JPS5834965B2 - Transistor Zoufuku Cairo - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a transistor amplifier circuit, and is particularly directed to a transistor amplifier circuit whose output circuit is constituted by a push-pull circuit.

Generally, a transistor's junction temperature increases due to collector loss, and if this temperature exceeds a specified value, it will be destroyed.

Therefore, the maximum collector loss that does not lead to destruction of the transistor, that is, the maximum allowable collector loss, is constant when the junction temperature is constant. Therefore, if the collector-emitter voltage increases, the collector current must be reduced. There is a risk that the loss will exceed the maximum allowable collector loss and the transistor will be destroyed.

By the way, a push-pull circuit as shown in FIG. 3 is conventionally known as an acoustic power amplifier.

In this circuit, when the power supply voltage rises above the rated value, or when a surge voltage occurs at the L of the power supply filter when the power is turned on or off, the power transistors Q1□ to Q15 of the class B push-pull output circuit are There was a problem with it being destroyed.

In addition, in the above push-pull circuit, if the load is short-circuited, only the output capacitor C becomes a load, and its impedance is close to zero, and it becomes a capacitive load, causing a phase shift between the collector voltage and collector current. When the collector voltage is maximum for a large level input, the maximum collector current flows, and the collector loss may exceed the maximum allowable collector loss.

On the other hand, conventionally, output transistor Q12 due to load short circuit
~As a load short-circuit protection circuit to prevent damage to Q15, the output transistor collector and
A protection circuit that controls the output transistor by simultaneously detecting the emitter voltage and collector current was published in Japanese Utility Model Publication No. 47-8162.
Although disclosed in the publication ``Transistor Amplifier Circuit,'' the above protection circuit requires a control transistor Q against fluctuations in the power supply voltage in order to protect the output transistors Q1□, Q14 from being destroyed due to power-on or increase in power supply voltage.
The limiting sensitivity of 16 t Q1□ was low, and it was not suitable as a protection circuit for protecting the output transistor against fluctuations in the power supply voltage.

On the other hand, Japanese Patent Publication No. 45-33201 "Circuit protection device"
In this case, a power amplifier circuit and a semiconductor substrate temperature detection control circuit are formed in the same semiconductor integrated circuit substrate, and when the temperature of the junction of the output transistor of the power amplifier circuit rises abnormally, the semiconductor substrate temperature detection control circuit It is disclosed that the output transistor is prevented from being destroyed by overheating by blocking an alternating current signal flowing into the base of the output transistor.

However, in the semiconductor integrated circuit disclosed in Japanese Patent Publication No. 45-33201, the semiconductor substrate temperature detection control circuit compares the power supply voltage division voltage with the forward voltage of the diodes connected in series, so the power supply voltage is If the temperature rises abnormally, this semiconductor substrate temperature detection control circuit operates, and as a result, the collector-emitter path of the output transistor that constitutes the push-pull output stage of the power amplifier circuit is connected between the power supply and the output terminal. However, it has become clear that the emitter-base junction of the output transistor is forward biased and a relatively large current flows through it, which poses a high risk of destruction of the output transistor.

On the other hand, Japanese Patent Application No. 11661/1989 related to the earlier application of the present invention
No. 5 (Japanese Patent Publication No. 54-22272) ``Outputless push-pull transistor power amplifier circuit'', when a surge of power supply voltage (abnormally high power supply voltage) is applied, a voltage is generated between the power supply terminal and the output terminal. The collector
It has been proposed to substantially short-circuit the base-emitter junction of the output transistor to which the emitter path is connected by means of a protection transistor.

However, according to studies conducted by the inventors of the present application, when the emitter-base junction of a transistor is short-circuited, the base current becomes almost zero, so the collector-emitter breakdown voltage becomes a value close to the base-open collector-emitter breakdown voltage BVOEO, which is satisfactory. It has become clear that the possible anti-destruction action cannot be achieved.

In the present invention, when the emitter-base junction of a transistor is reverse biased, the base-open collector-emitter breakdown voltage BVC! This was done by focusing on the phenomenon that the collector-emitter breakdown voltage is determined by the breakdown voltage close to the emitter open collector-base breakdown voltage BVOBO, which has a value higher than Eo, and its purpose is to achieve high sensitivity. An object of the present invention is to provide a transistor amplifier circuit equipped with a highly reliable overvoltage protection circuit.

The present invention will be specifically described below along with examples and with reference to the drawings.

FIG. 1 shows an example of a transistor amplifier circuit according to the present invention.

As shown in the figure, the input signal ■in amplified by the first stage amplifier 1 is applied to the driving transistor Q1, and the push-pull output transistors Q12 to Q15 are driven by the transistor Q1□.

In this transistor amplifier circuit, the power supply voltage V that destroys the output transistors Q12 to Q15 is
A Zener diode Dz for obtaining a reference DC constant voltage for detecting a rise in CC, and a voltage dividing resistor R1 for detecting fluctuations in the power supply voltage VOO.

R2o is arranged.

Furthermore, a transistor Q18 is arranged to perform a voltage comparison between the reference DC voltage and the power supply divided voltage.
The collector output signal of output transistor Q1
□, control transistor Q that controls the base current of Q13
1. The base of is driven.

In this way, collector loss destruction due to an increase in the power supply voltage VOO of the output transistor can be prevented.

In addition, in the above circuit example, the transistor Q16゜Q
The present invention is applied to a transistor amplifier circuit having a circuit for preventing destruction of the output transistor due to a load short circuit configured by 1□, but it is not particularly necessary for the present invention, and it is necessary to take load short circuits etc. into consideration. If the amplifier circuit does not have transistor Q16. Q17 can be omitted.

According to the present invention explained in the examples above, the object can be achieved for the following reasons.

In the transistor amplifier circuit according to the present invention, the divided voltage of the resistors R10 and R2o increases as the power supply voltage vcc increases,
Therefore, the emitter voltage of voltage comparison transistor Q18 increases.

On the other hand, a reference DC constant voltage for detecting the collector breakdown voltage due to the Zener diode D8 is stamped on the base of the transistor Q18, and the normal power supply voltage VOO
If so, the transistor Q18 is made non-conductive, and therefore the control transistor Ql is made non-conductive, so that normal operation is not affected in any way.

When the divided voltage rises above the reference DC constant voltage due to an abnormal rise in the power supply voltage VOO, the voltage comparison transistor Q18 becomes conductive and the control transistors Q1. conducts, output transistor Q12. The base input of Q13 is shorted to ground to cut off the output transistor Q1.
□～Q1. This prevents collector loss and destruction.

Therefore, since the output transistor protection circuit according to the present invention amplifies the breakdown voltage detection signal from the voltage comparison transistor Q18 using the control transistor Q19, the control sensitivity can be increased, and voltage detection is performed using the stable Zener voltage as the reference voltage. Therefore, reliable protection operation can be performed.

Control transistor Q1. The base voltage of the output transistor Q1□ is maintained at ground potential due to the conduction of Since the potential of the common connection point with the resistor R1□ is maintained at the DC operating point (approximately half the potential of the power supply voltage ■cc), the output transistor Q42 p Q43
The emitter-base junction of is reverse biased and is in a cut-off state.

When the emitter-base junction of the transistor is biased in the reverse direction, the collector-emitter voltage cE-collector current IC characteristic is shown by curve R in FIG.

The solid line portion of the curve R indicates the non-destructive primary yield region,
The broken line portion of the curve R indicates the secondary destruction area of permanent destruction.

V when the emitter-base junction of the transistor is open
The OE-IO characteristic is shown by curve O in FIG.

The solid line part of the curve O indicates the non-destructive primary yield region,
The broken line portion of the curve O indicates the secondary destruction area of permanent destruction.

The VOE-IC characteristic when the emitter-base junction of the transistor is forward biased is shown by curve F in FIG.

The solid line part of the curve F indicates the non-destructive primary yield region,
The broken line portion of curve F indicates the secondary destruction area of permanent destruction.

When the emitter-base junction of a transistor is short-circuited, the base current becomes almost zero, so the collector-emitter breakdown voltage is the base open collector-emitter breakdown voltage B
The value is close to YOEO.

In the present invention, when the power supply voltage VOO increases abnormally, the control transistor Q1. The emitter-base junctions of the output transistors Q1□ and Q13 are biased in the reverse direction due to the conduction of The value is close to .

Thus, according to the present invention, the output transistor Q1□ due to an abnormal rise in the power supply voltage VOO.

It becomes possible to improve the strength against secondary fracture yielding of Q13, and the initial objective can be achieved.

[Brief explanation of drawings]

FIG. 1 shows an example of a transistor amplifier circuit according to the present invention, FIG. 2 shows VOE-IC characteristics when the emitter-base junction is controlled under various conditions, and FIG. 3 shows a conventional transistor amplifier circuit. FIG. 4 shows a conventional push-pull circuit for preventing load short circuits. 1...First stage amplifier circuit, Q11-Q28...
・Transistor, R1, ~R20, R3o-R4,・
...Resistance, DIl~D, D-D...
Diode, D8...Zena 13 21
23 Ner diode, C... Capacitor, L...
...Choke coil, SP...Speaker.

Claims (1)

[Claims] 1. A first output transistor whose collector-emitter path is connected between a power source and an output terminal, and whose collector-emitter path is connected between the output terminal and a base potential. A second output transistor, a DC cutting capacitor provided between the output terminal and the base potential and to which the output of the output terminal is added, and the first output transistor and the second output In a transistor amplifier circuit configured such that a transistor operates in a push-pull manner, a control transistor Q9 whose collector-emitter path is connected between the base of the first output transistor and the ground potential, and a power supply voltage . C or a comparison means DZ, R18, R19, R2o that compares the power supply voltage and the reference voltage and forms an output that makes the control transistor conductive when the power supply voltage or the power supply voltage division exceeds the reference voltage. , Q18, such that a reverse bias voltage is applied between the base and emitter of the first output transistor by the potential applied from the capacitor to the output terminal when the control transistor is made conductive. A transistor amplifier circuit characterized by: