JPH01221010A - Automatic power control circuit - Google Patents

Abstract

PURPOSE:To reduce noise sent to an output terminal when an input signal is interrupted by providing a means controlling the amplifier to a minimum gain when the input signal level of the amplifier with variable gain function is lower than a preset level. CONSTITUTION:A signal inputted to the input terminal 1 is amplified by an amplifier 3 with gain variable function and outputted to an output terminal 2, part of the output is detected by a level detector 4 and then gain of the amplifier 3 is controlled at a control section 5 based on the detected value so as to keep the level of the output signal at the output terminal 2 constant against the fluctuation of the input signal. When the input signal level is lowered and the input signal is lowered from a point (a), it is detected by an input level detector 6 to control the amplifier 3 so as to attain minimum gain by a switch 7 for gain control. Moreover, when the input signal is increased and the level exceeds a point (b), the maximum gain is restored by the changeover device 7. Thus, the noise sent to the output terminal at input signal interruption is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic power control circuit used in a transmitting device.

[Conventional technology]

Automatic power control circuits used in conventional transmitting devices automatically control the output level to keep it constant despite fluctuations in the input signal level.

FIG. 3 shows an example of a block diagram of a conventional automatic power control circuit. 1 is an input terminal, 2 is an output terminal, 3 is an amplifier with a variable gain function, 4 is a level detector, and 5 is a gain control section.

The signal input to the input terminal 1 is amplified by the amplifier 3, a part of the output is detected by the level detector 4, and the gain of the amplifier 3 is adjusted by the control section 5 so that the output level of the output terminal 2 is constant. control to keep the output signal constant against fluctuations in the input signal. FIG. 4 shows the level characteristics of the output signal with respect to the input signal of this power amplifier.

[Problem to be solved by the invention]

In the conventional power amplifier described above, when the input signal is cut off, the amplifier attains maximum gain as shown in FIG.

At this time, the thermal noise N at the output end is expressed as follows.

N=KTBFXG K: Boltzmann constant, T: Absolute temperature B: Bandwidth, F: Amplifier noise figure G: Amplifier gain Therefore, even though there is no input signal, thermal noise N is sent to the output and interference occurs at the receiving end. Or it may cause it to be mistaken for a signal.

SUMMARY OF THE INVENTION An object of the present invention is to provide an automatic power control circuit having a circuit that reduces noise sent to an output terminal even when the input signal of a power amplifier is cut off.

[Means to solve the problem]

The automatic power control circuit of the present invention is characterized by having means for controlling the gain to a minimum when the input signal level of the variable gain amplifier is lower than a preset level.

[Effect]

In the above configuration, since the gain is set to the minimum when the input signal level is lower than the set level, the noise sent to the output terminal is reduced.

〔Example〕

Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 shows the characteristics of an output signal with respect to an input signal.

In Fig. 1, 1 is an input terminal and 2 is an output terminal.

3 is an amplifier with variable gain function, 4 is an output level detector, 5
6 is a gain control section, 6 is an input level detector, and 7 is a gain control switch.

A signal input to the input terminal 1 is amplified by the amplifier 3 and output to the output terminal 2. A part of this output is detected by the level detector 4, and based on this detected value, the control unit 5 outputs the signal to the amplifier 3. The gain of the output terminal 2 is controlled to keep the level of the output signal at the output terminal 2 constant against fluctuations in the input signal.

, when the input signal level decreases, the maximum gain is achieved as shown in Figure 2, but when the input signal decreases below point a,
It is detected by the input level detector 6, and the amplifier 3 is controlled by the switch 7 so as to have the minimum gain. When the input signal increases and exceeds point b, the switch 7 returns to the maximum gain.

Therefore, according to this configuration, when the input signal is above the specified value, the gain is controlled so that the output is constant, and when the human input signal is below the specified value, that is, when the input signal is disconnected, the gain is the minimum.

〔Effect of the invention〕

As explained above, the automatic power control circuit of the present invention can minimize the gain of the amplifier and reduce the noise sent to the output end even if the human input signal of the variable gain amplifier is cut off. This has the effect of reducing interference due to noise or misidentification with signals.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the automatic power control circuit of the present invention, FIG. 2 is a characteristic diagram of the output signal level with respect to the input signal of the power amplifier for detailed explanation of the present invention, and FIG.
The figure is a block diagram of an example of a conventional automatic power control circuit.
FIG. 4 is a characteristic diagram of the level of the output signal with respect to the input signal of the power amplifier in the conventional circuit. 1... Input terminal, 2... Output terminal, 3... Amplifier with variable gain function, 4... Output level detector, 5...
Gain control section, 6... Input level detector, 7... Gain control switch. Figure 2 ab: Many blind spots

Claims (1)

[Claims] 1. An automatic power control circuit equipped with an amplifier with a variable gain function, characterized in that the automatic power control circuit is provided with means for controlling the gain to a minimum when the input signal level is lower than a preset level.