JPH0744400B2 - Power amplifier - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power amplifier, and more particularly to a power amplifier suitable for parallel connection used when a plurality of devices are connected in parallel to increase output.

[Prior Art] Conventionally, when a power amplifier with a large output is required, for example, when the number of speakers is large in a broadcasting system used in factories or hotels, this requirement is satisfied by one power amplifier. Instead, it is common to meet this requirement by connecting a plurality of small capacity power amplifiers in parallel. In this type of usage, the weight of the amplifier alone can be reduced, so
In addition to being advantageous in management and transportation, it has the advantage that a broadcasting system with various power capacities can be freely configured as needed.

However, in the usage mode in which a plurality of power amplifiers are connected in parallel as described above, there are the following drawbacks. That is, when some power amplifiers out of the plurality of power amplifiers stop outputting the intended signal due to a failure or the like,
The output of another normal amplifier is added from the output terminal of the defective amplifier (hereinafter, referred to as reverse drive), and a large current flows through the power amplification element. As a result, the load impedance of the normal power amplifier is reduced and the power amplifier is overloaded, which causes a defect that the normal amplifier also fails. In particular, when each power amplifier has a negative feedback circuit, the internal impedance seen from each output terminal becomes very low, which causes a serious overload on other normal amplifiers. Therefore, in a usage mode in which a plurality of power amplifiers are connected in parallel and used, even if one power amplifier fails, the individual power amplifiers are also considered so as not to damage other power amplifiers. Safety design is required.

Against this background, the applicant of the present invention has previously disclosed that the Japanese Patent Laid-Open No. 55-1454
I proposed the power amplifier shown in 09. This power amplifier restricts the short-circuit current flowing to the power amplifying element when the failed power amplifier cannot output the desired signal and is reversely driven, so that the other power amplifier connected in parallel can be It was intended to prevent a faulty power amplifier from becoming overloaded for a normal power amplifier.

[Problems to be Solved by the Invention] However, in the above-mentioned conventional type, when the reverse drive is applied to the failed power amplifier, the voltage induced on the primary side of the output transformer of the power amplifier is used. Since it was intended to limit the short-circuit current flowing to the power amplification element, it was impossible to always make the internal impedance of the output end of the power amplifier sufficiently large.

In view of the above problems in the conventional type, the present invention sufficiently increases the internal impedance with respect to the reverse drive signal,
Therefore, an object of the present invention is to provide a power amplifier which can prevent normal operation of other power amplifiers from being damaged by a failure of the power amplifier even when a plurality of power amplifiers are connected in parallel. Is.

[Means for Solving Problems] A power amplifier according to the present invention has a voltage amplifying unit for amplifying an input voltage and a power transistor forming a drain or collector grounded push-pull amplifier circuit A power amplifier connected to the output of the voltage amplifier via the driver transformer, and an output coupled to the output terminal via the output transformer, and connected to the source or emitter of the power transistor in series with the output transformer. And the resistance and the voltage induced on the primary side of the output transformer when a drive signal is received from the output end side, the operation starts when the source or emitter of the power transistor drops below the ground potential. The bias voltage of the power transistor is changed according to the voltage generated across the resistor due to the current flowing through the power transistor. And a bias control unit for converting the same to a different one.

[Operation] With the above-described configuration, when the reverse drive is applied to the failed power amplifier from the output end, the bias control unit causes the voltage of the power transistor due to the voltage induced on the primary side of the output transformer. The operation is started when the source or the emitter drops to a potential lower than the ground potential, and the bias voltage of the power transistor is changed according to the voltage generated across the resistor due to the current flowing in the power transistor. As a result, the bias voltage of the power transistor is controlled so as to reduce the overcurrent that tends to flow to the power transistor, so that the bias voltage of the power transistor accurately flows to the power transistor without being affected by the magnitude of the reverse drive voltage and other conditions. The current is limited. For this reason, the load of the failed power amplifier becomes extremely light with respect to the normal power amplifier.

Embodiments Embodiments of the present invention will be described below with reference to the drawings. The accompanying drawings show a power amplifier according to an embodiment of the present invention, in which a plurality of such power amplifiers are connected in parallel at an input terminal and an output terminal, respectively, so that a desired output power amplification system can be obtained. Can be configured. The power amplifier shown in the drawing is a voltage amplification unit A1 that amplifies an input signal applied to an input terminal, a driver transformer T1 that couples the voltage amplification unit A1 and the power amplification unit, a field effect transistor TR4 as a power transistor, Constant voltage power supply unit A2 that supplies constant voltage current to TR5, output transformer T2, and voltage amplification unit A1
The amplifier circuit is configured based on the above. Although the power transistors TR4 and TR5 are shown as power FETs in the drawings, it is obvious that they may be bipolar type transistors. Each power transistor TR
4, the drain D of TR5 is connected to the power source + V, and the source S is connected to one of the corresponding primary windings of the output transformer T2 via resistors R8 and R9, respectively. The other terminal of these primary windings is grounded through each fuse F1 and F2. That is, the power amplification unit is configured by a drain (collector) grounded push-pull method, and a negative feedback circuit is not provided to increase the internal impedance seen from the output end. Also, resistors R1, R2, diodes CD1, CD2, transistor TR1, resistor R12, variable resistor
VR1 and the like are circuits that generate and adjust the bias voltage applied from the power supply + V to the power transistors TR4 and TR5.

Further, a circuit including a diode CD7 connected to the source S of the power transistor TR4, resistors R4 and R5, a transistor TR2 and a diode CD3, and a diode CD8 connected to the source S of the power transistor TR5, resistors R6 and R7, a transistor The circuit formed by TR3 and diode CD4 is a control circuit that detects the source currents of the power transistors TR4 and TR5 and controls the bias voltage accordingly. The Zener diode CD11, the field effect transistor TR6 in which the gate and the source are connected, the resistor R3, the diodes CD5 and CD6 are the transistors of these control circuits.
It forms a constant current circuit to help the conduction of TR2 and TR3. Further, each diode CD9 and CD10 is for preventing a reverse voltage from being applied between the gate and source of each power transistor TR4, TR5 when receiving a reverse drive from the output end, and for bypassing the reverse drive voltage to the power supply + V. Is.

Next, the operation of the above power amplifier will be described. When a plurality of power amplifiers as described above are connected in parallel and all the amplifiers are normal, the signal voltage applied to the input terminal is amplified by the voltage amplifier A1 and the driver transformer T1
Is applied to the gates of the power transistors TR4 and TR5 with the reverse polarity via. Each power transistor TR4, TR5 operates as a source follower circuit, and push-pull-drives a corresponding primary coil of the output transformer T2 via resistors R8, R9, respectively, according to the voltage applied to the gate. As a result, the power-amplified signal is output from the primary winding of the output transformer T2 and is combined with the output of another power amplifier to drive a load such as a speaker.

Also, since the voltage at points A and B does not drop below the ground potential, TR2, R4, R5 and TR3, R
The control circuit consisting of 6 and R7 does not work.

Next, if one of the power amplifiers connected in parallel shuts off the power supply + V due to, for example, the power fuse being cut, or the power amplifier A1 fails to output a signal, its output transformer T2 The output of a normal power amplifier is applied to the secondary winding. For example, when a voltage of the illustrated polarity is applied to the secondary coil T2 of the output transformer T2, the primary coil P11
A voltage of the illustrated polarity is induced in each of P12 and P12. In this case, the positive voltage induced in the primary coil P11 is bypassed to the power supply + V terminal side via the diode CD9. At this time, the source S of the transistor TR4 has a resistance R
Its transistor TR4 to rise in the positive direction through 8
No large current will flow through it. On the other hand, the negative voltage induced in the primary coil P12 acts so as to raise the potential at the point B to a potential lower than the ground potential and similarly lower the source voltage of the transistor TR5 via the resistor R9. Therefore, at this time, if the gate bias voltage of the transistor TR5 remains fixed, the transistor TR5 drops in proportion to the induced voltage of the source voltage due to the fixed gate voltage, and a large short-circuit current flows. Therefore, in the present invention, the transistor TR5 responds to the voltage drop of the source resistor R9 due to the current flowing at this time.
The bias voltage is controlled. That is, when the voltage at the point B drops below the ground potential due to the voltage induced in the primary coil P12, ground-CD
11 transistor TR6-R3-CD6-CD8-R9 path and ground-CD11 current flows in the circuit including the transistor TR6-R3-CD6-R7-R6 path, and when the voltage across the source resistor R9 exceeds a certain value, The transistor TR3 starts to turn on. When the transistor TR3 is turned on, a current flows from the power supply + V terminal to the path of R2-CD4-TR3. For this reason, the anode of the diode CD4, that is, the gate voltage of the transistor TR5 also decreases as the voltage at the point B decreases, limiting the current flowing from the drain to the source of the transistor TR5. The transistor TR6, whose gate and source are connected to each other, operates as a high-performance constant current circuit and becomes conductive when the voltage at the point B drops to a potential lower than the ground potential due to the reverse drive signal, and a current flows to the base of the transistor TR3. And serves to help the conduction of the transistor TR3.

