JPH0936679A - Gain control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gain control circuit which can stabilize an AGC operation by eliminating the variance of the DC offset. SOLUTION: The amplifies 11, 12 and 13 are connected in series at N stages, and the comparators 31, 32 and 33 compare the output levels of the N stages with a decision level signal. Then (H) and (L) are outputted to an automatic changeover switch 4 when the output signals of level detectors 21, 22 and 23 are larger and smaller than the decision level signal respectively. The switch 4 outputs the output signal (A) of the amplifier 11, the output signal (B) of the amplifier 12 and the output signal (C) of the amplifier 13 when the output signals of the comparators 31, 32 and 33 are equal to (H) (H) (H) and (L) (H) (H), (L) (L) (H), and (L) (L) (L) respectively. Every amplifier has no feedback loop and therefore the DC offset of the amplifier never varies. Then the DC offset can be eliminated for the AGC control. As a result, a fast and stable operation is secured for an AGC circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gain control circuit for controlling the gain of a signal level, and more particularly to an AGC circuit of a direct convergence receiver, for example.
The present invention relates to a gain control circuit that suppresses fluctuations in C offset.

[0002]

2. Description of the Related Art For example, when a direct convergence receiver is applied to burst signal reception, a baseband amplifier does not operate with a linear gain regardless of the input level of the receiver, that is, the received signal level. Don't Moreover, in order to operate this receiver in a normal indoor environment without any problem, the baseband gain needs to have a control range of 60 dB or more.

FIG. 3 shows a conventional voltage control gain amplifier circuit, which is composed of a voltage control gain amplifier 5 and a level detector 20. Among them, in the voltage control gain amplifier 5, the high impedance input operational amplifier 5 is used.
By using 1, the gain control with excellent linearity can be obtained. In this case, a feedback loop for the output signal is formed in the external part of the operational amplifier 51, and the level detector 20 for obtaining the amplitude level between the feedback level and the decision level signal is interposed in the feedback loop. Therefore, the voltage control gain amplifier 5 outputs an output level signal based on the determination level signal.

Since there is this feedback loop, FIG.
Smooth input / output characteristics as shown in are obtained.

[0005]

However, even if a large gain is obtained and smooth characteristics are obtained by the feedback loop as described above, there is a problem due to a change in DC offset. That is, when changing the amplification degree at the external part of the operational amplifier 51, it is unavoidable that the offset voltage changes according to the gain in the normal amplifier design.
Particularly in wireless communication, a DC offset of several microvolts becomes a problem when the output of the quadrature demodulator is set as low as possible in order to improve the adjacent signal rejection characteristic. Further, when the gain is changed by using a Gilbert cell in a normal differential amplifier, a DC offset change of several millivolts occurs, and this offset becomes equal to or higher than the signal level.

In any case, when the gain of the amplifier is changed, the DC level of the amplifier changes, and when the level is decreased due to the decrease in gain, the signal level and the DC level change cannot be separated and the AGC operation becomes unstable. Become.

Therefore, an amplifier whose DC offset does not change is required.

As a known technique, a baseband signal is detected from a reference signal and an output signal of a high frequency amplifying means, and the output signal is AC-amplified so that an average energy value above and below a vibration center of a generated beat is a macro. That the DC offset is removed by utilizing the fact that the level shift does not occur because they are equal to each other (JP-A-4-13354).
No. 6), a circuit for controlling the DC offset of the output and an AGC control circuit for keeping the amplitude constant are independent so that the DC offset becomes constant regardless of the AGC control error (JP-A-3-249811). However, in any case, it is different from the offset invariant means of the method according to the present invention.

In view of the above problems, it is an object of the present invention to provide a gain control circuit which eliminates the problems that the DC offset does not change even when the gain changes, the level decreases due to the decrease in gain, and the AGC operation becomes unstable. And

[0010]

The present invention which achieves the above object is characterized by the following constitution.

(1) N amplifiers for amplifying N stages of an input signal, N level detectors for obtaining an amplitude level of an output signal of each stage of the N amplifiers, a decision level signal and the level detection Number of comparators that compare the output signals of the comparators and output comparison signals, the output signals of each stage of the N amplifiers and the output signals of the N comparators, And an automatic changeover switch for outputting the output signal of the amplifier which outputs a corresponding amplitude.

(2) The automatic changeover switch is an automatic changeover switch for outputting the output signal of the last-stage amplifier that outputs an amplitude level equal to or lower than the judgment level signal except for the first-stage amplifier among the N-stage amplifiers. It is characterized by being.

Regarding each stage amplifier, a feedback loop is not formed, the amplification degree of a single amplifier does not change, and the DC offset does not change because the amplifiers are connected in series. Therefore, it is possible to remove only the DC offset.

[0014]

Next, an embodiment of the present invention will be described. Conventionally, a feedback loop is formed by an external part of an operational amplifier which is a differential amplifier to obtain a smooth input / output characteristic. However, the DC offset does not change at least in the state without the feedback loop.

For this reason, in this embodiment, at least a circuit that does not feed back is used. Instead, multiple amplifiers were cascaded to change the amplification.

FIG. 1 is a block diagram of an embodiment of the gain control circuit. As shown in the figure, amplifiers 11, 12, and 13 are connected in series, and the outputs of the amplifiers 11, 12, and 13 are
The amplifier 11, which is connected to the automatic changeover switch 4,
It is connected to level detectors 21, 22 and 23 for obtaining the amplitude levels of the output signals (A) (B) (C) of 12 and 13.
The level detectors 21, 22, 23 are connected to the comparators 31, 32, 33, respectively, and the comparators 31, 3 3,
In Nos. 2 and 33, the magnitudes of the detection levels of the detectors 21, 22 and 23 and the decision level signal are compared, and (H) or (L)
Is output. That is, each comparator 31, 3
2 and 33, the determination level signal and each level detector 21,
The level detectors 2 are compared by comparing the output signals from 22 and 23.
When the output signals from 1, 22, and 23 have a signal level higher than the determination level signal, (H) is output, and when the output signal is a low signal level, (L) is output. In this case, (H) (L) is
The comparator 31 outputs the output signal (α), the comparator 32 outputs the output signal (β), and the comparator 33 outputs the output signal (γ).

The automatic changeover switch 4 is for each amplifier 1
The outputs of 1, 12, and 13 are connected and a comparator 31,
The outputs of 32 and 33 are connected, and the automatic changeover switch 4
Then, the output signals (α) (β) of the comparators 31, 32, 33
The combination of (H) and (L) of (γ) makes the amplifiers 11 and 1
The outputs (A), (B) and (C) of 2 and 13 are selected and output. That is, in the automatic changeover switch 4, the output signals (α) (β) (γ) of the comparators 31, 32, 33 are
Is (H) (H) (H) and (L) (H) (H), the output signal (A) of the amplifier 11 is output, and if (L) (L) (H) is output signal of the amplifier 12 (B). ), (L) (L) (L)
Then, the output signal (C) of the amplifier 13 is selected and output.

As a result, in the AGC gain, each comparator 3
When the outputs of 1, 32, and 33 are (L), (L), and (L), the output signal (C) of the amplifier 13 is output from the automatic changeover switch 4, and the output signal (C) of the amplifier 13 reaches the determination level. That is, the output of each comparator 31, 32, 33 is (L)
When it becomes (L) or (H), the output of the automatic changeover switch 4 is switched to the output signal (B) of the amplifier 12, and
When the output signal (B) of the amplifier 12 reaches the judgment level, that is, the outputs of the comparators 31, 32, 33 are (L) (H).
When it becomes (H), the output of the automatic changeover switch 4 is switched to the output signal (A) of the amplifier 11. FIG. 2 illustrates this switching state. Comparator 31, 3
Amplifier output (A) (B) depending on the output state of 2, 33
(C) has been switched.

The gain control circuit operates at a constant level regardless of whether the received signal level is high or low. Then, as described above, the feedback loop does not exist in each of the amplifiers 11, 12 and 13, and the amplification factor is not changed,
The amplifiers are connected in series. As a result, the DC offset of the amplifier does not change, and the DC offset can be removed in the AGC control. In this way, the AGC circuit can obtain a high-speed and stable operation. Although FIG. 1 shows a three-stage amplifier, the number of stages is plural and the number is not limited to three.

[0020]

As described above, according to the present invention, since the gain control circuit does not change the amplification factor of each amplifier, the DC offset of the amplifier also does not change. Therefore, stable operation can be expected by removing the DC offset. Further, the burst operation has an effect that it can be operated at an extremely high speed as compared with a normal AGC.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a gain control circuit of the present invention.

FIG. 2 is an input / output characteristic diagram showing AGC control in the embodiment of the invention.

FIG. 3 is a block diagram of a conventional example.

FIG. 4 is an input / output characteristic diagram of AGC control in a conventional example.

[Explanation of symbols]

 11, 12, 13 Amplifier 20, 21, 22, 23 Level detector 31, 32, 33 Comparator 4 Automatic changeover switch 5 Voltage control gain amplifier

Claims (2)

[Claims] 1. N amplifiers for amplifying an input signal by N stages, N level detectors for obtaining an amplitude level of an output signal of each stage of the N amplifiers, a decision level signal and the level detector. N that outputs the comparison signal by comparing the output signals of
N comparators, N output signals of each stage of the amplifier and N
A gain control circuit for receiving an output signal of each of the comparators and outputting an output signal of the amplifier which outputs an amplitude corresponding to the determination level signal. 2. The automatic changeover switch is an automatic changeover switch for outputting an output signal of an amplifier at a final stage that outputs an amplitude level equal to or lower than the determination level signal except for an amplifier at a first stage of the N-stage amplifiers. The gain control circuit according to claim 1, wherein: