Protection Circuit for Amplifier Circuits

The present invention relates to a protection circuit for amplifier circuits for protecting the circuit components of an amplifier circuit from deterioration or destruction when power source voltage for the amplifier circuit or the operation temperature thereof rises to exceed a predetermined value.

In amplifier circuits used in audio apparatus, particularly power amplifier circuits, rise in the power source voltage for the amplifier circuits and rise in the operation temperature thereof due to heat generation of the circuit components per se possibly give rise to deterioration or destruction of the circuit components of the amplifier circuits, particularly those circuit components arranged at the output stage thereof. In this field, it is common practice to employ separate protection circuits to protect the circuit components from deterioration or destruction due to the power source voltage rise and the operation temperature rise, respectively. Such a scheme, however, needs larger number of circuit components for the protection circuit construction, thus resulting in expensive protection circuits.

Accordingly, an object of the present invention is to provide a protection circuit for amplifier circuits for preventing the amplifier circuits from deterioration or destruction of the circuit components when the power source voltage or the operation temperature thereof rises to exceed a predetermined value. To this end, the protection circuit uses a by-pass circuit or signal interruption circuit driven in such a situation.

According to one aspect of the present invention, there is provided a protection circuit for amplifier circuits comprising first and second power source terminals; a power source voltage detection circuit which is connected between the first and second power source terminals and produces an output signal when voltage between the first and second power source terminals reaches a predetermined value; an operation temperature detection circuit for producing an output signal when the operation temperature exceeds a predetermined value; and a signal interruption circuit which is connected at the output terminal to the input terminal of an amplifier circuit to be protected and which interrupts an input signal flow to the amplifier circuit, in response to the output signal from the power source voltage detection circuit or the operation temperature circuit.

Additional objects and advantages of the invention will be apparent from the following description in connection with the accompanying drawings, in which:

FIG. 1 shows a circuit diagram of an embodiment of a protection circuit for amplifier circuits according to the invention;

FIG. 2 shows a circuit diagram of an amplifier circuit into which the protective circuit shown in FIG. 1 is incorporated;

FIG. 3 shows a circuit diagram of another amplifier circuit with a protective circuit of the invention which is another embodiment; and

FIG. 4 shows a transistor circuit usable in place of the Zener diode for constant voltage used in FIG. 3.

Reference is made to FIG. 1 illustrating an embodiment of a protection circuit for an amplifier circuit according to the present invention.

As shown, a series circuit including resistors R.sub.1 and R.sub.2 and n Zener diodes D.sub.z1 to D.sub.zn is inserted between a power source terminal 1 and a ground terminal 2. Another series circuit including a resistor R.sub.3 and a Zener diode D.sub.Z0 is inserted between the terminals 1 and 2. Resistors R.sub.4 and R.sub.5 are connected in series between the ground terminal 2 and a junction between the resistor R.sub.3 and the Zener diode D.sub.z0. A junction between the resistors R.sub.4 and R.sub.5 is connected to the base of an NPN transistor TR.sub.1. The transistor TR.sub.1 is connected at the collector to a junction between the resistor R.sub.2 and the Zener diode D.sub.z1, and at the emitter to the ground terminal 2, through m diodes D.sub.1 to D.sub.m.

Connected to a junction between the resistors R.sub.1 and R.sub.2 is a by-pass circuit, or a signal interruption circuit 3. The output terminal 4 of the signal interruption circuit 3 is coupled with the signal input terminal of an amplifier output stage 5, for example. As will be described later, when the potential difference across resistors R.sub.1 and R.sub.2 reaches a predetermined level, the signal interruption circuit 3 is energized to couple its output terminal 4 to ground, for example. Accordingly, in such a case, the signal to be applied to the signal input terminal of the amplifier circuit is by-passed by way of the signal interruption circuit 3 to ground, thereby to interrupt the signal flow to the amplifier circuit.

The operation of the protection circuit is now explained with reference to FIG. 1.

It is assumed that the Zener voltages of the Zener diodes D.sub.z1 to D.sub.zn are so selected that the total Zener voltage nV.sub.z1 is equal to a power source voltage V.sub.cc supplied to the amplifier circuit. When some reason causes the power source voltage V.sub.cc to exceed a predetermined value, current flows through resistors R.sub.1 and R.sub.2 and the Zener diodes D.sub.z1 to D.sub.zn. This increases the voltage at the junction between the resistors R.sub.1 and R.sub.2, and energization of the signal interruption circuit 3. As a result, the output terminal 4, that is, the signal input terminal at the output stage 5 in the amplifier circuit, is grounded. Accordingly, the input signal to be applied to the signal input terminal of the output stage is by-passed through the signal interruption circuit 3 so that little input signal is applied to the output stage and thus the circuit elements in the output stage are quiescent. In this manner, these circuit elements are protected from deterioration or destruction occurring at an abnormal rise in the power source voltage.

Description will now be given about a case where the temperature of the circuit elements of the amplifier circuit, that is, the operation temperature, rises.

Designations for necessary factors will first be given as follows: V.sub.z0 for the Zener voltage of the Zener diode D.sub.z0 ; V.sub.F for the forward voltages of m diodes D.sub.1 to D.sub.m and the voltage between the base and emitter of the transistor TR.sub.1 ; .DELTA.V.sub.z0 for the change rate of the Zener voltage V.sub.z0 of the Zener diode D.sub.z0 with respect to temperature change, i.e. the temperature coefficient; .DELTA.V.sub.F for change in the voltage V.sub.F of each of the diodes D.sub.1 to D.sub.m or voltage V.sub.F between the emitter and base of the transistor TR.sub.1 with respect to temperature change, i.e. the temperature coefficient. Assume now that the operation temperature of the amplifier circuit rises from the normal temperature T.sub.a to a higher temperature T.sub.b and that, at this time, the temperature of the Zener diode D.sub.z0, the transistor TR.sub.1, the diodes D.sub.1 to D.sub.m also rise up to the temperature T.sub.b. Under this condition, the Zener voltage V.sub.zb of the Zener diode D.sub.z0 at the temperature T.sub.b is expressed by the following equation:

The minimum base voltage V.sub.BM to permit the conduction of the transistor TR.sub.1 at the temperature T.sub.b, is given: ##EQU1## In order to render the transistor TR.sub.1 conductive when the temperature at the PN junctions of the diodes D.sub.1 to D.sub.m and the transistor TR.sub.1 is T.sub.b, the resistors R.sub.4 and R.sub.5 are so selected that the voltage drop V.sub.d5 across the resistor R.sub.5 is equal to the minimum base voltage V.sub.BM. This is expressed by the following equation: ##EQU2##

In this manner, when the transistor TR.sub.1 is turned on at the temperature T.sub.b, a current path is completed including the resistors R.sub.1 and R.sub.2, the collector-emitter path of the transistor TR.sub.1 and the diodes D.sub.1 to D.sub.m so that current flows through the current path between the power source terminal 1 and the ground terminal 2. The current flow produces a voltage drop across the resistor R.sub.2 which in turn energizes the signal interruption circuit 3 to couple the output terminal 4 with the ground terminal 2. As a result, the signal to be applied to the signal input terminal of the output stage of the amplifier circuit is by-passed through the signal interruption circuit to interrupt the supply thereof to the amplifier output stage.

Turning now to FIG. 2, there is shown a power amplifier circuit of SEPP (single ended push-pull) with the protection circuit shown in FIG. 1. In the figure, a preamplifier section 10 is provided with a well known differential amplifier and an AC power source and amplifies an input signal applied to its input terminal 11 up to a given level. The signal amplified in the preamplifier section 10 is supplied to a power amplifier section 20. The positive component of the signal is applied to the base of the NPN transistor TR.sub.2 in the section 20 while the negative component thereof to the NPN transistor TR.sub.3. The emitters of these transistors TR.sub.2 and TR.sub.3 are connected to the bases of NPN type output transistors TR.sub.4 and TR.sub.5, respectively. The emitter of the transistor TR.sub.4 is connected to the collector of the transistor TR.sub.5 and to an output terminal 21 through which an amplified output signal is delivered.

As shown in FIG. 1, the protection circuit 30 is connected between a positive power source terminal 1 and a ground terminal 2. The signal interruption circuit 3 is provided with PNP transistors TR.sub.6 and TR.sub.7 of which the emitters are connected to a positive power source terminal 1 and the bases are connected to a junction between resistors R.sub.1 and R.sub.2. The collector of the transistor TR.sub.7 is coupled with the emitter of the transistor TR.sub.4 through the resistor R.sub.6 and directly to the base of an NPN transistor TR.sub.8. The emitter of the transistor TR.sub.8 is coupled with the emitter of the output transistor TR.sub.4 and the collector thereof is connected to the base of the transistor TR.sub.2 of the power amplifier section 20. As will be recalled, the base of the transistor TR.sub.2 receives the positive component of the signal fed from the preamplifier section 10. The collector of the transistor TR.sub.6 is connected to the ground terminal 2 by way of the resistor R.sub.7 and directly to the base of an NPN transistor TR.sub.9. The emitter of the transistor TR.sub.9 is connected to the ground terminal 2 and the collector thereof is coupled to the base of the transistor TR.sub.3 for receiving the negative component of the signal.

In the above-mentioned amplifier circuit, when the power source voltage rises or the operation temperature rises, current flows through the resistors R.sub.1 and R.sub.2, thereby to render the transistors TR.sub.6 and TR.sub.7 conductive and then the transistors TR.sub.8 and TR.sub.9 conductive, too. As a result, the positive component of the signal outputted from the preamplifier section 10 is by-passed to the output terminal 21 through the transistor TR.sub.8 in conductive state while the negative component thereof is by-passed to the ground terminal 2 through the transistor TR.sub.9 in conductive state. Therefore, little signal is supplied to the bases of the transistors TR.sub.2 and TR.sub.3 of the power amplifier section 20 so that these transistors TR.sub.2 and TR.sub.3 are inoperative. In this manner, when the power source voltage or the operation temperature rises in the SEPP push-pull amplifier circuit, the power transistors TR.sub.4 and TR.sub.5 are protected from deterioration or destruction.

FIG. 3 shows a power amplifier circuit into which a protection circuit according to the present invention is incorporated. The power amplifier circuit is similar to that shown in FIG. 2, except that the negative signal component of the power amplifier section 20 is applied to the PNP transistors TR.sub.10 and not to the NPN transistors TR.sub.2 and TR.sub.5 and that the signal interruption circuit 3 is comprised of transistors TR.sub.6 and TR.sub.9. The rise of the power source voltage or the operation temperature causes current with a predetermined value to flow through the resistors R.sub.1 and R.sub.2 and thus to render the transistors TR.sub.6 and TR.sub.9 conductive. As a result, the positive and negative signal input terminals of the power amplifier section 20 are electrically connected with each other, that is, the bases of the transistors TR.sub.2 and TR.sub.10 are directly coupled with each other, and therefore the signal flow from the preamplifier section 10 to the power amplifier section 20 is shut off.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the invention. For example, n Zener diodes D.sub.z1 to D.sub.zn used in the protection circuit 30 shown in FIGS. 2 and 3 may be substituted by n transistors TR.sub.z1 to TR.sub.zn arranged in diode connection shown in FIG. 4.