Wideband variable gain amplifier with high linearity operating in switch mode

Disclosed is a wideband variable gain amplifier with high linearity that operates in a switch mode. The variable gain amplifier includes a first amplifier unit, a second amplifier unit and a third amplifier unit. The first amplifier unit includes an amplifier, a wideband-matching element, an attenuator and first switching means. The first amplifier unit supports a high gain mode operation and a low gain mode operation. In the high gain mode, the first switching means is short-circuited and an input signal is amplified with a high gain. It is thus possible to reduce noise characteristics of the entire system by a rear stage. In the low gain mode, the first switching means is opened and the input signal is attenuated by the attenuator without an amplification operation. It is thus possible to reduce a non-linearity occurring by the rear stage of the first amplifier unit. According to another embodiment of the present invention, only the first amplifier unit can be independently used in order to amplify a signal. Furthermore, a variable gain amplifier can be implemented by connecting various combinations of the second amplifier unit and the third amplifier unit at the rear of the first amplifier unit.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable gain amplifier, and more particularly, to a wideband variable gain amplifier with high linearity that operates in a switch mode.

2. Background of the Invention

A variable gain amplifier refers to a device which functions to maintain a desired output signal level by controlling its gain. The variable gain amplifier is usually used in radio frequency-receiving devices in which a signal received through an antenna has a high dynamic range. In order for a signal of a constant level to be supplied to a base band terminal of the received signal, the variable gain amplifier must also have a high dynamic range.

Many researches have been actively made so far on a variable gain amplifier, and hence various variable gain amplifiers have been proposed. Researches are in progress to increase the dynamic range of the variable gain amplifier and improve a linearity of a signal.

One example of prior arts attempted to improve a linearity of a variable gain amplifier includes U.S. Pat. No. 5,949,286.

The prior art amplifier includes a differential amplifier transistor pair for amplifying an input signal and a diode pair connected between the emitters of the transistor pair to provide its gain control using emitter degeneration.

In other words, the amount of current applied to a diode pair can be controlled by connecting a transistor circuit to the diode pair and varying a control voltage applied to the transistor. Therefore, an impedance value of the diode pair is controlled by the control voltage, so that a gain control of the variable gain amplifier can be accomplished.

The variable gain amplifier disclosed in U.S. Pat. No. 5,949,286, however, has the diodes connected to its input terminal for performing a predistortion function, thereby improving non-linear characteristics of the diode pair connected between the emitters of the transistor pair. Furthermore, a dummy amplifier is connected to the differential amplifier transistor pair to cancel the capacitive effects generated by the diode pair in order to increase the gain control range of the amplifier at higher frequencies.

For the variable gain amplifier disclosed in U.S. Pat. No. 5,949,286, however, since it employs the dummy amplifier to improve its linearity, there has been a problem in that additional current and chip area are needed, and in that there is a limit in improving linear-characteristics using an active element.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to substantially obviate one or more problems due to limitations and disadvantages of the related art.

An object of the present invention is to provide a variable gain amplifier having an excellent linearity.

Another object of the present invention is to provide a variable gain amplifier that can perform an optimum operation in a high gain mode or a low gain mode.

Still another object of the present invention is to provide a variable gain amplifier having an excellent amplification operation at a wideband.

To achieve the above objects, according to one embodiment of the present invention, there is provided a variable gain amplifier having an input terminal and an output terminal, for amplifying a signal inputted to the input terminal to output the amplified signal, the amplifier operating in a high gain mode or a low gain mode, comprising: an amplifier element having a first terminal constituting the input terminal of the variable gain amplifier, a second terminal constituting the output terminal of the variable gain amplifier and a third terminal connected to a second power supply, wherein the amount and direction of current of the amplifier element, which flows from the second terminal to the third terminal, are varied based on the amount of a voltage applied to the first terminal; a wideband-matching element connected between the input terminal and the output terminal of the variable gain amplifier, for matching input impedance at a wideband upon the operation of the variable gain amplifier in the high gain mode; an attenuator connected between the input terminal and the output terminal of the variable gain amplifier, for attenuating the input signal to output the attenuated signal to the output terminal upon the operation of the variable gain amplifier in the low gain mode; means connected to the first terminal of the amplifier element, for activating the amplifier element upon the operation of the variable gain amplifier in the high gain mode; and a load resistor connected between the first terminal of the amplifier element and a first power supply.

Preferably, the wideband-matching element includes a capacitor, an inductor, a resistor and switching means, wherein one end of the capacitor is connected to the input terminal of the variable gain amplifier, the other end of the capacitor is serially connected to the inductor, the resistor and the switching means, and the other end of the switching means is connected to the output terminal of the variable gain amplifier.

Preferably, the attenuator includes a capacitor, a resistor and switching means, wherein one end of the capacitor is connected to the input terminal of the variable gain amplifier, the other end of the capacitor is serially connected to the resistor and the switching means, and the other end of the switching means is connected to the output terminal of the variable gain amplifier.

Also, the means for activating the amplifier element in the high gain mode preferably includes switching means and a bias voltage.

It is preferred that the amplifier element is a MOSFET transistor, the first terminal of the amplifier element is a gate, the second terminal thereof is a drain, and the third terminal thereof is a source.

It is also preferred that the variable gain amplifier further comprises a second amplifier circuit, including: first and second amplifier elements each having a first terminal, a second terminal and a third terminal, wherein the amount and direction of current of each of the first and second amplifier elements, which flows from the second terminal to the third terminal, are varied based on the amount of a voltage applied to the first terminal; first and second load resistors each connected between the second terminal of each of the first and second amplifier elements and a first power supply; first and second current sources connected to the third terminals of the first and second amplifier elements, respectively; a source degeneration variable resistor connected between the third terminals of the first and second amplifier elements; and a load degeneration variable resistor connected between the second terminals of the first and second amplifier elements, wherein the first terminal of the first amplifier element is connected to the output terminal of the variable gain amplifier, the first terminal of the second amplifier element is grounded, and the second terminals of the first and second amplifier elements form second negative(−) and positive(+) output terminals, respectively.

It is preferred that the variable gain amplifier further comprises a third amplifier circuit, including: first and second amplifier elements each having a first terminal, a second terminal and a third terminal, wherein the amount and direction of current of each of the first and second amplifier elements, which flows from the second terminal to the third terminal, are varied based on the amount of a voltage applied to the first terminal; first and second load resistors each connected between the second terminal of each of the first and second amplifier elements and a first power supply; first and second current sources connected to the third terminals of the first and second amplifier elements, respectively; and a source degeneration variable resistor connected between the third terminals of the first and second amplifier elements, wherein the first terminals of the first and second amplifier elements are connected to the second negative(−) and positive(+) output terminals, respectively, and the second terminals of the first and second amplifier elements form third negative(−) and positive(+) output terminals, respectively.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A variable gain amplifier according to the present invention employs a MOSFET transistor amplifier. The amplifier has a gate, a source and a drain. The MOSFET transistor has a property that the amount and direction of current flowing from its source to its drain, and vice versa are decided depending on the amount and polarity of a voltage applied to its gate. Such an amplifier may involve a bipolar junction transistor (BJT), a junction field effect transistor (JFET), a metal oxide semiconductor field effect transistor (MOSFET), a metal semiconductor field effect transistor (MESFET), and so on.

Furthermore, most of these amplifiers utilize two complementary devices that are complementary each other, i.e., a first complementary device, for example, an N type MOSFET, and a second complementary device, for example, a P type MOSFET. The first complementary device and the second complementary device have a property that the amount and direction of current flowing from their sources Ns and Ps to its drains Nd and Pd, and vice versa are decided depending on the amount and polarity of a voltage applied to its gates Ng and Pg.

