Amplifiers – With semiconductor amplifying device – Including protection means
Patent
1979-08-09
1982-05-18
Mullins, James B.
Amplifiers
With semiconductor amplifying device
Including protection means
330267, H03F 330
Patent
active
043307574
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a power amplification circuit using semiconductor devices and, in particular to a semiconductor power amplification circuit equipped with an output short-circuit protection circuit to protect a transistor which is a part of the circuit, from being destroyed when the output terminal is short-circuited with one terminal of the power supply means.
The conventional transistorized power amplification circuit has a circuit construction of the output stage in which an output transistor operating as an NPN-type transistor and an output transistor as a PNP-type transistor are connected in series between the power supply lines, and the common connecting point of these two transistors is used as an output terminal, and in which a series circuit of a large capacitance capacitor and a loud speaker is connected between the above output terminal and one of the power supply line. However, such a basic circuit configuration of a power amplification circuit output stage alone has a drawback that, when the output terminal is connected directly to the one of the power supply lines to form a short circuit, an unlimited current flows through the output transistor positioned opposite to the power supply line connected to the output terminal, resulting in a breakdown of this output transistor.
As conventional means for overcoming such a drawback, there has been proposed a protection circuit in which a resistor having a resistance of 0.5 to several ohms is inserted between the output electrode of the output transistor such as the emitter and the output terminal, and, by detecting the voltage drop across the above resistor, the base and the emitter electrodes of the transistor through which large current is flowing are short-circuited to cause it to be in a cut-off condition, and thereby the breakdown of the output transistor is prevented. Such a protective circuiit has another shortcoming that of an output power loss due to the presence of the resistor positioned between the output electrode of the output transistor and the output terminal.
SUMMARY OF THE INVENTION
The object of the present invention is to obtain a semiconductor power amplification circuit equipped with a protection circuit to protect the output transistor from the destruction without consuming the output power when the output terminal is directly connected to the power supply line to form a short circuit.
The present invention provides a semiconductor power amplification circuit comprising: first and second transistors operating respectively in accordance with respective parts of the input signal and serially connected between power supply lines so as to produce an output signal according to the input signal; an output terminal connected to the common connecting terminal of these first and second transistors; a third transistor having a base connected to the base of the transistor to be protected, which is either one of the first and second transistors, and an emitter connected to the emitter of the transistor to be protected directly or through a resistor, the conduction type of the third transistor being the same as that of the transistor to be protected; a current mirror circuit having an input-side circuit connected between the collector of the third transistor and the power supply line supplying a current to the collector of the transistor to be protected; a fourth transistor having its base electrically connected to the output terminal of the current mirror circuit, its emitter electrically connected to the output terminal and its collector connected to a circuit section supplying a base current to the base of the transistor to be protected; and current-voltage conversion means connected to the base of the fourth transistor.
According to the present invention, the current equal to that flowing in the emitter of the transistor to be protected also flows to the emitter of the third transistor, and a current approximately equal to the emitter current is supplied to the input circuit of the curren
REFERENCES:
patent: 3845405 (1974-10-01), Leidich
patent: 4092612 (1978-05-01), Schade, Jr.
Fukaya Hirokazu
Togari Hisashi
Mullins James B.
Nippon Electric Co. Ltd.
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