JPS6143891B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a device for detecting pulse noise contained in a received signal of a radio receiver or the like, and particularly relates to a device for detecting pulse noise contained in a received signal of a radio receiver, etc. The purpose is to prevent destruction. 2. Description of the Related Art A system is known that detects pulse noise in a received signal and blocks a gate provided in a signal transmission path during a period when the pulse noise is present to prevent the noise from occurring. As a pulse noise detection device used in the above system, one as shown in FIG. 1 can be considered. In FIG. 1, 1 is an input terminal to which high frequency components of continuous noise such as pulse noise and white noise obtained through a high-pass filter (not shown) are applied, and 2 is a signal applied to the input terminal 1. 3 is a double-wave rectifier circuit for double-wave rectification of the output signal of the differential amplifier circuit 2 ; 4 is a circuit for detecting pulse noise contained in the output signal of the double-wave rectifier circuit 3 ; 5 is a smoothing circuit for generating a DC signal corresponding to the continuous noise level included in the output signal of the double-wave rectifier circuit 3 ; 6 is a smoothing circuit for controlling the gain of the differential amplifier circuit 2 ; It is a control circuit. While the continuous noise applied to input terminal 1 is small,
The voltage across the smoothing capacitor 7 of the smoothing circuit 5 is not large enough to make the first and second control transistors 8 and 9 of the control circuit 6 conductive.
is in an immobile state. Thus, the signal applied to the input terminal 1 is amplified by the differential amplifier circuit 2 , and is transmitted from the collectors of the pair of transistors 10 and 11 to the impedance conversion transistors 12 and 1, respectively.
3 to the double-wave rectifier circuit 3 . The first rectifier transistor 14 of the rectifier circuit 3 conducts when the output signal of one transistor 10 of the differential amplifier circuit 2 is larger than the output signal of the other transistor 11, and the second rectifier transistor 15 conducts when the output signal of the other transistor 11 of the differential amplifier circuit 2 is larger than the output signal of the other transistor 11. The circuit is connected so that the output signal of the transistor 11 becomes conductive when the output signal of the transistor 11 is higher than the output signal of the transistor 10. Therefore, a double-wave rectified signal is obtained at the common collector of the first and second rectifying transistors 14 and 15. the first and second rectifying transistors 14 and 1;
Pulse noise among the signals obtained at the common collector 5 is detected by the pulse noise detection circuit 4 . That is, pulse noise generally has a sufficiently higher level than continuous noise. Therefore, by appropriately setting the voltage dividing ratio of the first and second voltage dividing resistors 16 and 17 of the pulse noise detection circuit 4 and setting a predetermined threshold level, it is possible to detect when a sufficiently large level of pulse noise exists. only detection transistor 18
is conductive and the output terminal 1 connected to its collector
A detection pulse is obtained at 9. At this time, pulse noise with little level difference compared to continuous noise is not detected, but such pulse noise may be regarded as continuous noise and ignored without causing any inconvenience. On the other hand, the first and second rectifying transistors 14
A signal related to continuous noise obtained at the common collectors of 1 and 15 is applied to the smoothing circuit 5 via the charging transistor 20 and charges the smoothing capacitor 7.
Therefore, the voltage across the smoothing capacitor 7 gradually increases. While the continuous noise level applied to the input terminal 1 is low, the voltage across the smoothing capacitor 7 does not become high enough to make the first and second control transistors 8 and 9 of the control circuit 6 conductive; When becomes large, the voltage across the smoothing capacitor 7 also increases and becomes the base-emitter voltage (V _BE ) of the first and second control transistors 8 and 9.
and 9 exhibit a conductive state. When the first and second control transistors 8 and 9 are rendered conductive, gain control of the differential amplifier circuit 2 is started. Now, if the collector current of the first control transistor 8 is I ₁ and the collector current of the second control transistor 9 is I ₂ , then I ₁ =I ₂ (1). Further, if the collector current of the transistor 22 of the current mirror circuit 21 is I ₃ , then I ₁ =I ₃ (2). Therefore, from equations (1) and (2), I ₂ = I ₃ (3). When the second control transistor 9 becomes conductive, the diodes 23, 24, 25, and 26 become conductive, and the internal impedance of the diodes 23 to 26 decreases, so that the input signal applied to the input terminal 1 is attenuated. As a result, gain control of the differential amplifier circuit 2 is achieved. At that time, as shown in equation (3), I ₂ = I ₃ , so the differential amplifier circuit 2
The negative effect on the base bias of is prevented. As described above, the circuit shown in FIG. 1 performs gain control, so even if the detection level of the pulse noise detection circuit 4 is fixed, it can effectively detect only pulse noise above a predetermined level without malfunctioning. can be detected. However, in the circuit shown in Figure 1,
There is a risk that the first and second control transistors 8 and 9 will be destroyed by excessive collector current. That is, as the continuous noise level applied to the input terminal 1 increases due to a decrease in electric field strength, etc., the degree of conductivity of the first and second control transistors 8 and 9 of the control circuit 6 increases accordingly, resulting in a very large Collector current flows. Therefore, there is a risk that the first and second transistors 8 and 9 will be destroyed by the very large collector current. The present invention has been made in view of the above points, and is provided with a current limiting circuit that limits the collector current of the control transistor of the control circuit that controls the gain in a pulse noise detection device as shown in FIG. The purpose of this invention is to prevent the control transistor from being destroyed. Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows an embodiment of the present invention, which is constructed exactly the same as the circuit shown in FIG. 1 except for a current limiting resistor 26 connected in series to the collector of the first control transistor 8. In FIG. However,
When a large continuous noise is applied to the input terminal 1 and the collector current _I1 of the first control transistor 8 increases, the voltage drop across the current limiting resistor 26 increases accordingly, and the collector voltage of the first control transistor 8 increases. descend. When the collector current I ₁ reaches a predetermined value, the first control transistor 8 is saturated and the collector current stops increasing. circuit 21
Prevention of destruction such as the following is achieved. As a simple transistor destruction prevention circuit, the one shown in FIG. 2 is sufficient. FIG. 3 shows another embodiment of the invention. The circuit shown in FIG. 2 can sufficiently prevent transistor destruction under normal operating conditions. However, the circuit shown in FIG. 2 has a drawback that when the power supply voltage (+Vcc) decreases, the first control transistor 8 becomes saturated while the collector current _I1 of the first control transistor 8 is small, and the control range becomes narrow. On the other hand, when the power supply voltage (+Vcc) increases, the transistor 8 is not saturated even if the collector current of the first control transistor 8 becomes considerably large, so that the first control transistor 8 is destroyed. There is danger. The circuit shown in FIG. 3 uses the power supply voltage (+
This circuit is designed to be able to respond to changes in Vcc), and is different from the circuit shown in FIG. A current limiting transistor 28 operated by the collector current of the second current mirror transistor 27 is provided. When the input signal applied to the input terminal 1 increases and the collector current of the first control transistor 8 increases, a current equal to the collector current flows through the second control transistor 9 and the second current mirror transistor 2.
It flows to 7. Then, a voltage is generated across the resistor 29 depending on the collector current of the second current mirror transistor 27. Therefore, if the value of the resistor 29 is appropriately set, the current limiting transistor 28 becomes conductive when a predetermined collector current of the second current mirror transistor 27, that is, a predetermined collector current of the first control transistor 8 flows. , bypassing the base current of the first control transistor 8 and the second control transistor 9, the collector current of the first control transistor 8 and the second control transistor 9 is limited, thereby limiting the collector current of the first control transistor 8 and the second control transistor 9. Prevention of destruction such as the following is achieved. The circuit shown in FIG. 3 can accomplish the main purpose of preventing destruction of the transistor as described above, and its operation is similar to that of the second current mirror transistor 27.
Since it depends only on the collector current of , it has the advantage that a stable protection operation can be performed regardless of changes in the power supply voltage. As described above, the pulse noise detection device according to the present invention has an excellent pulse noise detection function and is excellent in being able to prevent destruction of a transistor that performs gain control.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram for explaining the present invention, and FIGS. 2 and 3 are circuit diagrams each showing an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1...Input terminal, 2 ...Differential amplifier circuit, 3 ...Double wave rectifier circuit, 4 ...Pulse noise detection circuit, 5 ...Smoothing circuit, 6 ...Control circuit, 8, 9...Control transistor,
26... Current limiting resistor, 27... Second current mirror transistor, 28... Current limiting transistor.

Claims (1)

[Scope of Claims] 1. An amplifier circuit that amplifies pulse noise and continuous noise in a received signal, a first detection circuit that detects a continuous noise level in an output signal of the amplifier circuit, and a pulse noise in the output signal. a second detection circuit that detects
a control circuit driven by the output signal of the first detection circuit, the control circuit controls the gain of the amplifier circuit, and extracts pulse noise of a predetermined level or higher from the second detection circuit. In the pulse noise detection device, the control circuit includes:
The control transistor has a control transistor to which the output signal of the first detection circuit is applied to the base, and a limiting circuit that limits the input signal of the amplifier circuit according to the control transistor, and the control transistor is configured to prevent destruction due to an increase in continuous noise level. A pulse noise detection device comprising a current limiting circuit that limits the collector current of the control transistor for prevention. 2. The control circuit includes a first control transistor, a current mirror circuit, a second control transistor, and an input signal limiting circuit for the amplifier circuit, and the current mirror circuit controls the input signal of the first and second control transistors. The collector currents are set to be equal, and the input signal limiting circuit is configured to be controlled by the collector current of the second control transistor, and the current limiting circuit is configured to control the input signal limiting circuit by the collector current of the second control transistor.
A transistor that can flow a current equal to the collector current of the control transistor, and a current-limiting transistor driven by the current of the transistor, and the base current of the first control transistor is controlled by the current-limiting transistor. 2. The pulse noise detection device according to claim 1, wherein the pulse noise detection device is configured to limit the amount of noise.