It is said that the MOSFET Of the mentioned amplifiers has the least difference in characteristics between complementary devices of the same specification. It is thus preferred that the MOSFET is used. In view of the above, description on MOSFET only will be given in the present context. It is, however, to be noted that the spirit of the present invention can be applied to all devices that operate complementarily as well as MOSFETs. Furthermore, those skilled in the art will appreciate that the concept of the present invention can be applied to a P type MOSFET although an N type MOSFET is described in the present specification.

FIG. 1is a block diagram schematically showing the configuration of the variable gain amplifier according to one embodiment of the present invention.

As shown inFIG. 1, the variable gain amplifier according to one embodiment of the present invention includes first, second and third amplifier units101,103and105. First, second and third control signals Vc1, Vc2and Vc3are applied to the first, second and third amplifier units101,103and105, respectively. A gain of each amplifier unit is controlled by each control signal.

In the variable gain amplifier according to one embodiment of the present invention, the first amplifier unit101is constructed to operate in a high gain mode or a low gain mode depending on the first control signal Vc1applied thereto. In other words, if an amplification operation of a high gain is needed since a voltage level of a received signal is low, the first amplifier unit101supports a high gain mode operation. Meanwhile, if the linearity of a signal is critical since a voltage level of a received signal is high, the first amplifier unit101supports a low gain mode operation. The high gain mode and the low gain mode are determined by the first control signal Vc1applied thereto.

The second amplifier unit103consists of a differential amplifier circuit, and functions to amplify a voltage difference between an output voltage of the first amplifier unit101and a ground voltage with a given gain. A gain of the second amplifier unit103is determined by the second control signal Vc2.

The third amplifier unit105consists of a differential amplifier circuit, and functions to amplify a voltage difference between output voltages of the second amplifier unit103with a given gain. A gain of the third amplifier unit105is determined by the third control signal Vc3.

FIG. 2is a circuit diagram showing a detailed configuration of the first amplifier unit101of the variable gain amplifier shown in FIG.1.

As shown inFIG. 2, the first amplifier unit101according to one embodiment of the present invention includes an NMOS transistor MN21, a load resistor201, first switching means SW1, a wideband-matching element210, and an attenuator230.

Interconnection relationship of the constitutional elements in the first amplifier unit101will now be described in detail.

A gate of the NMOS transistor MN21forms an input terminal Vin of the first amplifier unit101. A drain of the NMOS transistor MN21is connected to one end of the wideband-matching element210and the attenuator230and one end of the load resistor201, thus constituting an output terminal Vout1of the first amplifier unit101. A source of the NMOS transistor MN21is grounded.

One end of the first switching means SW1is connected to the gate of the NMOS transistor MN21. The other end of the first switching means SW1is applied with a bias voltage Bias. The other end of the load resistor201is connected to a power supply VDD.

The wideband-matching element210is connected between the input terminal Vin and the output terminal Vout1of the first amplifier unit101. The wideband-matching element210has a first capacitor C21, a first inductor L21, a first resistor R21, and second switching means SW2. In more detail, one end of the first capacitor C21is connected to the input terminal Vin of the first amplifier unit101. The other end of the first capacitor C21is serially connected to the first inductor L21, the first resistor R21and the second switching means SW2. The other end of the second switching means SW2is connected to the output terminal Vout1of the first amplifier unit101.

The attenuator230is connected between the input terminal Vin and the output terminal Vout1of the first amplifier unit101, and includes a second capacitor C22, a second resistor R22and third switching means SW3. In the concrete, one end of the second capacitor C22is connected to the input terminal Vin of the first amplifier unit101, and the other end of the second capacitor C22is connected to one end of the second resistor R22. The other end of the second resistor R22is connected to one end of the third switching means SW3. The other end of the third switching means SW3is connected to the output terminal Vout1of the first amplifier unit101.

The operation of the first amplifier unit101according to one embodiment of the present invention will be described with reference to FIG.2.

The first amplifier unit101according to one embodiment of the present invention supports a high gain mode operation and a low gain mode operation. In other words, if a voltage level of the input signal Vin applied to the first amplifier unit101is less than a predetermined reference voltage, the first amplifier unit101supports the high gain mode operation. Meanwhile, if a voltage level of the input signal Vin applied thereto is more than a predetermined reference voltage, the first amplifier unit101supports the low gain mode operation.

In the case of the high gain mode, the first and second switching means SW1and SW2are short-circuited and the third switching means SW3is opened. Therefore, the gate of the NMOS transistor MN21is applied with the bias voltage Bias and the NMOS transistor MN21is activated accordingly. Further, the wideband-matching element210becomes active and the attenuator230becomes inactive.

The NMOS transistor MN21functions to amplify the input signal Vin by allowing a current proportional to the voltage Vin applied to its gate to flow into its drain.

The first capacitor C21of the wideband-matching element210cuts off the DC power supply VDDapplied to the wideband-matching element210. The first capacitor C21also adequately selects an impedance value of the first capacitor C21, the first inductor L21and the first resistor R21, so that the variable gain amplifier according to one embodiment of the present invention can be matched at a wideband and can obtain a flat gain at a high frequency. In other words, in general, since a MOSFET transistor has high input impedance, an impedance transformation process for impedance matching is required. As shown inFIG. 2, if the resistor, the inductor and the capacitor are serially connected in a feedback manner, it is possible to lower impedance of the MOSFET transistor and impedance matching is possible at a wideband.

In the case of the low gain mode, the first and second switching means SW1and SW2are opened and the third switching means SW3is short-circuited. Therefore, the NMOS transistor MN21and the wideband-matching element210become inactive and only the attenuator230operates.

In other words, as the input signal Vin is applied to the second capacitor C22, with a DC signal cut off, only an analog signal is outputted to the output terminal Vout via the second resistor R22and the third switching means SW3. Accordingly, the input signal Vin does not pass through the NMOS transistor MN21being an active element and is attenuated by the second resistor R22being a passive device and the third switching means SW3that is dominant in a resistor component. The attenuated signal is then transmitted to the output terminal Vout. As such, the signal has an excellent linearity.

Meanwhile, as both the second resistor R22and the third switching means SW3are dominant in a resistor component in a low gain mode, impedance variation depending on a frequency is very small. Therefore, wideband matching is possible by properly adjusting impedance of the second resistor R22and the third switching means SW3.

As described above, the first amplifier unit101according to one embodiment of the present invention reduces a noise figure of the entire system by a rear stage, by amplifying a signal with a high gain when a voltage level of the input signal Vin is low. On the contrary, if a voltage level of the input signal Vin is high, the first amplifier unit101reduces a nonlinear characteristic of the entire system by a rear stage of the first amplifier unit101, by reducing a gain of the first amplifier unit101. It is therefore possible to improve the linearity of the variable gain amplifier.

FIG. 3is a circuit diagram showing a detailed configuration of the second amplifier unit103of the variable gain amplifier shown in FIG.1.

As shown inFIG. 3, the second amplifier unit103according to one embodiment of the present invention includes first and second NMOS transistors MN31and MN32, first and second resistors R31and R32, first and second current sources I31, I32, a source degeneration variable resistor SD1and a load degeneration variable resistor LD1.

Gates of the first and second NMOS transistors MN31and MN32constitute + and − input terminals Vin2+ and Vin2− of the second amplifier unit103, respectively. Drains of the first and second NMOS transistors MN31and MN32are connected to one ends of the first and second resistor R31and R32, respectively, to form negative(−) and positive(+) output terminals Vout2− and Vout2+ of the second amplifier unit103. A bias current is applied to respective sources of the first and second NMOS transistors MN31and MN32by means of the first and second current source I31and I32, respectively.

The load degeneration variable resistor LD1is connected between the drains of the first and second NMOS transistors MN31and MN32. The source degeneration variable resistor SD1is connected between the sources of the first and second NMOS transistors MN31and MN32. Furthermore, the other ends of the first and second resistors R31and R32are connected to the power supply VDD, respectively.

