JP2515332B2 - Circuit arrangement for protecting the push-pull output stage from overload - Google Patents

Description

Description: INDUSTRIAL APPLICABILITY The present invention has a load impedance connected to a reference potential connected to the output side of a push-pull output stage, and measures the output current flowing through the push-pull output stage. Therefore, it relates to a circuit arrangement for protecting a transistorized push-pull output stage from overload, which is provided with at least one detection element.

2. Description of the Related Art A circuit for protection against overload of an output stage is described, for example, in German Patent Specification 3311955. In this specification, a voltage proportional to the output current is taken out via a detection resistor in the output stage, which causes the output stage to be deactivated via a switching device, which is measured when a predetermined output current is exceeded. . In this case, the circuit described therein only evaluates half the output current. The circuit thus does not operate with sufficient accuracy and is unsuitable for high quality output stages such as hi-fi devices. Furthermore, the known circuits have considerable tolerances, so that the predetermined breaking point of the output stage cannot be obtained.

SUMMARY OF THE INVENTION The object of the invention is therefore to achieve a precisely defined breaking point of the output stage during overload.

Means for Solving the Problem According to the present invention, the problem is that one detection element is provided in each branch of the push-pull output stage, and each detection element is measured with respect to a reference potential. A voltage is taken out, the voltage is applied to one input side of each differential amplifier stage, the output side of the differential amplifier stage is connected to one input side of the comparator circuit, and the other side of the comparator circuit is connected. The input side is placed at the reference potential, and when the input voltage supplied to the comparator circuit exceeds the reference potential, a signal is taken out from the output side of the comparator circuit, and the signal is supplied to the means for cutting off the push-pull output stage. When the input side of the push-pull output stage is short-circuited by the means and the push-pull output stage is overloaded, the output signal of the comparator circuit causes the time limit. A reference voltage input side of the comparator circuit is connected to a reference potential via an element and an electronic switch controlled by the timed element, and the timed element is provided between an output side of the comparator circuit and a control electrode of the electronic switch. This is solved by a series connection body of a resistor and a capacitor provided and a resistor inserted between the control electrode of the electronic switch and the reference potential.

The dependent claims show advantageous embodiments of the invention.

Next, an embodiment of the present invention will be described with reference to FIGS. 1a and 1b.

Example The circuit arrangement shown uses a known push-pull output stage configuration, which has output transistors 1, 2 whose emitters are connected to one another.
The collectors of these transistors have operating voltages + UB ₁ , -UB
Connected to terminal ₁ . A load impedance 3, for example in the form of a speaker, is connected to the output side of the push-pull output stage. The other end of the load impedance is
It is connected to a reference potential, for example ground. In order to measure the dark current flowing in the push-pull output stage, resistors 1 and 4 are respectively inserted and connected in the emitter line. A transistor 6 inserted between the bases of the output transistors 1 and 2 is used for adjusting the dark current. An input signal is supplied via the terminal 7, and the input signal controls the push-pull output stage via the preamplifier 8 and the driver transistor 9. The voltage measured with respect to the reference potential can be taken out from the resistors 4 and 5. The voltage that can be taken out by the resistor 4 (which is formed by the dark current and the load current that is superposed on the dark current and flows through the load impedance 3) is supplied to the non-inverting input side of the differential amplifier 10, and the resistor 5 The voltage extracted at is supplied to the inverting input side of the differential amplifier stage 10.

A signal as shown is generated at the output side of the differential amplifier stage 10. This signal consists of a value UR corresponding to the dark current and a value US corresponding to the signal current. By supplying the negative signal component to the inverting input side of the differential amplifier 10, the above signal is output at the output side.
Appears with a 180 ° phase rotation. Supply voltage U for differential amplifier 10
Since B ₂ is lower than the power supply voltage UB ₁ for the push-pull output stage, the signal voltage supplied to the differential amplifier 10 corresponds to the division factor (this corresponds to the ratio (R11 + R12) / R11 of the resistance values of the resistors concerned). Must be divided. Differential amplifier stage 10
Has an amplification degree V = R12 / R11.

The output signal of the differential amplifier 10 is applied to the positive input side of the comparator 20. The negative input side of this comparator is the reference potential
_{Placed in} U _ref (which is produced by means of a voltage divider formed by resistors 21, 22), said voltage divider being at the operating voltage +
It is inserted and connected between UB ₂ and the reference potential. As soon as the output voltage of the differential amplifier stage 10 exceeds the reference potential, the output side of the comparator 20 is placed at a high potential, which controls, for example, the transistor 30 connected to the input side of the push-pull output stage. It This transistor 30 short-circuits the input signal UE, which eliminates the overload.
Due to the disappearance of the overload condition, the transistor 30 is immediately put into the blocking state again and the clocked activation connection and disconnection of the output stage is effected. Muting control may also occur elsewhere in the circuit, for example in driver transistor 9
It can also be done by blocking.

Therefore, on the output side of the comparator 20, the reference potential appears in the normal operating state of the output stage, and on the contrary, in the case of overload, the potential depends on the value of the resistor 28 connected to the terminal of the operating voltage UB _2. Ascend to the given amount. This jumping potential change is transmitted to the base of the transistor 25 via the resistor 23 and the capacitor 24, and keeps this transistor conductive. As a result, the reference voltage input side of the comparator 20 is connected to the reference potential, and as a result, the output side of the comparator is reliably held at a high potential. This charges capacitor 24 while transistor 25 is held conductive by its base control.

Only when the capacitor 24 is fully charged,
The base-emitter voltage across the resistor 27 of the transistor 25 drops until the transistor 25 is de-energized and the reference voltage U _ref is again applied to the comparator. Due to the mute signal, the overload is no longer present at this point, so that the comparator output is at the reference potential.
This causes the capacitor 24 to be totally discharged via the diode 26, thus creating an initial condition for a new cycle.

When an overload situation occurs, the comparator output is immediately put back to the + UB ₂ − potential and the operating process is repeated until the overload disappears. This process is shown in the diagram
It is shown in Figures 1a, (b) and (c).

A corresponding signal for interrupting the dark current control, which is not shown in detail in the push-pull, can act via line 29 on the dark current control during overloading of the push-pull amplifier.

EFFECTS OF THE INVENTION In view of the difficulty in obtaining a predetermined breaking point in the past, according to the present invention, it is possible to achieve an accurate breaking point of the output stage at the time of overload.

[Brief description of drawings]

FIG. 1a is a configuration diagram of an embodiment of the present invention, and FIG. 1b is an explanatory diagram of changes in waveform during overload. 1,2 …… Output transistor, 3 …… Load impedance, 8 …… Preamplifier, 10 …… Differential amplifier

Continuation of the front page (56) Reference JP 59-168708 (JP, A) JP 54-157075 (JP, A) JP 57-206105 (JP, A) JP 59-79611 (JP , A)

Claims (2)

(57) [Claims] 1. A load impedance communicating with a reference potential is connected to the output side of the push-pull output stage, and at least one detecting element is provided for measuring an output current flowing through the push-pull output stage. In a circuit arrangement for protecting a transistorized push-pull output stage from overload, one detection element (4,5) is provided in each branch of the push-pull output stage (1,2). The voltage measured with respect to the reference potential is extracted from each of the detection elements, and the voltage is applied to one input side (+/−) of each differential amplifier stage (10), The output side of the amplifier stage (10) is connected to one input side of the comparator circuit (20), and the other input side of the comparator circuit (20) is placed at the reference potential (U _ref ). Comparator times When the input voltage supplied to the path (20) exceeds the reference potential (U _ref ), a signal is taken out from the output side of the comparator circuit (20), and the signal cuts off the push-pull output stage (1, 2). Is supplied to the means (30), the input side of the push-pull output stage (1, 2) is short-circuited by the means (30), and when the push-pull output stage is overloaded, the output signal of the comparator circuit (20) Time element (23,24,27)
And a reference voltage input side of the comparator circuit (20) is connected to a reference potential via an electronic switch (25) controlled by the timing element, and the timing element (23, 24, 27) includes a comparator circuit ( 20)
Of the series connection of the resistor (23) and the capacitor (24) provided between the output side of the electronic switch (25) and the control electrode of the electronic switch (25), and the control electrode of the electronic switch (25) and the reference potential. Circuit arrangement for protecting the push-pull output stage from overload, characterized in that it comprises a resistor (27) inserted in between. 2. A diode (26) is provided in the resistor (23) of the timing element, and its cathode is the comparator circuit (20).
The circuit device according to claim 1, wherein the circuit device is connected in parallel so as to be connected to an output side of the circuit device.