When the reverse drive voltage applied from the output terminal is in the opposite direction to that shown in the figure, the voltage at the point A on the source side of the transistor TR4 drops in the same manner as described above, and the transistor TR2 responds accordingly. Is turned on to prevent an overcurrent from flowing from the drain to the source of the transistor TR4.

In this way, the current flowing through each of the transistors TR4 and TR5 is limited, so that the defective power amplifier can be prevented from overloading the normal power amplifier.

The above operation is performed when the power supply + V is normally supplied and the input signal is not supplied due to the failure of the voltage amplifier A1 or the disconnection of the signal line, or when the input signal is supplied but the operation of the power protection fuse and the power supply are performed. The same operation is performed in any case where the power supply + V is not supplied due to the disconnection of the transformer or the like, and the overcurrent of the power transistor can be effectively limited.

If, for example, the drains and sources of the power transistors TR4 and TR5 are destroyed and short-circuited, the fuses F1 and F2
Is disconnected, each primary coil P11, P12 of output transistor T2
Since it is opened, the normal amplifier is not overloaded.

As described above, according to the present invention, the bias voltage of the power transistor is changed according to the voltage generated across the resistor due to the current flowing through the power transistor, and then the reverse voltage applied to the output terminal is applied. The drive voltage very accurately prevents short circuit currents flowing in the power transistors. Therefore, even when a plurality of power amplifiers are connected in parallel and used, even if a part of the plurality of power amplifiers connected in parallel fails, the normal power amplifier is overloaded. The normal power amplifier will not be damaged. Therefore, even if a part of a plurality of power amplifiers connected in parallel fails, the amplifier system can be used as it is without disconnecting the failed amplifier, and the failed amplifier can be repaired at an appropriate time.

[Brief description of drawings]

The accompanying drawings are electrical circuit diagrams showing a power amplifier according to an embodiment of the present invention. A1: Voltage amplifier, A2: Constant voltage power supply unit, T1: Driver transformer, T2: Output transformer, TR1, TR2 ..., TR6: Transistor, CD1, CD2 ..., CD10: Diode, F1, F2: Protective fuse, + V: Power supply , CD11: Zener diode, R1, R2,…, R12: Resistor, VR1: Variable resistor.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-55-145409 (JP, A) JP-B-52-43051 (JP, B1) JP-B-48-36203 (JP, Y1)

Claims (2)

[Claims] 1. A voltage amplifying section for amplifying an input voltage, and a power transistor constituting a grounded drain or collector type bush-pull amplifying circuit. An input is coupled to an output of the voltage amplifying section through a driver transformer. A power amplifier whose output is coupled to the output terminal via an output transformer, a resistor connected in series with the output transformer to the source or emitter of the power transistor, and a drive signal from the output terminal side. Then, when the source or emitter of the power transistor drops to a potential lower than the ground potential due to the voltage induced on the primary side of the output transformer, the operation is started, and the current flowing in the power transistor causes the operation to start. A bias control unit that changes the bias voltage of the power transistor according to the voltage generated across the resistor; A power amplifier comprising. 2. The power amplifier according to claim 1, wherein the power amplification section does not have a negative feedback circuit.