained at the output terminal, the output from the detection circuit being controlled to " off " the common transistor of the differential amplifier. Protection circuit of pulse width modulation amplifier.