JPH0691396B2 - Automatic gain control amplifier - Google Patents

Description

The present invention relates to an automatic gain control (hereinafter referred to as AGC) amplifier for a transmitter and the like.

[Prior Art] Large power amplifiers such as transmitters have an AGC function for stabilizing the average transmission power. Especially, in an AGC amplifier for a signal having an AM component, this component is generated by the AGC. It was often designed to have a sufficiently large time constant so that it would not be compressed.

[Problems to be solved] Here, when the transmitter input signal is suddenly changed from the disconnection state to the normal input state, the AGC amplifier is
Since it is in the maximum gain state of the GC range, it has a drawback that a large amount of power is output as it is, causing burnout and interference of parts.

In particular, when the response speed of AGC was slow, the transient state continued for a long time, and this effect was great.

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to provide an AGC amplifier which makes an output level constant with respect to a change in a signal input level.

[Means for Solving the Problems] As means for achieving the above-mentioned object, an AGC amplifier of the present invention includes a variable gain amplifier, a detector connected in parallel to the output terminal of the variable gain amplifier, and a detector output An error amplifier that outputs the control voltage of the variable gain amplifier by using the reference voltage as another input and the other input, a detector connected in parallel to the input terminal of the variable gain amplifier, and the detector output as one input As another input, a voltage comparator that outputs a control voltage that controls the gain of the variable gain amplifier when the input level of the variable gain amplifier is lower than a predetermined value, and a time constant circuit that determines the response speed of the voltage comparator. A combiner for combining the output of the error amplifier and the output of the voltage comparator, and by connecting the combiner output to the control terminal of the variable gain amplifier, at least The output of the voltage comparator side is configured to preferentially suppress the gain of the variable gain amplifier, and when the input signal is in the off state, the gain is controlled to be the minimum, and the input signal is rapidly input in the normal state. Even so, the required output level is set not to exceed the AGC setting.

Also preferably, a time constant circuit for determining a response speed is connected to the error amplifier side, further, the combiner is deleted, and two control terminals of the variable gain amplifier are provided.
The output of the error amplifier is connected to one of the control terminals, and the output of the voltage comparator is connected to the other.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. The signal input from the input terminal 1 is amplified by the variable gain amplifier 3 and output from the output terminal 2. The output level of the signal output from the variable gain amplifier 3 is detected by the detector 4 and input to the error amplifier 5. The error voltage between this detector output and the DC reference voltage equivalent to the required output level is
The required AGC characteristic can be obtained by being guided to the gain control terminal of the variable gain amplifier 3 via the combiner 10.

Time constant circuit 6 that determines the response speed of the AGC control loop
The value of the time constant is determined as necessary depending on the signal to be handled, but of course, in general, no special circuit is added,
The value as it is may be used. However, especially when the signal has an AM component, it is necessary to have a large time constant in order to prevent the signal from being distorted. In such cases,
If the input level of the AGC amplifier is a normal value, that is, if the value is within the range that AGC can follow, there is no problem, but if the signal is once cut off and the normal value is input again after maintenance,
Since the AGC control is in the maximum gain state, a large amount of power is transiently sent, and there is a risk of destroying the parts of the output part, the feeder system, and the like.

Further, the larger the time constant, the longer the time constant, and the larger the effect. Therefore, in the present embodiment, the detector 7 is connected in parallel to the input end of the variable gain amplifier 3 and the output thereof is led to the voltage comparator 8.

As the reference voltage of the voltage comparator 8, a value corresponding to a certain value of the input level of the present AGC amplifier is set, and when the input level becomes lower than this value, it is changed via the time constant circuit 9 and the synthesizer 10. A control voltage is sent to the gain control terminal of the gain amplifier 3 to set the AGC control to the minimum gain or a sufficiently low gain. As a result, even if the input changes from the disconnected state to the normal input, the gain is low during the transition period, and large power is not delivered. However, the response speed of the voltage comparator 8 needs to be slower than the speed controlled by the error amplifier.

If a pair of control terminals of the variable gain amplifier 3 are provided and one side is connected to the output of the error amplifier 5 and the other side is connected to the output of the time constant circuit 9, the combiner 10 may be omitted.

2 and 3 are explanatory views of the AGC operation in this embodiment, and FIG. 2 shows the input characteristic of the AGC amplifier. In FIG. 2, point a shows the output level at the time of transition in the conventional case, and point b shows the output level in the case of this embodiment, showing that a great improvement can be obtained.

FIG. 3A shows an input signal waveform, which shows that the signal is input at t ₀ .

FIG. 3 (B) shows an output signal waveform in the conventional case, which shows a large power output indicated by the point a at the time of transition, and eventually converges to the normal output level indicated by the point c.

Further, FIG. 3C shows the output signal waveform of this embodiment, which has a small power output as shown at the point b during the transition, and eventually converges to the normal output level shown at the point c.

[Effects of the Invention] As described above, according to the present invention, the input section detector and the voltage comparator are added to the conventional automatic gain control circuit so as to have an appropriate time constant. At the time of transition, there is an effect that it can be operated without outputting a large amount of power.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIGS. 2 and 3 are graphs showing operations of the embodiment of FIG. 1 and conventional AGC control in comparison. 1: Input terminal 2: Output terminal 3: Variable gain amplification 4: Detector 5: Error amplifier 6: Time constant circuit 7: Detector 8: Voltage comparator 9: Time constant circuit 10: Combiner

Claims (1)

[Claims] 1. A variable gain amplifier, a detector connected in parallel to the output terminal of the variable gain amplifier, and a control voltage of the variable gain amplifier with the detector output as one input and a reference voltage as the other input. Error amplifier, a detector connected in parallel to the input terminal of the variable gain amplifier, the detector output as one input and the reference voltage as the other input, and the input level of the variable gain amplifier is lower than a predetermined value. In this case, a voltage comparator that outputs a control voltage that controls the gain of the variable gain amplifier, a time constant circuit that determines the response speed of the voltage comparator, the output of the error amplifier, and the output of the voltage comparator are combined. By connecting the output of the synthesizer to the control terminal of the variable gain amplifier, the gain of the variable gain amplifier is preferentially suppressed at least at the output of the voltage comparator side. Automatic gain control amplifier and characterized by being configured to perform.