JPS6117620Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to a protection circuit for an output amplifier circuit suitable for use in the output stage of audio equipment, and is particularly designed to effectively prevent overload of the output amplifier circuit with a simple configuration. be. In general, in an output amplifier circuit, if the load is short-circuited, an excessive current flows through the load, damaging the elements that make up the output amplifier circuit and potentially destroying the load. Therefore, a protection circuit is installed to prevent overload. is provided.

Conventionally, a protection circuit for this output amplifier circuit using a relay as shown in FIG. 1 has been proposed.

That is, in FIG. 1, 1 indicates an audio signal input terminal, and the audio signal supplied to this audio signal input terminal 1 is transmitted via an input circuit to a first input terminal 3 of an integrated circuit 2 configured as an arithmetic circuit. The driving signal is supplied to the integrated circuit 2, and the driving signal is obtained from the first and second driving signal output terminals 4 and 5 of the integrated circuit 2, respectively. In this case, the integrated circuit 2 is constructed as shown in FIG. That is, this first input terminal 3 is connected to the base of one npn type transistor 6 constituting the differential amplifier, and the collector of this transistor 6 is connected to the first power supply terminal 8 via a resistor 7. And again 9
indicates a second input terminal, and this second input terminal 9 is connected to the base of the other npn transistor 10 constituting the differential amplifier, and this transistor 10
The collector of the transistor is connected to the first power supply terminal 8, the emitters of each of the transistors 6 and 10 are connected,
The mutual connection points of these emitters are connected to a second power supply terminal 12 via a resistor 11. Also transistor 6
The mutual connection point of the collector and resistor 7 is connected to the base of a pnp transistor 13, and the emitter of this transistor 13 is connected to the first power supply terminal 8,
The collector of this transistor 13 is connected to diode 1
4, 15 and a resistor 16 through a series circuit.
Connect to the power supply end 12 of the Also, this transistor 1
3 and the anode of the diode 14 are connected to the base of an npn transistor 17, and from the collector of this transistor 17, the first
The drive signal output terminal 4 of is derived, the connection point between the cathode of the diode 15 and the resistor 16 is connected to the base of the PNP transistor 18, the emitters of this transistor 17 and the emitters of the transistor 18 are connected together, From this mutual connection point, the detection end 1
9 is derived, and the second drive signal output terminal 5 is derived from the collector of this transistor 18.

Since the integrated circuit 2 is configured as described above,
When an audio signal is supplied to the audio signal input terminal 1, drive signals are obtained at the first and second drive signal output terminals 4 and 5, respectively.

The first power supply terminal 8 of the integrated circuit 2 is connected via a resistor 20 to a first power supply terminal 21 to which a positive DC voltage + _Vcc is supplied. The second power supply terminal 12 of the integrated circuit 2 is connected to a second power supply terminal 23 to which a negative DC voltage _-Vcc is supplied via a resistor 22. The negative DC voltage Vb required by the integrated circuit 2 is supplied to the second power supply terminal 12. In this case, the first
The voltages + _Vcc and _-Vcc supplied to the power supply terminal 21 and the second power supply terminal 23, respectively, are DC voltages having the same absolute value. The first drive signal output terminal 4 of this integrated circuit 2 is connected via a resistor 24 to the base of a pnp transistor 25 constituting the SEPP circuit, and a drive signal is supplied to the base of this transistor 25. A second drive signal output terminal 5 of the integrated circuit 2 is connected via a resistor 26 to the base of an npn transistor 27 constituting the SEPP circuit, and a drive signal is supplied to the base of the transistor 27. Further, an output terminal 28 is led out from the mutual connection point of the respective collectors of the transistors 25 and 27 constituting this SEPP circuit via a connection switch 41a of a relay 41 which will be described later and constituting a protection circuit. Further, a resistor 29 forming a feedback circuit connects the collectors of the transistors 25 and 27 to each other.
is connected to the second input terminal 9 of the integrated circuit 2 via the terminal 9, and the connection point of the respective collectors of the transistors 25 and 27 is connected to the detection terminal 19 via a series circuit of a capacitor 30 and a resistor 31. do. Further, this detection end 19 is grounded via a resistor 32. Further, the output signal obtained at the output terminal 28 is supplied to a load such as a speaker 33.

Further, the mutual connection point of the collectors of these transistors 25 and 27 is connected to the cathode of a diode 35 via a resistor 34.
The anode of the integrated circuit 2 is grounded via a smoothing capacitor 36, and the detection end 19 of the integrated circuit 2 is connected to a diode 3.
The cathode of this diode 37 is connected to the connection point of the diode 35 and the capacitor 36 through the resistor 38, and the detection of this integrated circuit 2 is performed at the connection point of the diode 35 and the capacitor 36. The positive side of the AC signal obtained at the terminal 19 is rectified and smoothed, and the transistor 25
and rectifying and smoothing the negative side of the output signal obtained at the connection point of each collector of 27,
This superimposed signal is supplied to the base of the npn transistor 39. In this case, the superimposed signal is normally set at approximately zero potential or negative potential. Also, the emitter of this transistor 39 is grounded, and the collector of this transistor 39 is connected to npn.
It is connected to the base of a type transistor 40. Further, the emitter of this transistor 40 is grounded, and the collector of this transistor 40 is connected to the first power supply terminal 21 to which a positive DC voltage + _Vcc is supplied via a coil 41b constituting a relay 41, and this power supply terminal 21 is grounded through a voltage dividing circuit made up of resistors 42 and 43, and the connection point of resistors 42 and 43 is connected to the base of transistor 40 through resistor 44. In this case, normally the transistor 40 is connected to the connection point between the resistors 42 and 43.
The structure is such that a bias voltage sufficient to turn on the transistor 40 is supplied to the base of the transistor 40.

The protection circuit using the relay 41 shown in FIG. 1 is normally constructed so that the base potential of the transistor 39 is zero or negative.
9 is off, and to keep the transistor 40 on, a current flows through the coil 41b constituting the relay 41. Therefore, the connection switch 41a remains on, and a current is obtained at the connection point of the collectors of each of the transistors 25 and 27. An output signal is provided to load 33.

Next, if the load 33 is short-circuited for some reason, the mutual connection point of the collectors of the transistors 25 and 27 will be at ground potential, and therefore the positive side of the AC signal obtained at the detection terminal 19 of the integrated circuit 2 will be rectified. Since only the smoothed positive voltage is supplied to the base of the transistor 39, the transistor 39 is turned on, and as a result, the base potential of the transistor 40 becomes approximately zero potential, so this transistor 40 is turned off, thus forming a relay 41. coil 41
Since no current flows through b, the connection switch 41a is turned off and the load 33 is cut off.

In this way, the protection circuit of the output amplifier circuit shown in FIG. 1 is such that if the load 33 is short-circuited for some reason and an overcurrent flows through the load 33, the relay 41 can shut off the load 33. This makes it possible to prevent overloading of this output amplifier circuit.

However, since the protection circuit for the output amplifier circuit shown in FIG. 1 uses a relay 41, the circuit configuration is complicated, and since it involves mechanical operation, it has the disadvantage of slow operation. It was hot.

In view of these points, the present invention provides a protection circuit for preventing overload of the output amplifier circuit with a simple circuit configuration and moreover effectively prevents overload.

The protection circuit of the output amplifier circuit of the present invention will be explained below with reference to FIG.

In FIG. 3, parts corresponding to those in FIG. 1 are designated by the same reference numerals, and detailed explanation thereof will be omitted.

In FIG. 3, the emitter of the output transistor 25 constituting the SEPP circuit is connected to the first power supply terminal 21 to which a positive DC voltage + _Vcc is supplied via a resistor 45 for overcurrent detection, and this output A diode 46 is connected to the emitter of the transistor 25 to prevent backflow.
The anode of this diode 46 is connected to the cathode of the npn transistor 4 constituting a protection circuit.
7, the emitter of this transistor 47 is connected to the first power supply terminal 21, the collector of this transistor 47 is connected to the anode of a diode 48 for backflow prevention, and this diode 48
The cathode of the transistor 47 is connected to the second power supply terminal 12 of the integrated circuit 2 through a resistor 49, and the collector of this transistor 47 is connected to the resistor 50 and the resistor 51.
It is connected to the second power supply terminal 23 via a series circuit. In this case, under normal conditions, overcurrent detection resistor 4
The resistor 45 is selected so that the voltage across the resistor 45 is lower than the sum of the forward drop voltage V _D of the diode 46 and the base-emitter voltage V _BE of the transistor 47. The value is chosen to be equal to the value of resistor 22. Further, the emitter of the output transistor 27 is connected to the second power supply terminal 23 to which a negative DC voltage _-Vcc is supplied via the overcurrent detection resistor 52, and the emitter of the output transistor 27 is connected to the second power supply terminal 23 to which a negative DC voltage -Vcc is supplied. diode 5
3, the cathode of this diode 53 is connected to the base of an npn transistor 54 constituting the protection circuit, the emitter of this transistor 54 is connected to the second power supply terminal 23, and the collector of this transistor 54 is connected to the second power supply terminal 23. Connected to the cathode of a diode 55 for backflow prevention, and this diode 5
The anode of the transistor 54 is connected to the first power supply end 8 of the integrated circuit 2 via a resistor 56, and the collector of the transistor 54 is connected to the first power supply terminal 8 of the integrated circuit 2 through a resistor 56.
It is connected to the first power supply terminal 21 through a series circuit of. In this case, the voltage across the overcurrent detection resistor 52 is lower than the sum of the forward drop voltage V _D of the diode 53 and the base-emitter voltage V _BE of the transistor 54 under normal conditions. This resistor 52 is selected and the value of resistor 56 is selected to be equal to the value of resistor 20. Further, the mutual connection point of resistor 50 and resistor 51 is connected to the base of transistor 54, and resistor 57 and resistor 51 are connected to each other.
The mutual connection point of 8 is connected to the base of the transistor 47.

Since the present invention is configured as described above, first, when a load current having a value within the normal range is flowing through the load 33, the first drive signal output terminal 4 of the integrated circuit 2
Since a drive signal in the normal range is supplied to the base of the output transistor 25, the voltage drop across the overcurrent detection resistor 45 is equal to the forward drop voltage V _D of the reverse current prevention diode 46 and the base voltage of the transistor 47.
The transistor ₄₇ remains off, so that the supply voltage Vb supplied to the second power supply terminal 12 of the integrated circuit 2 is also smaller than the sum of the emitter-emitter voltage VBE. maintain the voltage value. Similarly, since a drive signal in the normal range is supplied from the second drive signal output terminal 5 of the integrated circuit 2 to the base of the output transistor 27, the voltage drop across the overcurrent detection resistor 52 is caused by the backflow prevention diode 53. The transistor 54 remains off and the power supply voltage supplied to the first power supply end 8 of the integrated circuit 2 is less than the sum of the directional drop voltage V _D and the base-emitter voltage V _BE of the transistor 54 .
Va also maintains the voltage value required by this integrated circuit 2. When a normal current flows through the load 33 in this way, the first power supply terminal 8 and the second power supply terminal 12 of this integrated circuit 2 have a power supply voltage of a voltage value required by this integrated circuit 2. This output amplifier circuit operates stably as before.

Next, when the load 33 is short-circuited for some reason, the first drive signal output terminal 4 of the integrated circuit 2
When a drive signal is obtained in
A large current flows through the resistor 45, causing a large voltage drop across the resistor 45. This voltage drop is the forward drop voltage V _D of the reverse current prevention diode 56 and the base-emitter voltage V _BE of the transistor 47.
The transistor 47 is turned on because it is larger than the sum of the above. Therefore, the first power terminal 21
As a result, current flows to the second power supply terminal 23 via the emitter of the transistor 47 → collector → diode 48 for backflow prevention → resistor 49 → resistor 22, and the current flows from the first power supply terminal 21 to the emitter of the transistor 47. A current flows to the second power supply terminal 23 through the circuit of -> collector -> resistor 50 -> resistor 51. In this case, since the value of the resistor 49 and the value of the resistor 22 are selected to be equal, the power supply voltage Vb supplied to the second power supply end 12 of the integrated circuit 2 from the connection point of the resistors 49 and 22. becomes approximately zero voltage, and the operation of the integrated circuit 2 stops. However, in this case, current also flows through the series circuit of resistors 50 and 51, and the voltage drop across resistor 51 becomes larger than the base-emitter voltage V _BE of transistor 54. turns on,
From the first power supply terminal 21, resistor 21 → resistor 56
A current flows to the second power supply terminal 23 via the diode → collector of the transistor 54 → emitter circuit, and the current flows from the first power supply terminal 21 to the resistor 58 → resistor 57 → collector of the transistor 54 → emitter circuit. via the second power terminal 23
Current flows through the resistor 56 and the resistor 20
Since the power supply voltage Va supplied to the first power supply end 8 of the integrated circuit 2 becomes approximately zero voltage, the operation of the integrated circuit 2 is completely stopped. In this case, the operation of the integrated circuit 2 is stopped and no drive signal is supplied to the base of the output transistor 25, so the output transistor 25 is turned off, but current flows through the series circuit of the resistors 58 and 57.
The voltage drop across resistor 58 is the voltage drop across transistor 4.
Since the voltage V _BE between the base and emitter of the transistor 47 is higher than the voltage V BE between the base and the emitter of the transistor 47, the transistor 47 remains on.
Transistor 54 also remains on as described above.
Therefore, the power supply voltages Va and Vb supplied to the first power supply terminal 8 and the second power supply terminal 12 of the integrated circuit 2, respectively.
Since the value of is maintained at approximately zero voltage, this integrated circuit 2
The operation of the output amplifier circuit stops, and no current flows through the load 33. This prevents the output amplifier circuit from being overloaded, and the elements making up the output amplifier circuit and the load 33 are not damaged. do not have.

Furthermore, when the load 33 is short-circuited for some reason and a drive signal is obtained at the second drive signal output terminal 5 of the integrated circuit 2, this drive signal is supplied to the base of the transistor 27. A large current flows through this transistor 27 via the overcurrent detection resistor 52, and a large voltage drop occurs between both ends of this resistor 52. This voltage drop is greater than the sum of the forward drop voltage V _D of the reverse current prevention diode 53 and the base-emitter voltage V _BE of the transistor 54, so the transistor 54
4 is turned on. Therefore, a current flows from the first power supply terminal 21 to the second power supply terminal 23 via the resistor 20 → resistor 56 → diode 55 → collector of the transistor 54 → emitter, and the current flows from the first current terminal 21 to the resistor. device 58→resistor 57→
A current flows through the collector->emitter circuit of the transistor 54. In this case, the value of the resistor 56 is chosen equal to the value of the resistor 20, so that a voltage is obtained at the mutual junction of the resistors 20 and 56 and supplied to the first power supply terminal 8 of the integrated circuit 2. Power-supply voltage
Va becomes approximately zero voltage, and the operation of the integrated circuit 2 stops. However, in this case, current also flows through the series circuit of resistors 58 and 57, and the voltage drop across resistor 58 becomes larger than the base-emitter voltage V _BE of transistor 47.
is also turned on, and from the first power supply terminal 21 the emitter of the transistor 47 → the collector → the diode 48
A current flows to the second power supply terminal 23 through the circuit of → resistor 49 → resistor 22, and the current flows from the first power supply terminal 21 to the emitter of the transistor 47 → collector → resistor 50 → through the circuit of resistor 51. Therefore, current flows through the second power supply terminal 23, and the resistor 49
Since the value of Vb and the value of the resistor 22 are selected to be equal, the power supply voltage Vb supplied to the second power supply terminal 12 of the integrated circuit 2 becomes approximately zero voltage, and the integrated circuit 2 completely stops its operation. . In this case, the integrated circuit 2 stops operating and no drive signal is supplied to the base of the output transistor 25, so the output transistor 25 is turned off, but the resistors 50 and 51
Since current flows through the series circuit of the resistor 51 and the voltage drop across the resistor 51 becomes larger than the base-emitter voltage V _BE of the transistor 54, the transistor 54 remains on, and similarly the transistor 5
By keeping transistor 4 on, transistor 47 also remains on as described above. Therefore, since the values of the power supply voltages Va and Vb supplied to the first power supply terminal 8 and the second power supply terminal 12 of the integrated circuit 2 are maintained at approximately zero voltage, the operation of the integrated circuit 2 is stopped.
No current flows through the load 33, and the output amplifier circuit is prevented from being overloaded, so that the elements constituting the output amplifier circuit and the load 33 are not destroyed.

As described above, according to the protection circuit for the output amplifier circuit of the present invention, the operation of the output amplifier circuit is not affected in normal times, and the output amplifier circuit continues to operate normally. In addition, if the load 33 is short-circuited for some reason and an excessive current flows through the load 33, the power supply voltage Va supplied to the first power supply terminal 8 and the second power supply terminal 12 of the integrated circuit 2 as well as
Vb is set to zero voltage, which stops the operation of the integrated circuit 2 and prevents the output amplifier circuit from being overloaded, and there is no risk of destroying the elements constituting the output amplifier circuit or destroying the load. do not have. Furthermore, since the protection circuit of the output amplifier circuit of the present invention is controlled by electronic operation, the response is faster and more reliable overload prevention can be achieved than that using the relay shown in Figure 1. Since no circuit is used, the circuit configuration becomes simpler.

Note that the present invention is not limited to the above-described embodiments, and it goes without saying that various other configurations may be adopted without departing from the gist of the present invention.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing a protection circuit of a conventional output amplifier circuit, Fig. 2 is a connection diagram showing an example of an integrated circuit, and Fig. 3 is a block diagram showing an embodiment of a protection circuit of the output amplifier circuit of the present invention. It is. 1 is an audio signal input terminal, 2 is an integrated circuit, 3 is a first input terminal, 4 is a first drive signal output terminal, 5 is a second drive signal output terminal, 8 is a first power supply terminal, 9 is a a second input terminal, 12 is a second power supply terminal, 20, 22,
45, 49, 50, 51, 52, 56, 57 and 58 are resistors, 21 is a first power supply terminal, 23
is a second power supply terminal, 25, 27, 47, and 54 are transistors, 28 is an output terminal, 33 is a load, and 46, 48, 53, and 55 are diodes, respectively.

Claims (1)

[Scope of utility model registration request] The first and second signal input terminals are supplied with DC voltages having equal positive and negative absolute values, respectively.
an integrated circuit having an arithmetic circuit configuration having first and second power supply terminals and first and second drive signal output terminals respectively connected to power supply terminals of the integrated circuit; and first and second drive signals of the integrated circuit. In an output amplifier circuit comprising first and second output transistors having different conductivity types and forming an SEPP circuit to which a drive signal obtained from an output terminal is supplied,
the respective emitters of the output transistors and the first
A resistor for overcurrent detection is inserted between the power supply terminal and the second power supply terminal, and the emitter of the first output transistor is connected to the base of the third transistor via the first backflow prevention diode. and connect the emitter of the third transistor to the first transistor.
The collector of the third transistor is connected to the second power supply terminal of the integrated circuit via a second backflow prevention diode, and the emitter of the second output transistor is connected to the voltage terminal of the third transistor. is connected to the base of a fourth transistor via a backflow prevention diode, the emitter of the fourth transistor is connected to the second power supply terminal, and the collector of the fourth transistor is connected to the fourth backflow prevention diode. A protection circuit for an output amplifier circuit, characterized in that the protection circuit is connected to a first power supply terminal of the integrated circuit through a diode.