JPH02265307A - Audio output protection circuit - Google Patents

Abstract

PURPOSE:To suppress heat of an output circuit and to attain miniaturization and low cost by providing a detection means detecting an output current of an output circuit, a means decreasing a bias signal with a detection signal of the detection means and a means feeding back positively the output signal of a negative feedback amplifier circuit to the means reducing the signal. CONSTITUTION:When a bias current starts decreasing to raise a base and emitter voltage of a transistor(TR) Q1 in the positive direction, the base voltage of the TR Q1 starts decreasing. Since a negative feedback voltage from a resistor R2 starts decreasing in an operational amplifier IC1, the output voltage is increasing to correct the decrease, since a voltage is operated in a diode D2 in the reverse bias direction through a resistor R10, no voltage is delivered from the cathode to the node and the level of the cathode is increasing. When the cathode voltage is increased, a base current of a TR Q7 is increasing through a resistor R11 and the collector current is further increased, then the TR Q3 is turned off, resulting in applying positive feedback to the base of the TR Q7, which is turned on.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an audio output protection circuit that protects an audio output circuit from short circuits and overloads at its output terminals.

[Summary of the Invention] The present invention provides an audio output protection circuit that protects an audio output circuit from overload, detects an overload of its output current, reduces the bias current of a complementary output circuit by positive feedback, By instantaneously cutting off the output current, the heat generated by the output circuit during protective operation is reduced, making it possible to downsize and reduce the cost of the output circuit.

[Prior art] Generally, the output lines of various audio equipment for broadcasting stations are 6.
It can be received with a termination impedance of 00Ω, and in order to ensure a high dynamic range, the maximum output is, for example, +3
A very large value of up to 0 dBm (IW received by 600 Ω) is required. Although it is possible to configure a circuit to meet such requests and extract the output,
Because of the high levels, it is necessary to protect the output circuit against overloads or short circuits.

Various protection circuits have been devised and put into practical use, but a typical one is an output protection circuit as shown in the circuit diagram of a conventional audio output circuit in FIG. In the figure, 1 is the input (IN) terminal, 2 is the output (OUT)
The terminal IC is an operational amplifier, and RL is a load. resistance n
r, Re and the operational amplifier IC constitute a negative feedback amplifier circuit.

Q,,Q, are output transistors forming a complementary circuit, and each output transistor Q
1. For Q*, diodes D+, Dt.

D, ,D, and resistors R3, R4, R8, Rs, R? and,
A DC bias is provided by a constant current circuit for DC bias consisting of transistors Q, ,Q. R-1R1 is an emitter resistor that detects the output current, and this resistor Rs, R
* and transistors Q, , Q form a current limiter circuit that serves as an output protection circuit, and limit the output current so that it does not exceed a specified value, as shown by the dotted line in the output waveform diagram in Figure 2. This protected the output transistors Q, ,Q, etc.

[Problem M to be solved by the invention] However, in the output protection circuit of the audio output circuit in the above-mentioned conventional technology, since the apparent output impedance of the output terminal 2 is low especially against overload such as sitat, the output current is continues to flow almost to the limit value, and at this time the power supply voltage is applied to the output transistor Q1. Since almost all of it is applied to Q*, this output transistor Q+, Q
*Fever was unavoidable. Therefore, the final output stage transistor Q1. For Q*, it is necessary to use a large transistor with a large collector power, which has been a major obstacle to miniaturization and power saving.

Another method is to limit the maximum current during output, such as by inserting a resistor in series within a range that does not affect the output side, but in either case, the problem of heat generation cannot be avoided. .

The present invention has been devised to solve the above-mentioned problems, and therefore provides an audio output protection circuit that suppresses heat generation in the output circuit during protection operation to a minimum, thereby making it possible to downsize and reduce the cost of the audio output circuit. i1m! The purpose is to provide

[Means for Solving the Problems] The configuration of the audio output protection circuit of the present invention for achieving the above object includes: a negative feedback amplifier circuit; and a complementary output circuit that amplifies the output signal of the negative feedback amplifier circuit. and a bias circuit for applying a bias signal to the output circuit, the audio output protection circuit for an audio output circuit having a detection means for detecting an output current of the output circuit, and a detection signal of the detection means to reduce the bias signal. and means for positively feeding back the output signal of the negative feedback amplifier circuit to the reducing means.

[Operation] When an overload occurs, the present invention first reduces the bias signal of the complementary output circuit that amplifies the output signal of the negative feedback amplifier circuit. As a result, the output signal of the negative feedback amplifier circuit, which attempts to increase the output current, is positively fed back to the means for reducing the bias signal in L, reducing the bias signal like an avalanche phenomenon, and increasing the output current almost instantly. Cut off.

This interruption is restored when the polarity of the output current is reversed, and the protection operation is repeated in the same manner until the load reaches the proper load 4. The present invention
This protective operation by cutting off the output current suppresses heat generation in the output circuit.

[Example] Hereinafter, an example of the present invention will be described in detail based on the drawings.

FIG. 1 is a circuit diagram of an audio output circuit including an embodiment of the present invention. First, the audio circuit of this embodiment includes a negative feedback amplifier circuit 3 consisting of an operational amplifier IC6 and resistors Rl, nt, a pnp transistor Q, and an npn transistor Q.
) Complementary type consisting of transistor Q.
output circuit 4 and diode D1. D! , Da, D4 and resistance R@, R4, RII R@, R? , R.I.
I and pnp) transistor Q31npn) DC bias constant current circuit 5 consisting of transistor Q4. Further, the output protection circuit 6 of the audio circuit of this embodiment includes resistors Rs and Re that detect the output currents of the final output transistors Q, Q, as voltages, and the resistors R@,
A protection transistor pair Q that reduces the DC bias current that drives the transistors Q, , Q, by the detection voltage of the RI.
7, Q, and diodes D*, Ds that first operate the protection transistor pair Q-, Qm by the above detection voltage, and then operate the protection transistor pair Q7゜Q by positive feedback of the output signal of the operational amplifier C1. and resistance R1゜, R, l
, R, etc.

1 is the input (IN) terminal of this audio output circuit, 2 is its output (OUT) terminal, and R1 is a load connected between this output terminal and ground.

In the negative feedback amplifier circuit 6, an operational amplifier IC.

The non-inverting input terminal + is connected to the input terminal 1, and the inverting input terminal - is connected to the ground via the resistor R1 and to the output terminal 2 through the resistor R1. Further, positive and negative power supplies ±Vcc are supplied to the operational amplifier IC. In the output circuit 4, the two transistors Q have their collectors connected to the positive power supply Vcc, their emitters connected to the output terminal 2 through the resistor R1 of the output protection circuit 6, and their bases connected to the constant current circuit 5. Connect to the collector of transistor Q3. The other transistor Q has its collector connected to the negative power supply -Vcc, its emitter is connected to the output terminal 2 through the resistor R8 of the output protection circuit 6, and its base is connected to the collector of the transistor Q4 of the constant current circuit 5. Connect to. In the constant current circuit 5, the diode D, the resistors Rs, R,, rt, and the diode D4 are connected in series in the forward direction between the positive power supply +Vcc and the negative power supply -Vcc in this order, and the resistors It,, I' The connection point of t, is connected to the base of transistor Q, and the connection point of resistors R4 and R8 is connected to the base of transistor Q4. transistor Q,
The emitter of transistor Q4 is connected to the positive power supply +Vcc through a resistor R, and the emitter of transistor Q4 is connected to the negative power supply -Vcc through a resistor R7. This transistor Q InQ4
The collectors of diodes Da and Ds are connected in series in the forward direction, and the connection point of the diodes Dt and Ds is connected to a resistor R1.
Connect to the output terminal of operational amplifier IC8 through ゜.

In the output protection circuit 6, n p n ) transistor Q
, is connected to the emitter of the transistor Q of the constant current circuit 5 through the current limiting resistor RI3, and is connected to the emitter of the transistor Q of the constant current circuit 5.
The collector of transistor Q is connected to the emitter of transistor Q4 of constant current circuit 5 through current limiting resistor r (14), and the emitters of transistors Qt and Q- are connected to the output terminal 2, that is, the emitter resistance of transistors Q, Q, It is connected to the connection point of the output current detection resistors Ra and R@.The voltage due to overcurrent is divided between this connection point and the connection point of the diodes D* and Ds explained in constant current circuit 5. Connect the resistors R,,,R,, in series,
Its resistance 111. , R, , the above transistor Q? , connect the bases of Q horse mackerel respectively.

The operation and effect of the embodiment configured as above will be described. FIG. 2 is an output waveform diagram for explaining the operation of this embodiment. The explanation will be given below with reference to FIG.

In this embodiment, an input signal is normally applied to the non-inverting input terminal + of the operational amplifier ICI, and an amplified output is obtained at the output terminal 2. Transistors Q, ,Q, each constitute a constant current circuit 5, and this current is passed through a diode D*, which is connected to the output terminal of the operational amplifier, respectively.
The bias voltage for the output transistors Q, , Q is obtained by flowing it through Ds. The diodes DI and D serve as temperature compensation for the bias voltages applied to the transistors Q, , Q, respectively. Resistance R1°R? including resistances R* and Rt of constant current circuit 5? , n ++, n lt, nt
s, R+4 and transistor Q? .

Q forms the main output protection circuit 6 of the present invention,
The protection operation is a positive feedback operation that utilizes the operation of the entire audio output circuit.

First, considering normal operation, it is an operational amplifier IC.

The subsequent circuit is a voltage follower circuit that amplifies the output current, and the entire circuit including the operational amplifier IC is an output amplifier with an amplification factor determined by the resistors It, R,. This circuit has a specified value (e.g. 400Ω)
What if I connect a load with a larger impedance? It source voltage is completely problematic even to the level where it clips (it operates normally.

Next, considering the operation in an overload state (for example, when a load of Rt = l 50Ω is connected), the output protection circuit 6 will start working when the output current exceeds a certain value.

In other words, in the output waveform diagram in Figure 2, if the waveform at a proper load is the waveform shown by the solid line, the output waveform at the time of overload will instantly decrease to almost zero when it exceeds a certain value, as shown by the dotted line. This state is maintained until the polarity of the waveform is reversed. When the polarity is reversed, the normal operating state is restored, but once it exceeds a certain value, it instantly decreases to almost zero, and it remains in that state until the next time the polarity of the waveform is reversed. These operations will be explained with reference to the circuit diagram in FIG. Now consider the case where the waveform rises from zero in the positive direction. As the output voltage increases, the current in the resistor R6 increases due to the heavy load, and a voltage including the increased voltage of the hollow resistor R6 passes from the base of the transistor Q1 through the diode Dt to the resistor R1 and the resistor R1! applied to the series voltage divider circuit. Here, resistance R1! is placed between the base and emitter of the transistor Q, and due to an overload condition, the voltage across the resistor R1t increases to the base-emitter conduction voltage v of the transistor Q7.
61! When the voltage exceeds 0, the collector current of transistor Q7 starts to flow. Transistor Q.

