JPS6026321B2 - Power amplifier overload detection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a circuit that detects overload based on information on clipping of an output waveform when a power amplifier is overloaded.

Conventionally, as shown in FIG. 1, an overload detection circuit for a power amplifier includes a circuit in which a resistor is connected to the emitter of an output transistor, and a voltage drop across the resistor due to the output current is detected. In FIG. 1, Q and Q' are mother transistors, R and R' are resistors, DET is a detection circuit, 1 is a relay, 2 is a relay drive circuit, and RL is a load resistance. However, in such overload detection circuits, a resistor is inserted in the output circuit, so if you try to obtain a large detection voltage, you will not be able to avoid output loss, and if the output waveform starts to clip during an overload, the detection fin will fail. Not only does the voltage not rise very much, but in the case of the combinary OTL type power amplifier circuit shown in the figure, it is difficult to connect the control system because the detected voltage cannot be extracted with reference to the ground potential. . Therefore, an object of the present invention is to obtain an overload detection circuit for a power amplifier that does not have the above-mentioned drawbacks. Means for generating an increasing clip information voltage is used and overload is detected by comparing the clip information voltage with the output voltage of the power amplifier.

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 2 shows an example in which the present invention is implemented in a complementary OTL circuit. In the figure, Q, Q2 are drive transistors that drive output transistors Q and Q4, respectively, and the emitters are commonly connected, and the common connection point is
It is connected to the output side of the output transistors Q and Q4, that is, to the collector common connection point via a diode D, a resistor R, a diode D2, and a resistor R2. A capacitor C with the other end grounded is connected to the connection point between the resistor R and the diode D2.
, and one end of the discharging transistor Q5 is connected to the discharging transistor Q5.
base is connected. In addition, drive transistors Q,,
The emitter common connection point of Q2 is grounded via the detection resistor R3, and the output sides of the output transistors Q and Q are connected to one end of the load (resistance) RL via the relay contact S of the relay circuit 1. The other end of is grounded. Reference numeral 2 denotes a relay drive circuit, which is composed of, for example, a relay drive transistor Q6.

The base of the relay drive transistor Q is grounded via the collector and emitter of the discharge transistor Q5. The collector of the relay drive transistor Q6 is connected to a DC power supply (of 10 V) through the relay winding. DC power supply (10V
Resistors R4 and R5 are connected in series between cc) and Oike.
The connection point between both resistors is grounded via resistor R8 and capacitor C2, and the connection point between R6 and C2 is connected to the base of relay drive transistor Q. Q7. Q is a pair of transistors constituting a differential amplifier; an input signal is applied to the base of transistor Q7, and an output signal (the third one) of output transistors Q3 and Q4 is applied to the base of transistor Q8.
A portion of Figure A) (Figure 3B) is added with negative feedback NF and both signals are compared.

Q9 is an amplification transistor that amplifies the output of the differential amplifier and supplies the driving transistors Q, . In addition to Q2, this is controlled. Therefore, during normal operation, a voltage (FIG. 3C) having a small amplitude and a waveform similar to the output signal appears at one end of the detection resistor R3 on the emitter side of the drive transistors Q, , Q2. On the other hand, the output current of this drive transistor is charged to the capacitor C via the diode D, and the resistor R.
resistors R, and R in relation to the load end voltage, i.e. the output voltage, such that the current discharged through 2 is greater
By setting the relationship with R2 and R3, the terminal voltage of the capacitor C is maintained at a negative voltage, and the discharging transistor Q is maintained in an off state. Therefore, when the power is turned on, the contact S of -1 remains closed and normal operation continues. However, when an overload condition occurs due to a decrease in the load resistance RL, the output current does not increase immediately, but the output voltage decreases, and this is applied to the differential amplifier by the above-mentioned feedback, causing the output current to decrease. As a result, the positive and negative peak sides of the output waveform begin to be clipped as shown in FIG. A clipped information voltage that increases depending on the degree of clipping as shown in FIG. E passes through the amplifying transistor Q and appears across the resistor R3 on the emitter side of the driving transistors Q, Q2.

On the other hand, as shown in FIG. 3D, the amplitude of the output voltage of the output transistor is relatively reduced due to the decrease and clipping of the load resistance RL, and as a result, the amount of discharge through the diode D2 and resistor R2 is reduced. The amount of charge to capacitor C increases. Therefore, at a certain point when the increase in the clip information voltage and the relative decrease in the output working pressure become significant, the terminal voltage of the capacitor C becomes positive, turning on the discharging transistor Q5. Then, the capacitor C2, which was charged after the power was turned on, is discharged through the discharging transistor Q5, turning off the relay driving transistor Q6, deenergizing the relay, and opening its contact S. If the power is not turned off, the relay will operate again due to the time constant of the muting time constant circuit 3 to prevent clicks, and this will repeat, but by setting the time constant appropriately, the heat generation of the output transistors QxQ4 can be reduced. can be suppressed. That is, in the present invention, the clip information voltage that indicates the overload state is compared with the output voltage of the output transistor, which changes depending on the overload, using the diode D2, and the discharge transistor Q5 is changed depending on the change in the relative relationship between the two. It is designed to detect when the base voltage becomes positive.

The relationship between the clip information voltage and the output voltage is as follows. In the present invention, a load short circuit or an overload similar to this, in which the clip information voltage becomes large and the output voltage becomes small, is detected, so there is an advantage that the output circuit is not cut off every time even if a clip occurs due to a slight overload. be.

This is because when the load becomes reactive, not only does a slight clip in the output waveform have no significant effect, but there are also output transistors that can withstand some overload, so even a slight clip It is rather inconvenient that the output is cut off one after another. On the contrary,
In the conventional detection circuit shown in Figure 1, the output current does not increase much once clipping begins, so it is necessary to detect the level of the output current just around the time when clipping begins, and therefore the output current increases. Each time it grows and begins to clip, the output circuit will be cut off, even though it is safe.

Further, according to the present invention, unlike a subordinate detection circuit, the detection voltage can be extracted with reference to the ground potential, so that a control system can be easily connected, and output loss is also reduced.

Note that 3 is a muting constant circuit for preventing a click sound when the power is turned on, and as a result, the relay contact S closes the output circuit after a certain period of time has passed after the power is turned on.

In the embodiment shown in FIG. 2, an IJ relay for muting and an IJ relay for overload prevention are shown, but the invention is not limited to this example. A muting circuit based on an input muting method may also be operated, or a relay contact may be provided in the power supply circuit.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a subordinate overload detection circuit, FIG. 2 is a circuit diagram showing an embodiment of the present invention, and FIG. 3 is a waveform diagram showing the operation of the circuit shown in FIG. Q,,Q2...drive stage transistor, R3,D.,R,
,C. . . . Means for detecting the voltage compared to the current of the drive stage, D2, R2, Q5 . . . Comparative detection means. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. A method for detecting a voltage proportional to the current of the drive stage, which increases according to the degree of clipping of the output waveform during overload of a negative feedback power amplifier, and comparing this voltage with the output voltage of the power amplifier. and means for detecting overload.