JPS636885Y2 - - Google Patents

Description

[Detailed explanation of the idea] The present invention relates to an amplifier protection circuit.

If excessive voltage and current are applied to the output transistor in an amplifier, the transistor will be destroyed. Second
Curve A in the figure shows the relationship between the collector-emitter voltage V _CE and the collector current I _C at which this output transistor is destroyed, and is generally called the ASO line. Conventionally, a transistor protection circuit as shown in FIG. 1 has been used to prevent the transistor from operating outside the area indicated by the ASO line A (shaded area). In the figure, input terminals 1 and 2 are connected to the bases of NPN and PNP transistors 3 and 4, respectively, and
Connected to the collectors of transistors 15 and 16. The collectors of transistors 3 and 4 are connected to power supplies (+B) and (-B), respectively, and the emitters are connected to load end 7 via resistors 5 and 6, respectively. The bases of transistors 15 and 16 are connected to the emitters of transistors 3 and 4 through resistors 9 and 10, respectively, and
1 and 12 to the anodes and cathodes of diodes 13 and 14. diode 1
The cathodes and anodes of 3 and 14 are grounded. The emitters of transistors 15 and 16 are connected to load terminal 7, and load 8 is connected between load terminal 7 and ground.

In the conventional example as described above, the diodes 13 and 14 are connected when the emitter voltage of the transistors 3 and 4 is higher than the forward voltage of the diode, that is, the collector-emitter voltage (hereinafter referred to as operating voltage) of the transistors 3 and 4 is V _CE. It turns on when is below the power supply voltage _VB . In this case, the impedance of the diode is small, so if it is ignored, the resistor 5,
A first bridge circuit is formed by resistors 6, 10, 12 and load 8, and a second bridge circuit is formed by resistors 6, 10, 12 and load 8. If the balance of the first and second blits is lost due to a short circuit in the load 8, a detection voltage is generated between the base emitters of the transistors 15 and 16, respectively, and the transistors 15 and 16 are turned on. The input signals applied to the transistors 3 and 4 are cut off and the transistors 3 and 4 are protected. Here, curve B in Figure 2 is a protection operation curve, where the protection circuit as described above operates and transistor 15 or 16 is turned on.The collector-emitter voltage V _CE and collector current I _C of transistor 3 or 4 are This is a diagram illustrating the relationship between

If the load connected to the load end 7 is a pure resistance, the load curve will be a straight line, but if the load is a reactive load such as a speaker, it will be elliptical as shown in the load curve L shown in Figure 2. 2, the load curve L intersects with the protective operation curve B even though no protective operation is required, and unnecessary protective operation is performed, resulting in large distortion. Particularly near the low resonant frequency of the speaker serving as the load, the reactance of the impedance of the speaker becomes large, which has the disadvantage that the above-mentioned distortion is likely to occur.

The present invention has been devised to eliminate such drawbacks, and will be described in detail below with reference to embodiments.

FIG. 3 shows an embodiment of the present invention, in which bandpass filters 17 and 18 shown within dotted lines are inserted in place of the resistors 9 and 10 in the conventional example shown in FIG. Input terminals P are connected to the emitters of transistors 3 and 4, respectively, output terminals Q are connected to bases of transistors 15 and 16, respectively, and common terminal R is connected to load terminal 7. Bandpass filters 17 and 18 are resistors R ₁ , R ₂ , R ₃ and capacitors.
This is a parallel T-type trap circuit consisting of C ₁ , C ₂ , and C ₃ , and its trap frequency is adjusted to match the low-frequency resonant frequency of the speaker connected to the load end 7 . Note that the resistor _R4 is used for impedance adjustment as described later. In addition, the degree of attenuation and attenuation bandwidth at the trap frequency can be adjusted to match the resonance characteristics of the load speaker, and the degree of attenuation is usually 3.
Set it so that it is about ~6dB. Since the other parts are the same as those in FIG. 1, detailed explanation will be omitted.

In the above configuration, the low resonant frequency of the speakers serving as the load, that is, the bandpass filters 17 and 1
At frequencies well below or above the trap frequency of 8, the bandpass filters 17 and 18
There is only an inherent internal resistance component between the input and output terminals PQ. This band bus filter 17
If the values of the internal resistance components of and 18 are made equal to the resistance values of resistors 9 and 10 in the conventional example shown in FIG. 1, no effect will be caused by inserting the bandpass filters 17 and 18. For this purpose, the sum of the resistance values of resistors R ₁ and R ₂ and the resistance value of resistor R ₄ of the parallel T-type trap circuit described above may be made equal to the resistance value of resistor 9 or 10. In this way, at low frequencies, the resistances R ₁ and R ₂
However, at higher frequencies, resistor R ₄ acts as resistors 9 and 10, respectively, in FIG.

Although the attenuation of the bandpass filter circuits 17 and 18 does not increase when the resistor R ₄ is inserted in this manner, there is no problem because the attenuation does not require a very large value as described above. Also resistance R ₄
If not inserted, insert capacitor C ₄ in parallel with resistor R ₇ as shown by the dotted line, and in the high frequency band, form a voltage divider circuit with capacitor C ₁ and C ₂ in series and capacitor C ₄ . , it is sufficient to set the attenuation degree by this voltage divider circuit to be equal to the attenuation degree by the voltage divider circuit by resistors 9, 11 and resistors 10, 12 shown in FIG.

Note that the resistor R ₄ is not limited to the location shown in Figure 3, and may be inserted outside the bandpass filter circuit as R ₄ ′, in which case R ₄ ′≫R ₁ , R ₄ ′≫R ₂ Set resistances R ₁ ¹ and R ₂ ² , etc.

Next, at the low resonant frequency, the signals generated at the emitters of transistors 3 and 4 are attenuated by about 3 to 6 dB, as described above, and
Since the load is applied to the base, the protection operation curve B moves in the direction of arrow C, and the protection operation curve B and the load curve L no longer intersect, so the distortion as described above does not occur.

In the above embodiment, bandpass filters 17 and 18 were used in place of the resistors 9 and 10, but the invention is not limited to this. For example, in the conventional example shown in FIG. Bandpass filters may be inserted between the point and the bases of transistors 15 and 16, respectively. The point is to attenuate the divided voltage output of the voltage dividing circuit consisting of the resistors 9 and 11 and the resistors 10 and 12 at the low resonant frequency of the speaker to be the load.

It goes without saying that the bandpass filter is not limited to the one in the embodiment.

As described above, according to the present invention, it is possible to prevent the output of a transistor amplifier having a protection circuit from being distorted, especially in the vicinity of the low resonant frequency of the speaker.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional protection circuit, FIG. 2 is an explanatory diagram of its operation, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. In the figure, 3, 4, 15 and 16 are transistors, 1
7 and 18 are bandpass filters.

Claims (1)

[Scope of utility model registration request] An output transistor is connected to a load end via a first resistor, a switching element is inserted between a control electrode of the output transistor and the load end, and a speaker to be connected to the resistor and the load end is connected. A protection circuit which connects a voltage dividing circuit in parallel to the voltage dividing circuit and controls the switching circuit on and off using the voltage divided output of the voltage dividing circuit, further comprising means for attenuating the voltage divided output at a low resonant frequency of the speaker. Characteristic amplifier protection circuit.