WO2008044750A1 - Low-noise amplifier - Google Patents

Abstract

A low-noise amplifier includes a plurality of feedback resistors (RN1 to RNn) and analog switches (ASW1 to ASWn) for selecting one of the resistors so as to perform gain control in such a way that the drain current of an input transistor (N1)is constant and the switching between the feedback resistors (RN1 to RNn) does not change the open gain, which suppresses the lowering of the input dynamic range and improves the linearity. Moreover, by selecting one of the feedback resistors (RN1 to RNn), the input impedance can be set to a desired value. This eliminates the need of a rectifier circuit using an LC circuit.

Description

 Description Low Noise Amplifier Technical Field

 'The present invention relates to a low-noise amplifier, and is particularly suitable for use in a source-grounded low-noise amplifier having a feedback circuit. Background art

 In general, a radio receiver such as a radio receiver or a television receiver uses an amplifier that amplifies a weak high-frequency signal received by an antenna. However, if the noise output from the amplifier increases during amplification, the sensitivity of the receiver will deteriorate. Therefore, L N A (Low Noise Amplifier), which generates less noise, is often used as the receiver amplifier.

 L N A can be broadly divided into types that do not constitute a feedback circuit and types that constitute a feedback circuit. In the case of L N A that does not constitute a feedback circuit, the input resistance becomes high impedance, so when connecting an antenna circuit to such L N A, it is necessary to perform impedance matching using an input transformer or the like. On the other hand, since the desired input impedance can be obtained by using the feedback type configuration, it is possible to directly connect the antenna circuit and LNA without using an input transformer or the like.

FIG. 1 is a diagram showing a configuration example of a conventional feedback LNA. The conventional LNA shown in Figure 1 has a source grounded transistor (input transistor) N 1 connected between the signal input terminal IN and output terminal OUT, and the input terminal IN and output. The feedback resistor RN and capacitor CN connected to the terminal OUT It is configured with.

 Source ground Transistor N 1 gate is connected to signal input i IN via capacitor C 1 and drain is connected to signal output terminal o UT Source ground transistor N 1 drain Is connected to the power supply VDD via the load resistor RL.

—S is connected to ground.

 卜 Transistor N 2 and current source transistor 1 constitute a current mirror circuit. That is, the gate of transistor N 2 is diode-connected to its drain and the source ground transistor N

Connected to the gate of 1 through resistor R2. Transistor N 2

—S is connected to ground. By configuring the current mirror circuit in this way, the drain current flowing through the common-source transistor N 1 is determined by the size ratio between the common-source transistor N 1 and the transistor N 2. A radio receiver normally has an AGC (Automatic Gain Control) circuit power S to adjust the gain of the received signal. The RF (Radio Frequency) A G C circuit adjusts the gain of the high-frequency signal received by the antenna and keeps the level of the received signal constant.

 In other words, the R F—A G C circuit does not operate unless the field strength of the antenna input signal is greater than the threshold, and does not reduce the gain of the received signal. However, if a signal with a strong electric field is input to the antenna and the electric field strength exceeds the threshold, excessive power is applied to the wireless receiver by operating the RF-AGC circuit and reducing the gain of the received signal. Try not to. This la la r F-

The operation of AGC can be realized by controlling the gain of LNA, for example.

In the conventional LNA shown in Fig. 1, the LNA gain (by the control voltage applied from the control terminal PG to the gate of the transistor N2 via the resistor R1) The gain of the received signal is variable. Specifically, it is usually set so that the gain of the LNA is maximized, and when a large level signal is input, the control voltage is changed according to the level, so that the LNA gain is appropriately adjusted. The value is lowered to a reasonable value. A high-frequency amplifier circuit that makes the gain variable by switching the level of the gate voltage of the transistor is also disclosed in Patent Document 1, for example.

 Patent Document 1: FIG. 3 of Japanese Patent Laid-Open No. 8-30 17 15

 As another method for making the gain variable, there is a technique for making the gain variable by switching the magnitude of the source resistance of the amplifier (see, for example, Patent Document 2). In the variable gain amplifier circuit described in Patent Document 2, a plurality of switches are connected to the source side of a plurality of sets of MOS transistors constituting a plurality of source grounded amplifier circuits. Select the amplifier circuit to control the voltage gain.

 Patent Document 2: FIG. 2 1 of the Japanese Patent Application Laid-Open No. 2000-26 6 30 09

 However, in the method of making the gain of the amplifier variable by controlling the gate voltage as shown in Fig. 1 or Patent Document 1, the drain current of the input transistor changes, so the input transistor is changed. The dynamic range of the camera will be reduced. For example, in the case of Figure 1, changing the gate voltage of transistor N 2 changes the reference current flowing in transistor N 2 that forms the input circuit of the current mirror. The drain current that flows will also change. For this reason, there is a problem that a distortion characteristic when a signal of a large level is inputted deteriorates and a desired signal-to-noise ratio excellent in linearity cannot be realized.

