JPS6336746Y2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in a protection circuit for a power amplifier that achieves high efficiency by switching output stage transistors in accordance with the level of an input signal.

_The above _- mentioned conventional power amplifier, for example, as shown in _{FIG .} Output power supply 3, 13
The emitters of transistors 4 and 14 to which a voltage of The transistor switch changes the input signal voltage v _i to
When the input signal voltage is lower than V ₁ , the input signal voltage is amplified by transistors 2 and 12 to supply power to the load 6, and when the input signal voltage is between voltages V ₁ and V ₂ , the input signal voltage is amplified by transistors 4 and 14. It is configured to amplify the voltage and supply power to the load 6. In the above, the part in parentheses indicates one side of the push-pull amplifier, 9 and 19 are drive stage transistors, and 31 and 41 are backflow blocking diodes.

In such a power amplifier, transistors 2,1
In order to protect transistors 2, 4, and 14 from overload, transistors 10 and 20 are connected between the input terminal and the output terminal, and the bases of transistors 10 and 20 are connected to the emitters of transistors 2 and 12 to protect them from overload. The current flowing through emitter resistors 5 and 15 is detected by the voltage drop across emitter resistors 5 and 15, and when overloaded, transistors 10 and 20 are turned on to limit the input signal voltage.

However, in the power amplifier described above, that is, in the power amplifier in which the output stage transistors are connected in parallel and switched depending on the input signal level, when the output terminal is short-circuited, an excessive current flows only to transistors 2 and 12 to which a low voltage is applied. Further, when the load has a low impedance (for example, 1/2 of the rated value) that is not substantially zero (short circuit), the peak current of the excessive current flows only through the transistors 4 and 14 to which the high voltage is applied. Now transistor 1
If the limiter level at low loads where limiter is applied by 0,20 is set to 1/2 of the rated load, emitter resistors 5, 15 are common to transistors 2, 12 and 4, 14, so when the load terminals are short-circuited, The limiter level will also have the same value.

However, on the other hand, in the power amplifier described above, due to the original purpose of high efficiency, the transistors 2 and 1 are
It is desirable to use a transistor 2 with a current rating smaller than that of the transistors 4 and 14. However, when using the conventional protection circuit described above, transistors 2 and 1
Similar to transistors 4 and 14, transistors 2 and 14 had the disadvantage that it was necessary to use a transistor with an I _{c nax} higher than the peak current at low load of 1/2.

The present invention was developed in consideration of the above, and it eliminates the above drawbacks with a simple configuration, and allows the use of transistors with low current ratings that are compatible with the original purpose of high efficiency for transistors to which low voltage is applied. The purpose of this invention is to provide a protection circuit for a power amplifier.

The present invention will be explained below with reference to examples.

FIG. 2 is a circuit diagram of an embodiment of the present invention, in which the same components as those of the power amplifier protection circuit shown in FIG. 1 are denoted by the same reference numerals.

In this embodiment, transistors 2 and 1 are used in the conventional power amplifier protection circuit shown in FIG.
Resistors 32 and 42 are connected between the emitters of transistors 4 and 12 and the emitters of transistors 4 and 12, respectively.

In this embodiment configured as above, the load 6
becomes a low impedance, the voltages v _a(l) and v _{b(l) between} the emitters of transistors 2 and 12 and the output terminal P at the maximum current are v _{a(l )} = i _c4 R ₅ , and similarly v _b(l) = i _c14 R ₁₅ .

Here, i _c2 and i _c12 are currents flowing from the emitters of transistors 2 and 12 to resistors 32 and 42, and i _c4 and i _c14
is from transistors 4, 14 to emitter resistors 5, 1
R ₅ , R ₁₅ , R ₃₂ and R ₄₂ are the resistance values of resistors 5 and 15 and resistors 32 and 42, respectively.

Next, when the load is short-circuited, the voltages v _a(s) and v _b(s) between the emitters of transistors 2 and 12 and the output terminal P are i _c4 =
0, v _a(s) = i _c2 (R ₅ + R ₃₂ ), and likewise v _b(s) = i _c12 (R ₁₅ + R ₄₂ ).

Now, if the voltages at which the limiter transistors 10 and 20 operate are v _a and v _b , then v _a = v _a(s) = v _a(l) v _b = v _b(l) = v _b(s) Therefore, the relationship between the currents i _c2 , i _c12 and i _c4 , i _c14 is i _c2 = R ₅ /R ₅ +R ₃₂ i _c4 , i _c12 = R ₁₅ /R ₁₅ +R ₄₂ i _c14 , and the resistance 32, By setting 42, the currents _ic2 and _ic12 can be set smaller than _ic4 and _ic14 . Therefore, the transistors used for the transistors 2 and 12 can be transistors with a small current rating that meets the original purpose of the power amplifier, which is to obtain high efficiency.

In addition, the idle current when there is no input signal flows through transistors 2 and 12, and the emitter resistance value is resistor 3.
2, 42 and the resistors 5, 15 and becomes large, so the power amplifier becomes stable.

As explained above, according to the present invention, the emitter of the transistor to which the voltage of the low voltage power supply is applied,
With a simple configuration in which a resistor is connected between the emitter of the transistor to which the voltage of the high-voltage power supply is applied, the overcurrent protection level can be set separately for load short-circuit and low load. A transistor suitable for the original purpose of a power amplifier, which aims to obtain high efficiency, can be used as the transistor.

Another advantage is that the power amplifier is stabilized when no signal is input.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a conventional power amplifier protection circuit. FIG. 2 is a circuit diagram of an embodiment of the present invention. 1 and 11...Low voltage output power supply, 2, 4,
12 and 14...transistor for output stage, 3
and 13... high voltage output power supply, 5 and 15
...Emitter resistance, 6...Load, 10 and 11
...Limiter transistors, 32 and 42
……resistance.

Claims (1)

[Scope of utility model registration request] The emitter of the first transistor to which the voltage of the first power supply is applied is common to the emitter of the second transistor to which the voltage of the second power supply that outputs a voltage higher than the voltage of the first power supply is applied. The first transistor or the second transistor is connected to the load through an emitter resistor, and drives the load depending on the level of an input signal, and the emitter of the first transistor is connected to the base, so that when an overload occurs, the first transistor or the second transistor drives the load. In a power amplifier protection circuit including a third transistor that limits an input signal,
A protection circuit for a power amplifier, characterized in that a second emitter resistor is connected between the emitter of the first transistor and the emitter of the second transistor.