JPH0314813Y2 - - Google Patents

Description

[Detailed description of the invention] (a) Industrial application field The present invention relates to the improvement of amplifiers. In particular, the present invention is designed to prevent damage caused by overvoltage of the power supply to amplifiers equipped with a circuit to prevent shock noise when turning on the power. related to the amplifier. (b) Prior Art An amplifier as shown in FIG. 1 is known which is equipped with a circuit for preventing a shock noise when the power is turned on. In FIG. 1, 1 is a front-stage differential amplifier circuit, 2 is a rear-stage push-pull amplifier circuit, and 3 is a shock noise prevention circuit. The input signal applied to the input terminal 4 is amplified by the differential amplifier circuit 1 and then applied from the collector of the PNP transistor 5 to the front drive transistor 6 of the push-pull amplifier circuit 2. It is amplified to class A. Of the amplified signals obtained at the collector of the predrive transistor 6, the positive signals are further amplified by the first drive transistor 7 and the first output transistor 8, and the negative signals are further amplified by the second drive transistor 9 and the first output transistor 8. The signal is further amplified by the second output transistor 10 and applied from the output midpoint A to the speaker 11 in a push-pull relationship. At this time, since the current mirror circuit 16 consisting of the decoupling capacitor 12, the resistor 13, and the transistors 14 and 15 is connected to the negative feedback circuit of the amplifier, the resistance value of the resistor 13 and
If it is set equal to the sum of the resistance values of the resistors 17 and 18, the voltage at the output midpoint A will be equal to the voltage at one end (point B) of the decoupling capacitor 12, and its value will be 1/2V _CC . On the other hand, in the shock noise prevention circuit 3 , the base is at point B.
a second transistor 22 whose base is connected to the dividing point of the resistors 20 and 21; and a third transistor 2 whose base is connected to the collector of the first transistor 19.
3, the collector of the third transistor 23 and the first
A diode 24 is connected between the base of the drive transistor 7 and the decoupling capacitor 12 to prevent the occurrence of a shock noise when the power is turned on. be. The operation of this shock noise prevention circuit 3 will be explained in more detail. When the power is turned on with the charge in the decoupling capacitor 12 being zero, current flows first through the resistors 20 and 21, and then through the second resistor.
The base voltage of transistor 22 rises to a predetermined value. Therefore, the first transistor 19 of the first and second transistors 19 and 22 that are differentially connected
is on, the second transistor 22 is off, and all the output current of the first constant current source 25 becomes the collector current of the first transistor 19. When the first transistor 19 is turned on, the third transistor 23 is also turned on, and the current supplied from the second constant current source 26 to the resistors 20 and 21 is bypassed. Therefore, the voltage drop across the resistor 21 becomes small, and the base voltage of the second transistor 22 decreases. At the same time, the voltage drop across the resistor 20 becomes small, the cathode (point C) voltage of the diode 24 decreases, and the diode 24 turns on. The diode 24
When turned on, the output constant current of the third constant current source 27 flows through the diode 24 and the third transistor 23.
, the first drive transistor 7 and the first output transistor 8 are not turned on, and the output capacitor 28 is not charged. As time passes from this state, as the decoupling capacitor 12 is charged, the voltage at point B increases, the collector current of the first transistor 19 decreases, and the collector current of the third transistor 23 also decreases. . Therefore, the current flowing through the resistors 20 and 21 increases, and the base voltage of the second transistor 22 and the voltage at point C increase. However, the point C
When the voltage increases, the current flowing through the diode 24 decreases, and the decreased current is supplied to the base of the first drive transistor 7, so the first drive transistor 7 and the first output transistor 8 are turned on, and the output capacitor 28 charges are performed. The charging current at that time is the decoupling capacitor 1
2, the output capacitor 28 is charged slowly and no shock noise is generated. When the charging of the decoupling capacitor 12 progresses and the voltage at point B reaches 1/2V _CC (however, V _CC is the power supply voltage), the base voltage of the second transistor 22 also reaches 1/2V _CC , and the first and the second transistors 19 and 22 become in a balanced state. At that time, if the values of resistors 20 and 21 are made equal, the voltage at point C becomes V _CC , diode 24 is turned off, and the voltage at output midpoint A is also 1/1.
2V _CC and the amplifier is in steady state. Since the shock noise prevention circuit 3 is disconnected from the amplifier after the diode 24 is turned off, the shock noise prevention circuit 3 does not have an adverse effect on the amplifier in a steady state. As mentioned above, by adding the shock noise prevention circuit 3 , it is possible to reliably prevent the occurrence of shock noise when the power is turned on, so the circuit shown in FIG. 1 has been widely used in the past. However, when a protection circuit is added to the amplifier of FIG. 1 to prevent destruction of the output transistor due to overvoltage of the power supply, a new problem arises. That is, the protection circuit normally protects the first and second output transistors 8 and 10 during overvoltage of the power supply.
is turned off, and the first and second output transistors 8
However, if the overvoltage condition continues for a long time, the voltage at the output midpoint A decreases due to the natural discharge of the output capacitor 28. On the other hand, the terminal voltage of the decoupling capacitor 12 increases in response to the overvoltage of the power supply, and is higher than normal. Therefore, when the overvoltage disappears in that state and the protective operation of the protection circuit is released, the output capacitor 28 is rapidly charged by the first output transistor 8, and the voltage at the output midpoint A is the same as that of the decoupling capacitor 12. This has the disadvantage that the voltage increases beyond the terminal voltage, producing a loud shock noise and, in extreme cases, causing destruction of the output transistor and damage to the voice coil of the speaker. (c) Purpose of the invention The present invention has been made in view of the above points, and effectively utilizes the shock noise prevention circuit when the power is turned on.
This is intended to prevent the negative effects of adding a protection circuit to protect the output transistor against overvoltage of the power supply. (d) Structure of the invention The amplifier according to the invention includes an amplifier circuit that amplifies a signal, a shock noise prevention circuit that prevents the generation of shock noise when the power is turned on according to the terminal voltage of the decoupling capacitor, and an overvoltage of the power supply. a power supply voltage detection circuit that detects the detection circuit; a first control circuit that turns off the output transistor in accordance with an output signal of the detection circuit;
and a second control circuit that discharges the charge of the decoupling capacitor according to the output signal of the detection circuit. (E) Embodiment Figure 2 is a circuit diagram showing an embodiment of the present invention.
2 is a push-pull amplifier circuit including a pre-drive transistor 6, first and second drive transistors 7 and 9, and first and second output transistors 8 and 10;
11 is a speaker serving as a load connected to the output midpoint A of the push-pull amplifier circuit 2 via an output capacitor 28; 29 is a fourth transistor 31 having a Zener diode 30 connected to its base; A differential power supply voltage detection circuit comprising a fifth transistor 32 that is differentially connected; 33 is a first control transistor that operates according to the output signal of the detection circuit 29 ; 34 is an operation of the first control transistor 33; a first short-circuit transistor for turning off the first output transistor 8 in response to the output signal; 35 a second control transistor that operates in response to the output signal of the detection circuit 29 ; 36 a second control transistor that operates in response to the operation of the second control transistor 35; a second short-circuit transistor for turning off the second output transistor 10; 37, a third control transistor that operates in response to the output signal of the detection circuit 29 ; and 38, in response to the operation of the third control transistor 37; This is a discharge transistor for discharging the charge of the decoupling capacitor 12 shown in FIG. Note that the first and second control transistors 33 and 35 and the first and second shorting transistors 34 and 36 constitute a first control circuit, and the third control transistor 37
and discharge transistor 38 constitute a second control circuit. In addition, the front-stage differential amplifier circuit 1 of the amplifier
Since the circuit 2 and the shock noise prevention circuit 2 overlap, they are omitted in FIG. 2, and if an explanation is necessary, the circuits and drawing numbers shown in FIG. 1 will be used. Now, assuming that the power supply voltage is in a normal state and its value is V _CC , the fourth voltage of the power supply voltage detection circuit 29
The base voltage V ₁ of the transistor 31 is V _Z (where V _Z is the Zener voltage of the Zener diode 30), and the base voltage V ₂ of the fifth transistor 32 is R ₂ /R ₁ +R ₂ ·V _CC (However, R ₁ and R ₂ are the resistance values of resistors 39 and 40.) Therefore, resistors 39 and 40 are connected so that V ₁ >V ₂
If the resistance values R ₁ and R ₂ are determined, the fourth transistor 31 is turned on, the fifth transistor 32 is turned off, and the first to third control transistors 33 to 37 are turned on.
Both are off. Therefore, as long as the power supply voltage is in a normal state, no protection operation is performed. Then the power supply goes into overvoltage and the supply voltage drops to V _CC +
If it becomes △V _CC , the fourth transistor 31
The base voltage V ₁ of the fifth transistor 32 does not change, but the base voltage V ₂ of the fifth transistor 32 increases to R ₂ /R ₁ + R ₂ (V _CC +△ V _CC ), and if V ₁ <V ₂ then, Ba,
The fourth transistor 31 is turned off and the fifth transistor 32 is turned on. However, the fifth transistor 3
2 turns on, the first to third control transistors 33 to 37 turn on, and in response to the first control transistor 33 turning on, the first shorting transistor 34 connects the base of the first drive transistor 7 to the output midpoint A. The first drive transistor 7 and the first output transistor 8 are turned off. Further, in response to the second control transistor 35 being turned on, the second shorting transistor 36 short-circuits the base of the second drive transistor 9 to the output midpoint A, and turns off the second drive transistor 9 and the second output transistor 10. do. Therefore, the first and second output transistors 8 and 10 are protected from destruction due to overvoltage of the power supply. Furthermore, the discharge transistor 38 is turned on in response to the third control transistor 37 being turned on. The collector (point D) of the discharge transistor 38 is
Since it is connected to one end (point B) of the decoupling capacitor 12 in FIG. 1, when the discharge transistor 38 is turned on, discharging of the decoupling capacitor 12 is started. The discharge time constant at this time is determined by the capacitance of the decoupling capacitor 12 and the resistance value of the resistor 41, and the discharge time constant may be made smaller than the discharge time constant of the output capacitor 28. Even if the overvoltage condition continues for a long time and the voltage at the output midpoint A sufficiently drops due to the natural discharge of the output capacitor 28 and reaches the ground potential, the present invention does not cause any drawbacks such as the generation of new shock noise.
In other words, the discharge transistor 38
Due to this action, the terminal voltage of the decoupling capacitor 12 is also maintained at the ground potential. For that reason,
When the overvoltage is released after a long period of overvoltage, the fourth transistor 31 is turned on again, the fifth transistor 32 is turned off, and the first to third control transistors 33 to 37 and the first and second shorting transistors are turned off. 34 and 36, discharge transistor 38
is turned off, but at that time the shock noise prevention circuit 3 immediately starts operating and the output constant current of the third constant current source 27 is bypassed, so the output capacitor 28 is not charged suddenly and the shock noise is suppressed. There are no drawbacks such as generation. The operation of the shock noise prevention circuit 3 is exactly the same as when the power is turned on, and since it has been described in detail previously, the description thereof will be omitted here. The permissible range of power supply voltage change, ie, how much power supply voltage is considered to be an overvoltage, is determined by the withstand voltages of the first and second output transistors 8 and 10, etc. Accordingly, the fifth transistor 32
By determining the values of resistors 39 and 40 connected to the bases of , the desired overvoltage protection is achieved. (f) Effects of the invention As described above, the amplifier according to the invention can reliably prevent the occurrence of shock noise when the power is turned on. Furthermore, the amplifier according to the present invention can reliably protect the output transistor from destruction due to overvoltage of the power supply.
Furthermore, the amplifier according to the present invention can prevent defects such as the generation of shock noise even when a long-term overvoltage condition is released, and the circuit configuration for preventing the above-mentioned defects is simple. It is a practical product with various advantages such as.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional amplifier, and FIG. 2 is a circuit diagram showing an embodiment of the present invention. Explanation of main figure numbers, 1 ...Differential amplifier circuit, 2 ...
Push-pull amplifier circuit, 3 ... Shock noise prevention circuit, 8, 10... Output transistor, 12... Decoupling capacitor, 28... Output capacitor, 29 ... Power supply voltage detection circuit, 33, 35, 3
7... Control transistor, 34, 36... Short circuit transistor, 38... Discharge transistor.

Claims (1)

[Scope of utility model registration request] In an amplifier equipped with a shock noise prevention circuit that operates when the power is turned on and gradually charges the output capacitor according to the terminal voltage of the decoupling capacitor to prevent the occurrence of shock noise when the power is turned on, the power supply voltage exceeds a predetermined value. a first control circuit that turns off the output transistor of the amplifier in accordance with the output signal of the detection circuit; and a first control circuit that turns off the output transistor of the amplifier in response to the output signal of the detection circuit; and a second control circuit for discharging the charge of the ring capacitor, and operates the first control circuit to protect the output transistor while the power supply voltage is at or above the predetermined value. An amplifier characterized in that when the decoupling capacitor becomes lower than the decoupling capacitor, the shock noise prevention circuit is operated in accordance with charging of the decoupling capacitor to prevent generation of shock noise.