Also, the gain of the amplifier can be controlled by controlling the source resistance as in Patent Document 2. In the method with variable, the open-circuit gain of the amplifier changes when the source resistance is changed, which reduces the dynamic range of the input transistor. For this reason, there is a problem that a desired signal-to-noise ratio with excellent linearity cannot be realized.

 In addition, the field α using the amplifier of Patent Document 2 for FΜ radio receivers> LNA, etc. of the machine, and the impedance matching between the NA and the antenna input circuit from the viewpoint of noise figure can be realized by resistance. In this case, an impedance matching is required by tuning to the frequency of the desired station that is rarely received from multiple stations. There is also a problem that the configuration of the receiver becomes complicated.

 The present invention has been made to solve such problems. The receiver configuration is not complicated, and the input dynamics of the amplifier can be obtained even when a large level signal is input. The purpose is to achieve a desired signal-to-noise ratio with excellent linearity by preventing the range from decreasing.

 In order to solve the above-described problems, in the present invention, in a source-grounded amplifier including a source-grounded transistor in which a gate is connected to a signal input terminal, the signal input terminal is connected to the output terminal. In addition, a plurality of feedback resistors' and a switch for selecting one of the plurality of feedback resistors are selected, and gain control is performed by selecting one of the feedback resistors by switching the switch. I am trying to do it.

According to the present invention configured as described above, the gain is controlled by switching the feedback resistance of the feedback amplifier, instead of switching the gate voltage and source resistance of the ranstar. The gate bias voltage of the input transistor is constant, the drain current is constant, and the open-circuit gain of the amplifier does not change by switching the feedback resistor. For this reason, the input dyna The linearity can be improved by suppressing the decrease of the microphone range, and the desired signal-to-noise ratio can be realized. In addition, the input impedance can be set to a desired value by configuring a feedback amplifier with multiple feedback resistors selectable, eliminating the need for a matching circuit using an LC circuit. Thus, the configuration of a receiver using an amplifier can be simplified. Brief Description of Drawings

 FIG. 1 is a diagram showing the configuration of a conventional LNA.

 FIG. 2 is a diagram showing a configuration example of LNA according to the present embodiment.

 FIG. 3 is a diagram showing another configuration example of LNA according to the present embodiment.

 FIG. 4 is a diagram showing another configuration example of the feedback resistor and the analog switch used in the present embodiment. Best Mode for Invention

 Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 2 is a diagram showing a configuration example of LNA according to the present embodiment. Please refer to Fig. 2.

, Components having the same functions as those shown in Fig. 1 are given the same reference numerals, and

 In FIG. 2, N 1 is a common-source input transistor, and its gate is connected to the signal input terminal IN through the capacitor C 1, the source is connected to the drain, and the drain is the signal output. Connected to terminal OUT. R is a load resistance, and is connected between the output (K line) of the common source transistor N 1 and the power source V DD.

N 2 is a transistor that forms a current mirror circuit together with the source ground transistor N 1. That is, the gate of transistor N 2 is diode-connected to its own drain and source grounded transistor N Connected to 1 gate via resistor R2. The drain of transistor N 2 is connected to power supply VDD through resistor R 1 and the source is connected to ground. By configuring the current mirror circuit in this way, the drain current flowing through the source grounded transistor N 1 is determined by the size ratio of the source grounded transistor N 1 and the transistor N 2.

 Further, a feedback circuit is provided between the signal output terminal OU T (drain of the common source transistor N 1) and the signal input terminal I N (gate of the common source transistor N 1). The feedback circuit includes a plurality of feedback resistors RN 1, RN 2,..., RN n (where n is an integer greater than or equal to 2) provided in parallel between the signal input terminal IN and the output terminal OUT, .., ASW n, which are connected in series to the feedback resistors RN 1 to RN n, respectively, of the analog switches A SW l, A SW 2,.

 Here, the resistance values of the plurality of feedback resistors R N l to R N n are different. The plurality of analog switches ASW 1 to AS Wn are turned on / off by the control voltage applied from the control terminals PG 1 to PG n, and only one of them is selectively turned on. It has become so.

 In the present embodiment, the gate bias voltage of the common source transistor N 1 is constant when the gain of L N A is changed. Instead, the analog switches ASW l to ASW n are turned on / off to selectively select one of the feedback resistors RN 1 to RN n connected in series with the analog switches ASW l to ASW n. By using it, LNA gain control is performed.

In this case, if the negative feedback amount (feedback resistance value) is changed by selecting feedback resistors RNl to RNn, the input impedance of LNA will change, which may cause a loss due to impedance mismatch. However, when the LNA gain is reduced in AGC operation, the input signal level is high, and the above-mentioned loss problem does not occur for the following reasons. That is, if the signal source voltage is E, the signal source resistance is R s, and the input impedance of the LNA is R i, then the maximum power when R s = R i

P max = E ² / (4 R s)

Is supplied to L N A. This is when impedance matching is achieved.To change the gain of the LNA, select one of the feedback resistors RN 1 to RN n with the analog switch ASW l to AS Wn and set the feedback resistance value. Suppose it has changed. If impedance mismatch occurs at this time and R i = l 5 R s, then the power P 1 supplied to L N A is

P 1 = E ² / (6.25 R s)

It becomes.

 Therefore, the loss due to impedance mismatch is

 P loss = 1 0 XL0G (P max / P 1) = 2 [d B]

At a level that does not become a problem. Therefore, for example, if the resistance values of the feedback resistors RN1 to RNn are set so that the value of the input impedance Ri is 1.5 Rs at the maximum, the loss due to impedance mismatch Is almost negligible.

 As described above in detail, in the present embodiment, in a source grounded LNA having a feedback circuit, the feedback circuit is constituted by a plurality of feedback resistors RN 1 to RN n, and any of them is selected. An analog switch ASW l to ASW n is provided to select either of them, and the LNA gain is controlled by selecting one of the feedback resistors.

In this way, since the gain is controlled by switching the feedback resistance value of the feedback amplifier, the gate bias voltage of the source grounded transistor N1, which is the input transistor, is constant, and the drain current is constant. . In addition, switching the feedback resistance value does not change the open gain of the LNA. others Therefore, even when a large level of signal is input, the desired signal-to-noise ratio with excellent linearity can be realized by suppressing the decrease of the input dynamic range of the LNA. In particular, the signal-to-noise ratio can be improved when two interference signals are input.

 Furthermore, according to the present embodiment, a plurality of feedback resistors RN l to RN n having different resistance values are provided as a feedback circuit, and any one of these is selected and connected. Thus, the input impedance of the LNA can be set to a desired value. This eliminates the need for a matching circuit using an LC circuit and simplifies the configuration of a receiver using an LNA.

 FIG. 3 is a diagram showing another configuration example of the LNA according to the present embodiment. In FIG. 3, since components having the same reference numerals as those shown in FIG. 2 have the same functions, redundant description is omitted here. In the example in Fig. 3, the amplification stage is composed of two transistors connected in cascade. In this case, a feedback circuit is provided from the output (drain) of the gate ground transistor located above the cascode connection to the signal input terminal I N.

 In FIG. 3, N 3 is a gate-grounded transistor, which is cascade-connected to the source-grounded transistor N 1 and has its drain connected to the signal output terminal OU T. The drain of the gate ground transistor N 3 is also connected to the power supply V DD via the load resistor R L. The common source transistor N 1 functions as a first-stage amplifier, and the common gate transistor N 3 functions as a next-stage amplifier.

N 4 and N 5 are transistors constituting the bias circuit, and determine the bias of the gate ground transistor N 3. One end of this bias circuit is connected to the power supply VDD via the resistor R3, and the other end is connected to the ground. Other configurations are the same as those in FIG. As mentioned above, Cascade connection of the registers Nl and N3 makes it possible to obtain a stable gain over a wide band.

 In the above embodiment, the configuration in which a plurality of analog switches ASW 1 to A SW n are connected in series to the plurality of feedback resistors RN 1 to RN n connected in parallel has been described. Is not limited to this. For example, a selector that selects any one of a plurality of signal lines including a plurality of feedback resistors R N 1 to R N n connected in parallel on the path may be used.

 In the above embodiment, a configuration has been described in which a plurality of feedback resistors R N1 to R N n having different resistance values are provided in parallel and any one of them is selected. However, the present invention is not limited to this. For example, as shown in FIG. 4, a plurality of feedback resistors RN l to RN n and a plurality of analog switches A SW l to A SWn are connected in a ladder shape, and one of the analog switches A SW l to A SWn is connected. By selecting, the combined resistance value of one or more feedback resistors may be made variable. In this case, the resistance values of the plurality of feedback resistors R N l to R N n may be different from each other or the same.

 In addition, the above-described embodiment is merely an example of the embodiment for carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the spirit or the main features thereof. Industrial applicability

 The present invention is useful for a common source low noise amplifier having a feedback circuit.

Claims

The scope of the claims 1. a common source transistor having a gate connected to a signal input terminal, a plurality of feedback resistors provided between the signal input terminal and the signal output terminal;  A switch for selectively connecting any one of the plurality of feedback resistors,  A low-noise amplifier configured to perform gain control by switching the switch.  2. The plurality of feedback resistors are provided in parallel between the signal input terminal and the signal output terminal,  2. The low noise according to claim 1, wherein the switch includes a plurality of analog switches respectively connected in series to the plurality of feedback resistors. amplifier.