The operation of the second amplifier unit103will now be described.

The first and second NMOS transistors MN31and MN32of the second amplifier unit103each control the amount of current flowing from its drains to its sources by the voltages Vin2+ and Vin2− applied to its gates. Voltage drop by the first and second resistors R31and R32continues to vary depending on the amount of the controlled current. Thereby, a difference between voltages applied to both the + and − input terminals Vin+ and Vin− is amplified, which is thus represented at both the negative(−) and positive(+) output terminals Vout− and Vout+.

The source degeneration variable resistor SD1and the load degeneration variable resistor LD1control the gain and linearity of the second amplifier unit403by varying its resistor values. In other words, the more the resistor value of the source degeneration variable resistor SD1is high, the more the gain of the second amplifier unit103is low and the more the linearity is good. Furthermore, as the load degeneration resistor LD1can be considered as a resistor connected in parallel to the first and second resistors R31and R32, a load resistor value of the second amplifier unit103can be varied by controlling the resistor value of the load degeneration resistor LD1. In more detail, the more the resistor value of the load degeneration resistor LD1is low, the more a total value of the load resistor is low and the more the gain of the second amplifier unit103is low. On the contrary, if the resistor value of the load degeneration resistor LD1is increased, influence by the load degeneration resistor LD1is reduced and the gain of the second amplifier unit103is rarely affected.

It is therefore possible to control the gain and linearity of the second amplifier unit103, by controlling the second control signal Vc2applied to the source degeneration variable resistor SD1and the load degeneration variable resistor LD1according to the amount of the input signal.

FIG. 4is a circuit diagram showing a detailed configuration of the third amplifier unit105of the variable gain amplifier shown in FIG.1.

As shown inFIG. 4, the third amplifier unit105according to one embodiment of the present invention includes first and second NMOS transistors MN41and MN42, first and second resistors R41and R42, first and second current sources I41and I42, and a source degeneration variable resistor SD2.

Gates of the first and second NMOS transistors MN41and MN42constitute + and − input terminals Vin3+ and Vin3− of the third amplifier unit105, respectively. Drains of the first and second NMOS transistors MN41and MN42are connected to one ends of the first and second resistor R41and R42, respectively, to form negative(−) and positive(+) output terminals Vout− and Vout+ of the third amplifier unit105. A bias current is applied to sources of the first and second NMOS transistors MN41and MN42by means of the first and second current sources I41and I42, respectively.

The source degeneration variable resistor SD2is connected between the sources of the first and second NMOS transistors MN41and MN42. Furthermore, the other ends of the first and second resistors R41and R42are connected to a power Supply VDD.

The third amplifier unit105functions to amplify, once more, the output signal amplified in the second amplifier unit103, thereby expanding a gain control range of the variable gain amplifier. The gain and linearity of the third amplifier unit105are controlled by a third control signal Vc3.

The amplifier circuit shown inFIG. 4, which constitutes the third amplifier unit105, is well known in the art and the source degeneration resistor SD2has been described with reference to FIG.2. Therefore, description on them will be omitted for simplicity.

As described above, the amplifier circuit according to one embodiment of the present invention includes the first to third amplifier units101to105, and amplifies the signal of each amplifier unit with a given gain, thus expanding its operating range.

Furthermore, the first amplifier unit101can improve the noise figure and linearity of the amplifier by supporting a high gain mode operation and a low gain mode operation. In other words, if a low signal is applied, the first amplifier unit101reduces the noise figure of the entire system by a rear stage by increasing a gain of the signal. Meanwhile, if a high signal is applied, it attenuates an input signal regardless of an amplification operation and then applies the attenuated signal to the second amplifier unit103. Therefore, the first amplifier unit101can reduce the non-linearity of the entire system affected by rear stages. Furthermore, if a signal is low, it allows a variable gain amplifier according to the present invention to perform a wideband amplification operation by connecting a wideband-matching element in a feedback manner.

