JPS5941618Y2 - Transistor amplifier protection circuit - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a protection circuit for a transistor amplifier that protects a power transistor in a transistor OTL circuit and limits the output.

In general, in audio amplifiers using transistors, and in particular in transistor OTL circuits, power transistors are often destroyed due to load short circuits, excessive input application, etc., and conventionally there has been a demand for ways to prevent this.

FIG. 1 is a circuit diagram of a transistor amplifier using a conventional protection circuit of this type.

In this figure, when the load RL is short-circuited and an excessive current flows through the power transistor Tr3, the voltage across the protective resistor RE rises and passes through the bias circuit 1 to one of the detection transistors Trl(/, ,) increases the base bias.

This transistor Trl is therefore conductive, limiting the power transistor T r s OJ current and protecting this transistor T r 3.

Furthermore, if an excessive current flows through the other power transistor Tr4, the other detection transistor T
r2 operates and protects power transistor Tr4.

In FIG. 1, D is a diode, C is a DC blocking capacitor, and R is a resistor.

However, as can be seen from FIG. 1, such a conventional protection circuit requires a detection circuit including detection transistors Tr1 and Tr2 for detecting excessive current of each of the power transistors Tr3 and Tr4. It has the drawbacks of high cost and high cost.

This invention was made in order to eliminate the above-mentioned drawbacks, and it is low cost and fully functional. ．． -) The purpose is to provide a protection circuit for transistor amplifiers that can protect transistors.

A practical example of this invention will be described below with reference to FIGS. 2 and 3.

FIG. 2 is a diagram showing an example of a transistor amplifier to which the protection circuit according to this invention is applied. In this figure, Trl and Tr2 are NPN type and PNP type detection transistors, and Tr 3 and Tr 4 are complementary transistors. NPN type and PN type that form the main configuration of OTL circuit
The P-type power transistors and RE are their protective resistors, respectively.

A bias circuit 1, which includes a capacitor C1 and resistors R1 and R2, is provided at both ends of the protection resistor RE of the power transistor Tr4, and provides a base bias to the detection transistor Tr1.

Detection transistor T r 10. ) The collector is connected to the base of the power transistor Tr, 0) via a reverse current blocking diode D1 having the polarity shown, and is also connected to the base of the detection transistor Tr 20).

The collector of this detection transistor Tr2 is connected to the base of the power transistor Tr4, and the emitter is connected to the amplifier midpoint (here, the connection point of resistors R3 and R4 provided corresponding to the bias circuit 1). .

Note that the other parts are the same as those in FIG. 1, so the same reference numerals are given and the explanation thereof will be omitted.

In such a protection circuit, when the resistance value of the load RL decreases due to a short circuit or the like, the current increase at that time is detected by the protection resistor RE.

The bias circuit 1 then detects the detection transistor Tr.
1 is given a base bias that makes it conductive, the base bias of the power transistor T r 30) is pulled in the non-conductive direction.

Furthermore, when the transistor Tr1 becomes conductive, the detection transistor Tr2 also becomes conductive, and the power transistor Tr4
The CD base bias will also be pulled in the non-conductive direction.

Therefore, the current of both power transistors Tr3 and Tr4 is limited, and their protection and output are limited.

In the above embodiment, the bias circuit 1 of the detection transistor Tr4 is provided across the protection resistor RE of the power transistor Tr4, that is, the power transistor T
Although a current increase 0 detection circuit is provided on the r4 side, for example, as shown in FIG.
r30:) The protection resistor RE may be provided at both ends, and the detection circuit may be provided on the power transistor Tr3 side.

In FIG. 3, the same parts as in FIG. 2 are designated by the same reference numerals, and their explanation will be omitted.

As described above, according to this invention, it is not necessary to provide a circuit for detecting an increase in current for both power transistors as in the conventional case, and only one of them can be used, which makes it possible to reduce costs. As with the conventional method, both power transistors can be protected.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of a transistor amplifier using a conventional protection circuit, Figure 2 is a circuit diagram showing an example of a transistor amplifier to which the protection circuit of this invention is applied, and Figure 3 is another example. It is a circuit diagram. Trl p Tr2...Detection transistor,
Tr3 +Tr4...Power transistor, R
E: Protection resistor, C: DC blocking capacitor, RL: Load, Dl:
Backflow blocking diode, R# R3M R4...
・Resistance, 1...Bias circuit.

Claims (1)

[Scope of utility model registration request] In a transistor OTL circuit, a bias circuit is provided across a protective resistor of any one of the power transistors, and a base bias is applied by this bias circuit, and the collector is connected to the base of the other power transistor via a backflow blocking diode. A connected first detection transistor, a base connected to the collector of the first detection transistor, a collector connected to the base of one of the power transistors, and an emitter connected to the midpoint of the amplifier. A protection circuit for transistor amplifiers featuring: