JP2834051B2 - Over-input protection circuit for transistor amplifier - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an over-input protection circuit for a transistor amplifier, and more particularly to an over-input protection circuit for protecting a transistor amplifier whose operating point changes due to self-bias from over-input.

[0002]

2. Description of the Related Art FIG. 3 shows a circuit example of a transistor amplifier whose operating point changes due to self-bias. Here, the high-frequency amplification transistor TR1 whose emitter is grounded is
A bias voltage is supplied to the base from the collector power supply Vcc via the resistors R1 and R2, and a signal input to the base is amplified.

When the input signal level is low, for example, as shown in FIG. 4 (a), the base-emitter voltage Vbe of the transistor TR1 is maintained substantially constant to perform a class A amplification operation. When the input signal level increases, as shown in FIG. 4B, Vbe decreases due to the current rectified by the diode characteristics between the base and the emitter, so that class B or class C amplification is performed.

For example, if the relationship between the base-emitter voltage Vbe of the transistor TR1 and the signal level is as shown in FIG. 5, if the signal level is about 0 dBm or less, Vbe becomes about 0.7 [ V], and FIG.
A class A amplification operation as shown in FIG. In addition, Vbe decreases as the signal level increases beyond 0 dBm. Then, when the signal level becomes 10 dBm, Vbe drops to about 0.4 [V], and as shown in FIG. 4B, the signal peak reaches the emitter-base breakdown voltage Vebo. Perform the amplification operation.

[0005]

As described above, in a transistor amplifier in which the operating point changes due to self-bias, when the input signal level increases, Vbe decreases to perform a class B or class C amplifying operation. However, when the transistor is operated for a long time with the signal peak reaching the breakdown voltage Vebo between the emitter and the base, the characteristics of the transistor deteriorate over time and over time, and a breakdown phenomenon occurs due to a decrease in the current amplification factor hfe. And the output of the transistor amplifier is reduced.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an over-input protection circuit that protects a transistor amplifier that amplifies by a self-bias from inputting an excessive signal and improves the reliability of the transistor amplifier.

[0007]

An over-input protection circuit for a transistor amplifier according to the present invention is provided with a variable attenuator on the input side of a first-stage transistor of the transistor amplifier, and controls a base-emitter voltage Vbe and an input signal level of the transistor. Detects and starts attenuation control of the variable attenuator when Vbe falls below a preset value based on the breakdown voltage between the emitter and the base, and then changes when the input signal level falls below a predetermined value. Stop the attenuation control of the attenuator. The variable attenuator includes a fixed attenuation circuit that attenuates a large signal without distortion and a variable attenuation circuit that varies an amount of attenuation according to an applied voltage.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. Here, the transistor amplifier 2 has the same configuration as that shown in FIG.
Is provided with an over-input protection circuit 1.

The over-input protection circuit 1 includes a CM coupler 11 for extracting an input signal S1, a detection unit 12 for detecting an output of the CM coupler 11 and outputting a detection voltage Vs indicating an input signal level, and a transistor. Base-emitter voltage Vbe of first stage high frequency amplification transistor TR1 of amplifier 2
, A variable attenuator 14 for attenuating the input signal S1 according to the control signal Cs,
A control unit 15 that generates a control signal Cs based on the emitter-to-emitter voltage Vbe and the detection voltage Vs and controls the variable attenuation unit 14
And

The CM coupler 11 is formed by a strip line, and a coupling strip line having a terminating resistor is provided at one end in parallel with the strip line transmitting the input signal S1, and the CM coupler 11 performs the CM coupling. A part of the input signal S1 is extracted.

Next, the operation of the over-input protection circuit 1 will be described.

Here, it is assumed that the base-emitter voltage Vbe of the transistor TR1 of the transistor amplifier changes as shown in FIG. 5, and that a signal higher than the emitter-base breakdown voltage Vebo is not applied. Therefore, control is performed so that Vbe does not drop below 0.6 [V], that is, the level of the signal S2 does not become 5 dBm or more.

Further, when the level of the input signal S1 is
As shown in (a), it sequentially increases from t1 to t3 after the start of the operation, reaches a peak of 10 dBm at t3, and then increases from t3 to t3.
It is assumed that it gradually decreases at t5. That is, the detection voltage V
s becomes v0 (0 dBm) when t1 and t5, and t
2 and t4, v1 (level of signal S1 is 5 dBm), and during the period from t2 to t4, v1 or more (5 dBm).
Above).

By the way, the base of the transistor TR1
As shown in FIG. 5, the voltage Vbe between the emitters decreases when the level of the signal S2 input to the transistor TR1 exceeds 0 dBm. Therefore, the signal level becomes excessive by monitoring the decrease of Vbe. Can be detected. The control unit 15 detects the detection voltage Vs from the detection unit 12 and the base-emitter voltage Vb from the Vbe detection unit 13.
The amount of attenuation of the variable attenuator 14 is controlled in accordance with e, so that a signal higher than the emitter-base breakdown voltage Vebo is not applied to the transistor TR1. Note that the amount of attenuation of the variable attenuation unit 14 at the start of the operation is set to 0 dB.

The variable attenuator 14 at the start of the operation
Is 0 dB, the signal S2 of the transistor TR1 at the beginning of the operation sequentially increases as shown in FIG. 2B, similarly to the input signal S1. Signal S2 is 0
If it exceeds dBm, the base-emitter voltage Vbe of the transistor TR1 of the transistor amplifier becomes
As shown in FIG. Then, when the level of the signal S2 becomes 5 dBm (time t
2), Vbe decreases to 0.6 [V].

The control unit 15 starts the attenuation control of the variable attenuation unit 14 when Vbe drops to 0.6 [V], and sends out a control signal Cs according to the detection voltage Vs from the detection unit 12. , The amount of attenuation is controlled as shown in FIG.

The variable attenuator 14 has a fixed attenuator using a resistor for attenuating large signals without distortion and a variable attenuator using a semiconductor element capable of varying the amount of attenuation according to an applied voltage. After being attenuated by the fixed attenuating circuit, it is attenuated by the variable attenuating circuit. For example,
The attenuation of the fixed attenuation circuit is 5 dB, the attenuation of the variable attenuation circuit is 0 to 15 dB, and when the attenuation operation is stopped, the fixed attenuation circuit is bypassed to make the overall attenuation 0 dB. By setting the attenuation of the circuit to 5 dB and the attenuation of the variable attenuation circuit to 0 to 15 dB, the entire attenuation can be 5 to 20 dB. With this configuration, the large signal is attenuated by the fixed attenuation circuit, and then the low-level signal is attenuated in the variable attenuation circuit. Therefore, the large signal can be attenuated without distortion.

At time t2, the control unit 15 starts the attenuation control of the variable attenuation unit 14, and the level of the signal S2 input to the transistor TR1 changes to the level shown in FIG.
As shown in (b), the power level suddenly decreases from 5 dBm to 0 dBm, and is thereafter maintained at 0 dBm. Thus, the base-emitter voltage Vbe of the transistor TR1 is
As shown in (c), the voltage returns from 0.6 [V] to 0.7 [V] and is maintained.

Thereafter, the level of the input signal S1 is 5 dBm
When the detection voltage Vs has decreased to v1 (time t4), the control unit 15 stops the attenuation control of the variable attenuation unit 14, and reduces the attenuation amount as shown in FIG. 0 dB. At this time, as shown in FIGS. 2B and 2C, the signal S2 input to the transistor TR1
Temporarily increases and Vbe sharply decreases to 0.6 [V], but since the level of the input signal S1 sequentially decreases to 0 dBm (time t5), Vbe is 0.7.
It recovers to [V].

In this manner, control can be performed so that an excessive signal higher than the emitter-base breakdown voltage Vebo is not applied to the transistor TR1.

The variable attenuator 14 may be composed of only a variable attenuator using a semiconductor element for attenuating a large signal without distortion.

[0023]

As described above, according to the present invention, a variable attenuator is provided on the input side of the first-stage transistor of a transistor amplifier, and the base-emitter voltage V
be and the input signal level are detected, and when Vbe falls below a preset value based on the emitter-base breakdown voltage, the attenuation control of the variable attenuator is started. By stopping the attenuation control of the variable attenuator when the voltage drops below, it is possible to control the excessive signal exceeding the breakdown voltage between the emitter and the base so that it is not applied to the first-stage transistor. Can be improved.

Further, the variable attenuator includes a fixed attenuator for attenuating a large signal without distortion and a variable attenuator for varying the amount of attenuation according to an applied voltage, so that a large signal can be variably attenuated without distortion.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a diagram for explaining an operation of the over-input protection circuit 1 shown in FIG.

FIG. 3 is a circuit diagram illustrating an example of a transistor amplifier that performs an amplification operation by a self-bias;

FIG. 4 is a diagram illustrating an amplification operation of the transistor illustrated in FIG. 3;

5 is a diagram showing a relationship between a base-emitter voltage Vbe and a signal level of the transistor shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Over-input protection circuit 2 Transistor amplifier 11 CM coupler 12 Detection part 13 Vbe detection part 14 Variable attenuation part 15 Control part S1 Input signal TR1 Transistor Vbe Base-emitter voltage Vs Detection voltage

Claims (2)

(57) [Claims] 1. An over-input protection circuit for protecting a first-stage transistor of a transistor amplifier whose operating point changes due to a self-bias from an over-input, a signal level detecting means for detecting a level of an input signal, and an input side of the first-stage transistor. A variable attenuating means for attenuating the input signal, a Vbe detecting means for detecting a base-emitter voltage Vbe of the first-stage transistor, and an output from the signal level detecting means and an output from the Vbe detecting means. Control means for controlling the amount of attenuation of the variable attenuating means, wherein the control means reduces the base-emitter voltage Vbe below a preset value based on the emitter-base breakdown voltage of the first-stage transistor. At this time, the attenuation control of the variable attenuation means is started, and thereafter, the control is performed in accordance with the input signal level. Excessive input protection circuit of the transistor amplifier the input signal level is characterized by stopping the damping control when drops below a predetermined value and performs the attenuation control Te. 2. The transistor according to claim 1, wherein said variable attenuating means includes a fixed attenuating circuit for attenuating a large signal without distortion and a variable attenuating circuit for varying an amount of attenuation according to an applied voltage. Amplifier over input protection circuit.