JPS5842964B2 - limiter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic noise limiter circuit suitable for integrated circuits such as in-vehicle AM receivers.

FIG. 1 shows a conventionally commonly used noise limiter circuit.

In the figure, 1 is a limiter diode, 2 is a detection diode, 3, 4.5 is a capacitor, and 6, 7.8 are terminals.

In the circuit shown in the figure, a negative half wave of the AM output from the detection coil DET-C is detected by a diode 2, and a carrier level negative voltage and an audio signal are output to a terminal 6.

In this case, the voltage at terminal 6 is resistively divided and applied to the anode of limiter diode 1, while the voltage at terminal 6 is applied to the cathode of diode 1 via audio frequency bypass capacitor 3. terminal 8
diode 1 is normally forward biased and passes the audio signal.

If sufficiently large pulse noise, such as ignition noise, is mixed into the audio signal, the limiter diode 1 is reverse biased, and the diode 1 is cut off, producing a limiter effect.

In the circuit shown in FIG. 1, capacitors 3 and 4.5 both require relatively large capacitance values, and cannot be built into a semiconductor integrated circuit chip, so they must all be externally attached.

Therefore, the number of external parts increases, and terminals 6 and 7
．． 8 becomes a pin of the integrated circuit for external connection of the capacitor, and a limited number of pins of the integrated circuit are occupied for this purpose, making it impossible to incorporate multiple functions into the integrated circuit.

Figure 2 is a diagram showing the operation of the conventional noise limiter circuit shown in Figure 1, where 9 is an audio signal, 10° 11
is the noise, and 12 is the limiter level.

As you can see from this diagram, limiter level 12 is AM1
Since the output signal is fixed at a position where it is not clipped even during 00% modulation, noise 11 is output larger than noise 10.

That is, there is a problem in that the limiter effect changes depending on the position where the noise is superimposed on the audio waveform.

An object of the present invention is to eliminate the problems of the conventional noise limiter circuit and to provide a noise limiter circuit suitable for semiconductor integrated circuit implementation.

In order to achieve the above object, the present invention utilizes the limiter action of a differential amplifier supplied with current from a constant current source, and changes the current value of the constant current source in accordance with the full-wave rectified waveform of the input signal. This allows the limiter level to change in accordance with the input signal waveform, and at the same time, it facilitates integration by avoiding the use of capacitors in the parts to be integrated.

FIG. 3 shows a practical device of the present invention.

In the same figure, 13, 14, 15, 16, 17, 18° 19 are transistors, 20, 21 are resistors, 22 are input terminals, 23 are control voltage terminals, 24 are output terminals, 25 are full-wave rectifier circuits, 26 is a low-pass filter, and 34° and 35 are resistors.

Transistor 19 becomes a constant current source, and transistor 15
, 16 is supplied with a constant DC bias current.

This constant current is divided into transistors 15 and 16 according to the control voltage applied to control voltage terminal 23.

Transistor 15 operates as a constant current source that supplies a DC bias current to the second differential amplifier made up of transistors 13 and 14.

Transistor 13 or 14 determined by the input signal voltage applied to input terminal 22 and emitter resistances 20, 21
Even if the collector current of transistor 13 or 14 is about to exceed the DC bias current, one transistor is cut off and the other transistor reaches a state where all the current supplied from constant current source transistor 15 flows. The current can no longer increase.

That is, the second differential amplifier exhibits a limiter effect.

1st. The third differential amplifier also has a similar limiter effect.

The control voltage applied to the base of the transistor 15 is obtained by passing the input signal through a full-wave rectifier circuit 25 and a low-pass filter 26 for removing pulse noise (high frequency components). Therefore, transistor 1
The DC bias current supplied by 5 to the second differential amplifier changes depending on the magnitude of the amplitude of the audio signal in the input signal.

That is, the limiter level of the second differential amplifier changes depending on the input waveform.