When the collector current of transistor Q3. Since the emitter voltage of the bias constant current generating circuit formed by the diode D1, resistor R3, resistor R4, and resistor IN begins to decrease, the bias current begins to decrease. When this bias current for raising the base and emitter voltages of transistor Q in the direction of positive voltage begins to decrease, the base voltage of transistor Q begins to decrease, and as a result, the output voltage begins to decrease. When this happens, the negative feedback voltage from the resistor R2 begins to drop in the operational amplifier IC1, so it operates to increase the output voltage to compensate for this, but at this time, the voltage of the diode D moves in the direction of reverse bias through the resistor Rto. Therefore, voltage is not transmitted from the cathode to the anode, and the cathode moves in the direction of increasing voltage. As the cathode voltage increases, the base current of transistor Q7 increases through resistor RIl, further increasing its collector current, so transistor Q3 becomes even more OF
F, and eventually positive feedback is applied to the base of transistor Q7, turning it on. When transistor Q is turned off, the base current of transistor Q disappears, and therefore the output voltage becomes almost zero. Since the above operation occurs instantaneously like an avalanche phenomenon, the output waveform becomes like the output waveform indicated by the dotted line in FIG. Furthermore, when the output voltage is close to zero, the output voltage of the operational amplifier 1C1 naturally remains at a high level close to the positive power supply voltage. In addition, in this state, a positive voltage is applied to the base of transistor Q through resistor R1 and diode D, and a nearly positive power supply is applied between the base and emitter of transistor Q, which is a pnp transistor. Reverse biased with voltage. In this case, the base and emitter of the transistor Q will act as a Zener diode-like portion and a current will flow therein, but a resistor R1° is inserted to limit this current.

The above explanation is based on the case where the input waveform is in the positive direction, but this circuit is completely complementary, and when the input waveform is in the negative direction, the operation is completely opposite and the same, and the output waveform is the dotted line in Figure 2. It looks like a waveform.

Even when output terminal 2 is short-circuited, the operation is the same, but since output terminal 2 is fixed at Ov, the output waveform remains 0■, and the voltage at the output terminal of the operational amplifier IC is positive and negative, respectively. The power supply voltage will swing significantly to near VCC. Also, the current consumption of the circuit at this time is the transistor Ql.

Q! Since the percentage of time that is conductive becomes very small,
becomes close to no signal and shows a small value. That is, the output transistors Q+ and Qt generate almost no heat, and the collector power dissipation of the output transistors can be reduced to a small value.

Therefore, in this embodiment, it is possible to select small transistors as the transistors Q, , Q, and it is possible to achieve miniaturization and cost reduction. In the above, the operation starting point of the output protection circuit 6 is the value of the resistors Its, Re and the resistors R, , , R,
An arbitrary point can be selected depending on how to take the ratio of .

[Effects of the Invention] As is clear from the above description, the audio output protection circuit of the present invention provides the following effects.

(1) Since the output current is instantaneously cut off when the output protection circuit operates due to an overload, it is possible to suppress heat generation in the output circuit at this time, making it possible to reduce the size and cost.

(2) When the output protection circuit is in operation, the heavier the load (the smaller the impedance), the smaller the average current consumption 711 of the output circuit, and the more reliable the operation.

(3) Even though it is an output current cutoff method, it can automatically recover when the abnormal load is removed.

(4) The starting point of protection operation can be set arbitrarily.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an embodiment of the present invention, Fig. 2 is an output waveform diagram of the present embodiment, Fig. 3 is a circuit diagram showing a conventional technique, and Fig. 4 is an output waveform diagram of the conventional technique. . ! ... input terminal, 2 ... output terminal, 3 ... negative feedback amplifier circuit, 4 ... output circuit, 5 ... constant current circuit for DC bias, 6 ... output protection circuit, Q7. Q...
Protection transistor pair, R,, rt,... Resistor for output current detection, n 1 t *R1,... Voltage dividing resistor.

Claims (1)

[Claims] (1) In an audio output protection circuit for an audio output circuit that includes a negative feedback amplifier circuit, a complementary output circuit that amplifies the output signal of the negative feedback amplifier circuit, and a bias circuit that applies a bias signal to the output circuit, a detection means for detecting the output current of the output circuit; a means for reducing the bias signal using the detection signal of the detection means; and a means for positively feeding back the output signal of the negative feedback amplifier circuit to the reducing means. An audio output protection circuit comprising: