JPS6141162B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a protection device for detecting abnormalities such as grounding and short circuits occurring in the output circuit of a balanced direct-coupled amplifier and for preventing destruction of the amplifier.

[Technical background of the invention and its problems]

In general, in a high-output amplifier, in order to increase the output when two voltages are set for the power supply, it is necessary to select a transistor with a low collector saturation voltage for the output stage transistor, and to reduce the loss between the output and the load as much as possible. In particular, in-vehicle amplifiers for automobiles and the like use storage batteries as power sources, so they must be operated at relatively low voltages. ) tends to be preferred. In addition, when using a coupling capacitor for DC blocking on the output side, due to the relationship with the load impedance, a capacitor with a large capacity must be used to guarantee the amplification frequency characteristics down to low frequencies, and in addition, the signal current must pass through the capacitor. Such capacitors require a large current capacity, are large in size, and are expensive, so it is most advantageous to use an OCL circuit (output coupling capacitor-less circuit) together with a BTL circuit.

An example of a balanced direct-coupled amplifier of such an OCL-BTL circuit is shown in FIG.

The figure shows the configuration of a basic power amplifier PA using a balanced direct-coupled circuit. The signal e obtained from the front stage amplification etc. is applied to the input terminal I, and becomes an excitation signal with opposite phases in the buffer amplifier BA. , a single-ended push-pull power amplifier circuit constituted by transistors Q1, Q3, Q5, and Q7 and a similar power amplifier circuit constituted by transistors Q2, Q4, Q6, and Q8 are excited with opposite phases to each other. Note that the voltage of output terminals O1 and O2 is set to approximately 1/2 of the voltage of power supply B.
Since they are excited in opposite phases, when the voltage of one increases, the other decreases, and when one decreases, the other increases, and accordingly, the signal current is passed to the load L, supplying amplified signal power. .

Therefore, the output terminals O1, O2 and the load L must all be used in a floating state with respect to the earth E, that is, in an insulated state.

However, in the case of such a balanced direct-coupled amplifier in the OCL-BTL circuit, the DC voltage from the power supply Vb is applied to the output terminals O1 and O2, and since the DC voltages at both terminals O1 and O2 are equal, the load The DC component does not pass to L. If the load circuit including the output terminals O1, O2 and the load L such as the speaker is short-circuited or has a fault such as grounding, an overcurrent will flow to each circuit in the amplifier PA, especially the power transistor Q in the output stage.
5, Q6, etc. are destroyed relatively quickly, and it often happens that the fuse F inserted between it and the power supply B cannot be fused in time.

As a countermeasure against such accidents, it has been proposed to insert a low resistor for overcurrent detection in series between power supply B and operate a protection circuit by the voltage drop between the two. This is not desirable because it causes an increase in the power supply impedance, a decrease in the maximum output due to an increase in the power supply voltage fluctuation rate, an increase in distortion, an increase in power supply ripple, and a deterioration in stability.

In addition, the output of the amplifier contains not only DC components but also AC components, so only the DC components can be detected from the output without any time delay. It was difficult to detect abnormalities such as grounding and short circuits.

Therefore, although such balanced direct-coupled amplifiers have ideal characteristics, they have not yet been put into practical use as general-purpose equipment.

[Purpose of the invention]

The present invention aims to solve the above problems, and in order to detect the neutral point voltage of the load circuit of the amplifier in a balanced direct-coupled amplifier, a a voltage circuit; a comparison means for comparing a divided voltage output from the voltage dividing circuit with a reference voltage and generating a detection output when the divided voltage changes with respect to the reference voltage; A protective device for a balanced direct-coupled amplifier is characterized in that it is equipped with a protective means that performs a protective operation of the amplifier in a controlled manner, and is capable of detecting abnormalities sharply and reliably, and can quickly protect the amplifier. This is what we provide.

[Embodiments of the invention]

Fig. 2 shows the detection circuit in the protection device of the present invention.
This figure shows a configuration in which DT is connected to the power amplifier PA of FIG. 1.

In the circuit shown in Figure 1, the voltage at each terminal across the load L always changes in the opposite direction, and the voltage at each terminal across the load L always changes in the opposite direction.
If you look at it as a reference, there is a point between both terminals where the voltage does not change, and this is approximately 1/2 of the voltage of power supply B.
If a short circuit, ground, etc. occurs in the ground somewhere in the load circuit or in any part of the load L itself, the voltage at this point will drop. Therefore, either provide a neutral point on the load L itself, or connect a high impedance voltage divider circuit in parallel with it, extract the divided voltage, and compare it with a reference voltage that is approximately 1/2 of the voltage of power supply B. Then, abnormality can be detected.

FIG. 2 is a circuit diagram of an embodiment based on the above-mentioned idea, in which a resistor R6 of the same resistance value is connected in parallel with the load L.
1, connect R62, and connect resistor R6 from the midpoint.
3, a divided voltage is applied to the input terminal I1 of the detection circuit DT, and a voltage drop at the intermediate point is detected by a differential circuit of transistors Q61 and Q62. In other words, the voltage at the power supply terminal Vb is divided to a level slightly lower than 1/2 by resistors R66 and R67 and is applied as a reference voltage to the transistor Q62, but during normal operation of the power amplifier PA, the input terminal Since a voltage slightly higher than the reference voltage is applied to I1, transistor Q61 is on, and therefore transistors Q62 and Q63 are both off, and no detection output is generated at detection terminal P.

However, as mentioned above, when an abnormality occurs in the load circuit, the voltage at the input terminal I1 drops below the reference voltage.
Transistor Q61 turns off, and transistors Q62 and Q63 turn on accordingly, producing a detection output at detection terminal P, which is used to apply a trigger current to the gate of thyristor CR via current-limiting resistor R21. , the thyristor CR is turned on and the short-circuit current is passed from the power supply B to blow the fuse F, thereby protecting all the circuits including the power amplifier PA.

In addition, this detection output also controls the control terminals T1 and T.
2 can also be controlled to perform a protective operation.

[Other embodiments of the invention] Figures 3 and 4 show a stereo power amplifier PA.
This is an embodiment in which the output terminals O1 and O2 of the L channel are connected to the output terminals O11 and O12 of the R channel, and the load L1,
Since L2 is connected to each one, each load L1,
For each L2, resistors R61, R62, similar to those shown in FIG.
The voltage applied by R71 and R72 is taken out, and in FIG.
A comparison is made using the reference voltage and transistors Q71 and Q72, and the input terminal I1 or I11 is
When the voltage to the transistor Q71 or Q72 decreases, the transistor Q71 or Q72 turns on and produces a detection output.

In addition, Fig. 4 shows a diode D8 for backflow prevention.
Detection is performed by one circuit using transistors Q81 and D82, and a detection output is generated when both transistors Q81 and Q82 are turned on.

Note that the resistor R6 in FIGS. 2 to 4
1, R62, R71, R72 midpoint, that is,
The neutral point voltage of loads L, L1, L2 is the power amplifier
Depending on the adjustment status of the PA, the voltage may not necessarily be 1/2 of the voltage of the power supply B, and in this case, the reference voltage may be determined accordingly.

FIG. 5 shows an embodiment used for a power amplifier PA using two positive and negative power supplies +B and -B. In this case, since the neutral point voltage of the load L is the same as the earth E, The ground E is used as the reference voltage of the differential circuit, and when the voltage at the input terminal I1 changes to either positive or negative due to an abnormality in the load circuit, the transistor Q91 or Q92 turns on, which causes the diode D9
1 or D92, transistor Q93 is turned on and produces a detection output.

In addition, in order to control the control terminals T3 and T4 shown in Fig. 1, a switching circuit such as a transistor that is turned on by the detection output shown in Fig. 2 and thereafter can be inserted between the control terminals T3 and T4 and the ground E. often,
Any configuration can be selected by combining each of the above-mentioned detection circuits DT, control terminals T1 to T4 as protection circuits, thyristor CR, etc.
Further, in the case of using a stabilized power supply circuit, the power supply circuit may be controlled to lower the output voltage by the detection output, and various changes are possible depending on the amplifier to be protected and the surrounding conditions. In addition,
In the above description, the detection circuit DT is provided separately, but it may be incorporated into the power amplifier PA.

〔Effect of the invention〕

As is clear from the above explanation, according to the present invention, the alternating current components of the voltages at each terminal at both ends of the load circuit in a balanced direct-coupled amplifier have the same magnitude and opposite phases, and are therefore canceled out by the voltage divider circuit. Only the DC component is detected, making it possible to detect abnormalities such as grounding and short circuits. Therefore, it is possible to quickly and reliably detect the occurrence of a fault such as grounding or short circuit in the output circuit of a balanced direct-coupled amplifier, and to take protective action. The system can be operated as an amplifier system only by repairing the output circuit without replacing the transistors, and this is highly effective in improving the reliability of the device and reducing maintenance costs. It goes without saying that the amplifier can be applied not only to the balanced direct-coupled type, but also to any electronic circuit that operates in a similar manner.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing the configuration of a balanced direct-coupled amplifier, Fig. 2 is a detection circuit of a protection device in an embodiment of the present invention, and Figs. 3 to 5 are circuit diagrams of other embodiments of the invention. be. PA...Power amplifier (balanced direct-coupled amplifier), DT
...Detection circuit, O1, O2, O11, O12...
Output terminal, L, L1, L2...load, P...detection terminal, T1-T4...control terminal, CR...thyristor, F...fuse.

Claims (1)

[Claims] 1 In a balanced direct-coupled amplifier, a voltage divider circuit connected in parallel to the load circuit to detect the neutral point voltage of the load circuit of the amplifier, and a divided voltage that is the output of the voltage divider circuit are set as a reference voltage. Comparing means for comparing the divided voltage with respect to the reference voltage and generating a detection output when the divided voltage changes with respect to the reference voltage, and a protection means for performing a protective operation of the amplifier under control by the detection output. A protection device for a balanced direct-coupled amplifier.