Although the control voltage is somewhat distorted by the low-pass filter 26, this distortion is not applied to the output terminal 24 as will be described later.

That is, the collectors of transistors 17, 18 of the third differential amplifier are connected to the collectors of transistors 13, 18 of the second differential amplifier, respectively.
It is connected in parallel with the collector of No. 14, and is supplied with a power supply voltage +vCC via a resistor.

Moreover, a constant current source that supplies a DC bias current to the third differential amplifier is the transistor 16, and a constant value DC bias current is supplied from the transistor 19 to the first differential amplifier composed of this transistor and the transistor 15. ing.

Changes in the control voltage applied to the base of transistor 15 can therefore be compensated for by the action of transistors 16, 17, 18 and prevented from appearing at output terminal 24.

Here, the emitter resistor 20.21 is used to improve the linearity of the output with respect to the input signal, and the emitter resistor 34.3
5 serves to adjust the limiter effect on the control voltage.

FIG. 4 is an input/output characteristic diagram in the embodiment shown in FIG. 3, with the control voltage as a parameter, the horizontal axis representing the input level, and the vertical axis representing the output level.

As the control voltage increases, the input/output characteristic curves change sequentially as shown in FIGS. 27 to 30, indicating that the limiter level increases as the control voltage increases.

FIG. 5 shows the audio signal output waveform 9 and limiter level 33 in the circuit shown in FIG.

From this figure, it is clearly seen that the limiter level 33 changes depending on the amplitude of the audio signal in the input signal, as described above.

Therefore, according to the present invention, the noise 3
Even if 2 occurs at the phase shown in Figure 5 with respect to the signal, the output will be approximately the same as the noise 31, and in the conventional limiter circuit, as shown in Figure 2, the output will be different from the noise at the time of noise generation. The problem that noise 11 is output larger than noise 10 due to the phase relationship with the signal is eliminated.

Further, as shown in FIG. 3, in the circuit of the present invention, no capacitance is used except for the external low-pass waveform generator 26, so that it can be easily integrated into an integrated circuit.

As described above, according to the present invention, there is no problem that noise is particularly large in output no matter what phase it occurs with respect to the signal, and it is possible to obtain the effect of facilitating integration into an integrated circuit.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of a conventional noise limiter circuit, FIG. 2 is a diagram showing the operation of the conventional noise limiter circuit shown in FIG. 1, and FIG. 3 is a diagram of an embodiment of the present invention. Fig. 4 is an input/output characteristic diagram of the embodiment circuit shown in Fig. 3, and Fig. 5 is a diagram of the input/output characteristics of the embodiment circuit shown in Fig. 3.
FIG. 3 is a diagram showing the relationship between the signal output waveform and limiter level in the circuit shown in the figure. 1... Limiter diode, 3, 4, 5...
... Capacitor, 9 ... Audio signal,
12...Limiter level, 13, 14, 15
,16,17゜18.19...transistor,
22... Input terminal, 23... Control voltage terminal, 24... Output terminal, 25... Full wave rectifier circuit, 26... Low-pass filter, 31.
32...Noise, 33...Limiter
level.

Claims (1)

[Claims] 1. A first differential amplifier supplied with a bias current from a constant current source, and second and third differential amplifiers each using a pair of transistors of the first differential amplifier as constant current sources,
Resistors are inserted between the emitters of the pair of transistors of the differential amplifier and their constant current sources, and the collectors of one transistor of the second differential amplifier, the collector of one transistor of the third differential amplifier, and the The collectors of the other transistors of the two differential amplifiers and the collectors of the other transistors of the third differential amplifier are connected in parallel, respectively, and connected to the power supply through separate resistors, and output from any one of the collectors. the input signal to the base of one transistor of the second differential amplifier, and the input signal is further passed through a full-wave rectifier circuit and a low-pass filter to serve as a constant current source for the second differential amplifier. A limiter circuit characterized in that a voltage is applied to the base of a transistor of a first differential amplifier.