JPH07326939A - Limiter circuit - Google Patents

Abstract

PURPOSE:To provide a limiter circuit for which gain between input and output is not changed even when a limiter level is changed. CONSTITUTION:This circuit is provided with a first variable gain amplifier circuit 5 for amplifying input signals, a limiter 6 for limiting the amplitude of the output signals of the first variable gain amplifier circuit and a second variable gain amplifier circuit 7 for amplifying the output signals of the variable type limiter. By changing the gain of the first variable gain amplifier circuit, the limiter level for the input signals is changed and the gain of the second variable gain amplifier circuit is changed in the opposite direction of the gain change of the first variable gain amplifier circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a limiter circuit which limits the amplitude of an input signal, and more particularly to a limiter circuit which does not change the gain between input and output even if the limiter level is changed.

[0002]

2. Description of the Related Art FIG. 2 is a circuit diagram showing a conventional limiter circuit, in which an input signal from an input terminal (1) is amplified by a variable gain amplifier circuit (2) and amplitude-limited by a limiter (3). And is led to the output terminal (4). The limiter level can be changed by changing the limiter level itself of the limiter (3). However, in consideration of IC implementation, it is problematic to directly change the limiter level. Therefore, the variable gain amplifier circuit (2) is provided in front of the limiter (3), and the limiter level is changed by changing the amplification factor. Changing the amplification factor is a simple matter in an IC.

For example, if the gain of the variable gain amplifier circuit (2) is increased, the limiter level for the input signal at the input terminal (1) will be lowered. On the contrary, if the gain of the variable gain amplifier circuit (2) is reduced, the limiter level is increased. Therefore, according to the circuit of FIG. 2, the limiter level can be changed.

[0004]

However, when the limiter level is changed in the circuit of FIG. 2, the gain between the input and output terminals of a low level signal which does not reach the limiter level also changes. Depending on the type of input signal, it may be desired to change only the limiter level, which is a problem.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and limits a first variable gain amplifier circuit for amplifying an input signal and an amplitude of an output signal of the first variable gain amplifier circuit. And a second variable gain amplifier circuit for amplifying an output signal of the variable limiter, and by changing the gain of the first variable gain amplifier circuit, the limiter level for the input signal is changed, The gain of the second variable gain amplifier circuit is changed in a direction opposite to the change of the gain of the first variable gain amplifier circuit.

[0006]

According to the present invention, the limiter level for the input signal is changed by changing the gain of the first variable gain amplifying circuit, and the limiter level is changed in the opposite direction to the gain change of the first variable gain amplifying circuit. Since the gain of the second variable gain amplifier circuit is changed, even if the limiter level is changed, the amplification factor of the signal that does not reach the limiter level does not change.

[0007]

1 shows a limiter circuit according to the present invention, in which (5) is a first variable gain amplifier circuit for amplifying an input signal, and (6) is an output of the first variable gain amplifier circuit (5). A limiter for limiting the amplitude of the signal, (7) is the limiter (6)
2 is a second variable gain amplifier circuit for amplifying the output signal of.

In the initial state of FIG. 1, assuming that the gain of the first variable gain amplifier circuit (5) is 6 dB and the gain of the second variable gain amplifier circuit (7) is -6 dB, the limiter (6) swings from 500 mV. Suppose you want to put restrictions. In this case, the level relationship between the input terminal (1) and the output terminal (4) is
As shown in FIG. That is, when the input signal level exceeds 250 mV, the limiter (6) limits the amplitude, and the output level is constant at 250 mV. When the input signal level is 250 mV or less, for example, 100 mV, the output level is 100 mV, and the gain between the input and output terminals is 1.

Next, the limiter level is set to 5 from the above state.
Suppose that it is changed to 00 mV. In this case, for example, the gain of the first variable gain amplifier circuit (5) is set to 0 dB and the gain of the second variable gain amplifier circuit (7) is set to 0 dB. The amplitude limit level of the limiter (6) remains at 500 mV.
Then, the level relationship between the input and output terminals is as shown in FIG.

When the input signal level exceeds 500 mV, the limiter (6) limits the amplitude, and the output level becomes constant at 500 mV. Input signal level is 500 mV
For example, if it is 100 mV, the output level will be 100 mV, and the gain between the input and output terminals will be 1. Therefore, as is clear from the relationship between FIG. 3 and FIG.
According to this circuit, only the limiter level can be changed without changing the gain between the input and the output.

FIG. 5 shows a specific circuit example of FIG.
As shown in FIG. 5A, the first variable gain amplifier circuit (5) and the limiter (6) include first and second transistors (8) and (9) whose emitters are commonly connected, and the first and second transistors. First and second diodes (10) and (11) connected in opposite directions between the collectors of the transistors
And a variable current source (12) for supplying a current to the common emitter of the first and second transistors.
The second variable gain amplifier circuit (7) is as shown in FIG. 5B.
When the current value of the variable current source (12) is changed, the internal impedances of the emitters of the first and second transistors (8) and (9) are changed to change the gain. Similarly, for the variable current sources (13) and (14), the transistors (15) and (16) can be controlled to control the gain in the opposite direction.

[0012]

As described above, according to the present invention, it is possible to provide a limiter circuit in which the gain between input and output does not change even if the limiter level is changed.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a limiter circuit of the present invention.

FIG. 2 is a circuit diagram showing a conventional limiter circuit.

FIG. 3 is a characteristic diagram for use in explaining FIG.

FIG. 4 is a characteristic diagram for use in explaining FIG.

5 is a circuit diagram showing a specific circuit example of FIG.

[Explanation of symbols]

 (5) First variable gain amplifier circuit (6) Limiter (7) Second variable gain amplifier circuit

Claims (2)

[Claims] 1. A first variable gain amplifier circuit for amplifying an input signal, a limiter for limiting an amplitude of an output signal of the first variable gain amplifier circuit, and a second variable gain amplifier circuit for amplifying an output signal of the limiter. And changing the gain of the first variable gain amplifying circuit to change the limiter level for the input signal, and changing the gain level of the first variable gain amplifying circuit in the opposite direction to the second variable gain amplifying circuit. A limiter circuit characterized by changing the gain of the circuit. 2. The first variable gain amplifier circuit and the limiter include: first and second transistors having emitters commonly connected; and first and second transistors connected in opposite directions between collectors of the first and second transistors. The limiter circuit according to claim 1, further comprising: a first and a second diode; and a variable current source that supplies a current to a common emitter of the first and the second transistors.