JPS6247367B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transistor circuit, and includes a protection circuit that prevents the transistor from being destroyed by an abnormally large current when an output terminal contacts one end of a power supply while the circuit is operating in a steady state. Regarding amplifiers.

FIG. 1 shows a conventional amplifier with a built-in protection circuit. In Figure 1, R ₁ ... R ₉ are resistors, R _L are load resistances, C ₁ ... C ₃ are capacitors, Q ₁ ... Q ₁₀ are transistors, D ₁ ... D ₃ are diodes, and a is a signal An input terminal, b is a signal output terminal, e and g are positive and negative terminals of a power supply, I ₀ is a constant current source with a current of I ₀ , and Vcc ₁ is a power supply with a voltage of Vcc ₁ . This conventional protection circuit allows a current proportional to the collector current of the output transistor _Q7 to flow through the collector of the transistor _Q10 ,
The phase of _{the collector potential and collector current of output transistor Q 7 is} changed by transistor Q _9.
During normal operation, these phase relationships are the same, so transistor Q ₉ remains cut off, but during abnormal operation such as an output short circuit, this phase relationship changes significantly and transistors Q ₉ and Q ₈ are turned on and driven. This is a protection circuit that turns off transistor _Q3 . Now, during normal operation with no output short circuit,
When the output waveform is similar to the input waveform, the entire audio frequency band, and when the output waveform is saturated, the frequency
In the frequency band above 50Hz, the transistor shown in Figure 4
The base _{of transistor Q 9 ,}
Since the emitters are always in phase and reverse biased, transistor _Q9 is not conductive and the transistor
The protection circuits of _Q8 to _Q10 will not malfunction. However, when the output waveform is saturated and the frequency is below 50 Hz, a phase difference occurs between the collector potential and collector current of transistor Q ₇ due to the difference in the charging and discharging time constants of capacitors C ₂ and C _{3 .} As shown by the solid line showing the emitter potential and the dashed-dotted line showing the base potential, between A and B, the base and emitter of transistor Q ₉ temporarily become forward biased, transistor Q ₉ becomes conductive, and transistors Q ₈ - The drawback was that the _Q10 's protection circuit was activated. Here, for this type of protection circuit: (1) During normal operation, the protection circuit must not operate at any current value.

(2) In the event of an abnormality, power consumption (or current) must be limited to the lowest possible level to prevent element destruction.

Conventional protection circuits may not be able to satisfy condition (1).

The present invention provides a transistor circuit which prevents the malfunction of the conventional protection circuit during normal operation as described above.

An embodiment employing the present invention is shown in FIG. 2, and the principle of the present invention will be explained with reference to FIG. In Figure 2, R ₁ ... R ₉ is a resistance, R _L is a load resistance, C ₁ ...
... _C3 is a capacitor, _Q1 ... _Q10 is a transistor,
D ₁ ...D ₄ is a diode, a is a signal input terminal,
b is the signal output terminal, e and g are the positive and negative terminals of the power supply,
I ₀ indicates a constant current source with a current of I ₀ , and Vcc ₁ indicates a power source with a voltage of Vcc ₁ . During steady operation with no output short circuit, I ₀ = ( _{V BEQ7 − V BEQ10 )} /
A current _R9 flows. (Here, V _BEQ7 , V _BEQ10
represents the base-emitter voltage of transistors Q ₇ and Q ₁₀ . ) Now, when the voltage drop across the resistor R ₈ is smaller than the breakdown voltage V _ZD4 of the diode D ₄ (V _ZD4 > I ₀ R ₈₎ , the base potential V _BQ9 of the transistor Q ₉ is V _BQ9 = Vcc ₁ − I ₀ When V _ZD4 ≦I ₀ R ₈ , where the voltage drop across resistor R ₈ , represented by R ₈ , is greater than the breakdown voltage V _ZD4 of diode D ₄ , the base potential V _BQ9 of transistor Q ₉ is V _BQ9 = Vcc ₁ −
Since it is expressed as V _ZD4 , the highest value of the base potential of transistor _Q9 is determined by V _ZD4 . Therefore, the sixth
As shown in Figures 1 and 7, the base potential _VBQ9 of transistor _Q9 is shown by a dashed line and the emitter potential _VBQ9 is shown by a solid line. Set the breakdown voltage V _ZD4 of transistor Q ₉ to a value that does not reverse the phase of the base-emitter voltage of transistor Q ₉ .
In other words, select the potential at point C (Vcc ₁ - V _ZD4 ), and select a value that can sufficiently drive transistor Q ₉ during abnormal operation such as an output short circuit, that is, (Vcc ₁ - V _BEQ11 ).
≫(Vcc ₁ - V _ZD4 ) (Here, V _BEQ11 indicates the voltage between the base and emitter of transistor _Q11 )
If selected, the protection circuit will not malfunction during normal operation, and will be able to fully perform its function during abnormal operation such as an output short circuit. As described above, by setting the breakdown voltage V _ZD4 of the diode _D4 to an appropriate value, it is possible to realize an amplifier with a built-in protection circuit that does not malfunction under all conditions of actual use during steady operation.

Next, another embodiment is shown in FIG. In Fig. 3, R ₁ ...R ₁₀ is a resistance, R _L is a load resistance, C ₁ ...
... _C3 is a capacitor, _Q1 ... _Q10 is a transistor,
_D1 ... _D5 is a diode, a is a signal input terminal,
b is the signal output terminal, e and g are the positive and negative terminals of the power supply,
I ₀ indicates a constant current source whose current is I ₀ , and Vcc ₁ indicates a power supply whose voltage is Vcc ₁ . The output stage has a generally known class B single end pushable configuration. now,
During steady operation in this circuit, the output transistor Q ₇
A current of I ₀ =(V _BEQ7 −V _BEQ10 )/R ₉ flows through the collector of the transistor Q ₁₀ in proportion to the collector current of . (Here, V _BEQ7 and V _BEQ10 represent the base-emitter voltages of transistors Q ₇ and Q _10. ) Now, the voltage drop across resistor R ₈ is the voltage drop across diode D _4.
The breakdown voltage of V _ZD4 is smaller than V _ZD4 >
When I ₀ R ₈ , the base potential V _BQ9 of the transistor Q ₉ is expressed as V _BQ = Vcc ₁ - _{I 0} R ₈ , and the voltage drop across the resistor R ₈ is the breakdown voltage of the diode D ₄ V _ZD4
When V _ZD4 ≦I ₀ R ₈ , the base potential V _BQ9 of transistor Q ₉ is expressed as V _BQ9 = Vcc ₁ − V _ZD4 , so the highest value of the base potential of transistor Q ₉ is equal to the breakdown voltage V _ZD4 . It is decided. Therefore, as shown in Figures 6 and 7, during steady operation, even when the output waveform is oversaturated in all frequency bands, the breakdown voltage V _ZD4 of diode _D4 is equal to the base-emitter voltage of transistor _Q9 . The potential at point C (Vcc ₁ −V _ZD4 ) is set to a value that does not reverse the phase.
and a value that can sufficiently drive transistor Q ₉ during abnormal operation, that is, (Vcc ₁ −V _BED5 )≫
(Vcc ₁ - V _ZD4 ) (where V _BED5 indicates the forward conduction voltage of diode D ₅ ), the protection circuit will not malfunction during normal operation and will be sufficiently protected during abnormal operation. function can be activated.

As described above, by setting the breakdown voltage V _ZD4 of the diode _D4 to an appropriate value, it is possible to realize an amplifier with a built-in protection circuit that will not malfunction under all conditions of actual use during steady operation.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional example, FIGS. 2 and 3 are circuit diagrams showing an embodiment of the present invention, and FIG.
5 and 5 are voltage waveform diagrams illustrating the operation of a conventional circuit, respectively, and FIGS. 6 and 7 are voltage waveform diagrams illustrating the operation of a circuit according to an embodiment of the present invention. R ₁ ...R ₁₀ ...Resistance, R _L ...Load resistance, C ₁ ...C ₃
...Capacitor, Q ₁ ...Q ₁₀ ...Transistor, D ₁
...D ₅ ...diode, I ₀ ...constant current source, Vcc ₁ ...
Bias power supply, a, b, e, g...terminals.

Claims (1)

[Claims] 1. In a transistor circuit including a protection circuit that limits the base current of the output transistor when the output current of the output transistor connected to the output terminal exceeds a set value, the protection circuit is connected to the collector of the output transistor. A transistor that generates a collector current according to the current, a resistor and a constant voltage element connected in parallel between the collector of this transistor and one end of a power supply, and a transistor that responds to the potential difference between the output terminal and the collector of the transistor. and means for limiting a base current of the output transistor, and the constant voltage element is configured to conduct when the collector current of the transistor exceeds a predetermined value and clamp the potential of the collector of the transistor to a predetermined value. Characteristic transistor circuit.