JPS6158045B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION 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. Figure 2A shows the relationship between the collector-emitter voltage and the collector current that causes this output transistor to break down.
It is commonly 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 NPN and PNP, respectively.
It is connected to the bases of transistors 3 and 4 and to the collectors of NPN and PNP 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 terminal 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 to the anodes and cathodes of diodes 13 and 14 through resistors 11 and 12, respectively. The cathodes and anodes of diodes 13 and 14 are grounded. Transistors 15 and 16
The emitter of is connected to the load end 7, and the load 8 is connected between the load end 7 and ground.

In the conventional example as described above, the diodes 13 and 14 are connected when the emitter voltages of the transistors 3 and 4 are higher than the forward voltage of the diodes, that is,
The collector-emitter voltage of transistors 3 and 4 (hereinafter referred to as operating voltage) V _CE is the power supply +B and -B.
It turns on when the voltage is below _VB . In this case, since the impedance of the diode is small, it is ignored, and a first bridge circuit is formed by the resistors 5, 9, 11 and the load 8, and a second bridge circuit is formed by the resistors 6, 10, 12 and the load 8. If the balance of the first and second bridges is disrupted 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, and the input terminals 1, 16 are turned on. The input signal to be applied to the transistors 3 and 4 is cut off by the transistor 2, and the transistors 3 and 4 are protected. Now let the resistance values of resistors 5, 9 and 11 and the resistance values of resistors 6, 10 and 12 be R _E , R ₁ and R ₂ respectively, and the base-emitter voltage of transistors 15 and 16 be V _BE .
The collector current I _C of transistors 3 and 4 for turning on transistors 5 and 16 is I _C ≧R ₁・V _B +R ₂・V _BE / _RE (R ₁ +R ₂ ) −R ₁ /R _E (R ₁ +R ₂ )・V _CE (1). In addition, when the emitter voltage of diodes 3 and 4 is lower than the forward voltage of the diodes, that is, the operating voltage V _CE of transistors 3 and 4 is equal to the voltage V _B of the power supply +B and -B.
It turns off in the above cases. In this case, transistors 15 and 16 are turned on when the voltage drop across resistors 5 and 6 due to the collector current I _C flowing through transistors 3 and 4 exceeds the base-emitter voltage V _BE of transistors 15 and 16. Therefore, the collector current I _C in this case becomes I ₁ expressed by the following equation.

I ₁ =V _BE /R _E (2) _The protection line B shown in FIG _. This diagram illustrates the relationship between Here, when the load is inductive, the load curve L becomes an ellipse as shown in the figure, so when the conventional protection circuit as described above is used, the protection line B will meet the above load curve at the bend. Intersects with L. However, this part
Although it is located inside the ASO line and does not originally require protection, the protection circuit operates as described above, resulting in disadvantages such as distortion in the output signal.

The present invention has been made to eliminate such drawbacks, and by bringing the protection line closer to the ASO line, the range in which an amplifier having a protection circuit can operate normally is expanded. FIG. 3 shows an embodiment of the present invention, and FIG. 4C shows a limit line obtained by this embodiment. In Fig. 3, the same parts as in Fig. 1 are given the same reference numerals and their explanations are omitted. The cathodes of the Zener diodes 20 and 23 are connected to the base of the transistor 16 via the resistors 18 and 21, respectively, and the anode of the Zener diode 23 is connected to the base of the transistor 16 via the resistor 25, respectively. Similarly, a series circuit of Zener diodes 19 and 24 is connected to the negative power supply (-B), and each anode side is connected to the negative power supply (-B).
Connect the above Zener diode 1 so that it faces towards
Anodes 9 and 24 are connected to resistors 17 and 2, respectively.
2, and the anode of the Zener diode 24 is connected to the transistor 15 through the resistor 26, respectively.
Connect to the base of

In the above configuration, the power supply (+B) and (-
B) voltages are V _B and -V _B , and the Zener voltages of the Zener diodes 19, 20, 23, and 24 are respectively _VZ . Here the zener diode 19
To turn off both Zener diodes and 24, the voltage applied to both Zener diodes (2V _B - V _CE ) is equal to the Zener voltage V
It is sufficient if it is less than or equal to _Z. Therefore, the operating voltage V _CE of the transistor 3 at this time becomes V _CE >2V _B −V _Z (3), and if 2V _B −V _Z =V _T , it becomes the range shown in FIG. 4C. Therefore, in this range, one side of the first bridge circuit becomes the resistor 17 connected between the negative power supply (-B) and the base of the transistor 15. Therefore, if this resistance value is R ₃ , then the collector current I _C of transistors 3 and 4 for turning on transistors 15 and 16 is calculated using V _B in the first equation.
2V _B and R ₂ as R ₃ I _C ≧2R ₁ V _B +R ₃ V _BE /R _E (R ₁ +R ₃ )−
R ₁ V _CE /R _E (R ₁ +R ₃ ) (4), and the protection line C for the output transistor 3 has a gentle slope in this range.

Next, in order to turn on both Zener diodes 19 and 24, it is sufficient that the voltage applied to both Zener diodes (2V _B - V _CE ) is equal to or higher than the two Zener voltages (2V _Z ). At this time, the operating voltage V _CE of the transistor 3 becomes V _CE <2V _B −2V _Z (5), and if 2V _B −2V _Z =V _S , it becomes the range shown in FIG. 4A. Therefore, in this range, if the resistances of the Zener diodes 19 and 24 are ignored, one side of the first bridge circuit 9 is connected between the negative power supply (-B) and the base of the transistor 15 and the resistances 17, 22 and 24. 26 and a DC bias circuit including Zener diodes 19 and 24. Now, assuming that the current _IX flows through the resistor 9 (resistance value _R1 ), the base voltage _VX of the transistor 15 becomes _VX = _{VB - IXR1} - _VCE (6). Next, the current _I
is the sum of the currents flowing through 6. Since the voltage V ₃ applied to the resistor 17 is V _X +V _B , the current I ₃ flowing through the resistor 17 (R ₃ ) is I ₃ =V _X +V _B /R ₃ . Also, since the voltage V ₄ applied to the resistor 22 is lower than V ₃ by the Zener voltage V _Z , the current I ₄ flowing through the resistor 22 is I ₄ = V _X + V _B - V _Z /, where the resistance value is R ₄ . It becomes _R4 . Similarly, the voltage V ₅ applied to resistor 26 is V ₄
Since the zener voltage V _Z is lower than that of the resistor 2
The current I ₅ flowing through 6 becomes I ₅ =V _X +V _B -2V _Z /R ₅ when the resistance value is R ₅ . Therefore, the current V _X flowing through the resistor 9 is I _X = V _X + V _B /R ₃ + V _X + V _B - V _Z /R ₄
+V _X +V _B -2V _Z /R ₅ (7).

Here, in order for the transistor 15 to turn on, the voltage drop across the resistor 5 is greater than the voltage drop across the resistor 9, and the difference needs to be greater than or equal to the base-emitter voltage V _BE of the transistor 15. I _C R _E −I _X R ₁ ≧V _BE (8). Therefore, by substituting V _X in equation (6) into equation (7) to find I _X , and substituting it into equation (8), we get becomes. Here, R _P is the parallel resistance value of the resistors 17, 22, and 26, and 1/R _P =1/R ₃ +1/R ₄ +1/R ₅ . Therefore, since R _P <R ₃ , equations (9) and
As is clear from comparing the second terms on the right side of equation (4), the protection line C has a steep slope in this range.

Next, in order for the Zener diode 24 to turn on as described above, the Zener diode 19 must first be turned on to cause a voltage drop across the resistor 22. Therefore, the operating voltage V _CE of transistor 3
When V _S <V _CE <V _T (10) is in the middle of the ranges indicated by A and C above, only the Zener diode 19 is turned on, resulting in the range shown in FIG. 4B. Therefore, in this range, one side of the first bridge circuit is constituted by a DC bias circuit including resistors 17 and 22 connected between the negative power supply (-B) and the base of the transistor 15, and a Zener diode 19. will be done. Therefore, as in the above case, the current I _X flowing through the resistor 9 is the sum of I ₃ and I ₄ , and I _X = V _X + V _B /R ₃ + V _X + V _B - V _Z /R ₄ (
11) becomes. Therefore, by substituting V _X in equation (6) into equation (11), _I
By finding and substituting into equation (8), we get becomes. Here, R _Q is the parallel resistance value of the resistors 17 and 22, and 1/R _Q =1/R ₃ +1/R ₄ . Therefore, since R _P < R _Q < R ₃ , the
As is clear from comparing the second terms on the right side of equations (4), (9), and (12), the slope of the protection line C in this range is between the slopes of A and C above. become.

As mentioned above, the protection line C has a bending point of 2
Therefore, the protection line can be more accurately approximated to the ASO line A.

The above explanation relates to the operation of the transistor 15 for protecting the transistor 3, but the same applies to the operation of the transistor 16 for protecting the transistor 4, so a detailed explanation will be omitted. Note that the number of Zener diodes connected in series is not limited to two as in the above embodiment, but a large number of Zener diodes may be connected in series, and a resistor is connected between the connection point of each Zener diode and the base of the protection transistor. By connecting the lines, it is possible to provide multiple bending points, and in this case, the protection line can be more accurately brought closer to the ASO line. Furthermore, the Zener voltages of the Zener diodes may be different from each other, and the Zener diodes do not necessarily have to be Zener diodes, and when a large number of bending points are provided, general rectifying diodes may be connected with opposite polarities. Further, the series circuit of Zener diodes does not necessarily need to be connected to a power source for driving each output transistor, but may be connected to another power source having a different voltage.

As described above, according to the present invention, even more than the conventional
Since the protection circuit can be operated close to the ASO line, an excellent effect can be obtained in that the range in which the amplifier can operate normally without being subject to unnecessary protection operations can be expanded.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional amplifier protection circuit, FIG. 2 is a diagram explaining its operation, FIG. 3 is a circuit diagram showing an embodiment of the present invention, and FIG. 4 is a diagram explaining its operation. In the figure, 3, 4, 15 and 16 are transistors, 1
9, 20, 23 and 24 are Zena diodes, 8
is the load.

Claims (1)

[Claims] 1 Connect the emitter of the output transistor to the load end via the emitter resistor, connect the collector and emitter of the protection transistor between the base of the output transistor and the load end, and connect the base of the protection transistor to the load end via the detection resistor. In the amplifier connected to the emitter of the output transistor, connect one end of the series circuit formed by connecting multiple diodes in series to the power supply, and connect the other end and the connection point of each diode to the base of the protection transistor. An amplifier protection circuit characterized by connecting a resistor.