The second amplifier unit103can amplify the gain of the signal outputted from the first amplifier unit101by controlling the resistor value of the source degeneration variable resistor SD1and the load degeneration variable resistor LD1. Further, it can improve the linearity of the entire system by controlling the gain and linearity of the signal depending on a level of a received signal.

The third amplifier unit105can expand a gain control range of the variable gain amplifier by amplifying, once more, a signal outputted from the second amplifier unit103. It can also control the gain and linearity of the output signal by controlling a resistor value of the source degeneration variable resistor SD1, thereby improving the linearity of a variable gain amplifier.

Although it has been described, in the above description, that the variable gain amplifier includes the first to third amplifier units101to105, it is only one of preferred embodiments of the present invention. Therefore, those skilled in the art will appreciate that the variable gain amplifier may include only one amplifier unit or two amplifier units depending on a specific embodiment.

That is, the variable gain amplifier may be constructed to perform two gain mode operations by using only the first amplifier unit101. The first amplifier unit101and the second amplifier unit103, or the first amplifier unit101and the third amplifier unit105may be connected to implement a variable gain amplifier. Furthermore, only a plurality of amplifications stages being a kind of the second amplifier unit103can be connected to implement a variable gain amplifier and only a plurality of amplifications stages being a kind of the third amplifier unit105can be connected to implement a variable gain amplifier, at the rear stage of the first amplifier unit101. In other words, all kinds of combinations of the second amplifier unit103and the third amplifier unit105can be connected at the rear of the first amplifier unit101. As the number of the amplification stage is increased, the gain control range of the variable gain amplifier is increased.

FIG. 5is a block diagram showing the configuration of a variable gain amplifier and a switching mixer according to another embodiment of the present invention.

As shown inFIG. 5, depending on a case, a switching mixer507may be connected to the output terminal of the variable gain amplifier according to one embodiment of the present invention. The variable gain amplifier includes first, second and third amplifier units501,503and505. A gain of the switching mixer507is controlled by a fourth control signal Vc4applied thereto.

FIG. 6is a circuit diagram showing an internal configuration of the mixer circuit507shown in FIG.5.

The mixer circuit507shown inFIG. 6represents the concept of the present invention applied to a typical Gilbert mixer circuit. A source degeneration resistor SD3and switch means SW61are connected between sources of first and second NMOS transistors MN61and MN62.

In other words, the gain of the mixer circuit507can be controlled depending on a voltage level of inputted signals Vin4+ and Vin4−, by adding the source degeneration resistor SD3and the switching means SW61. In more detail, if the voltage levels of the input signals Vin4+ and Vin4− are high, the switching means SW61is opened, thereby improving the linearity of the mixer by means of the source degeneration resistor SD3. Meanwhile, if the voltage levels of the input signals Vin4+ and Vin4− are low, the switch means SW61is short-circuited, thereby improving a gain characteristic of the mixed507.

In another embodiments shown in FIG.5andFIG. 6, the gain of the inputted signal is amplified through the first to third amplifier units501to505in the variable gain amplifier. The gain of the signal outputted from the variable gain amplifier is then controlled once more in the mixer507. It is therefore possible to improve the linearity and gain characteristics of a receiving apparatus.

INDUSTRIAL APPLICABILITY

According to the present invention described above, the gain and linearity of each amplifier unit included in a variable gain amplifier is controlled according to a voltage level of an input signal. Therefore, the present invention has an effect that it can improve the linearity of a variable gain amplifier.

Also, a high gain mode operation and a low gain mode operation are supported. In the high gain mode, an inputted signal is amplified with a high gain, thereby improving a noise figure of the entire system. In the low gain mode, an inputted signal is attenuated without an amplification operation, thereby improving the linearity of the entire system.

Furthermore, a wideband-matching element to which a capacitor, an inductor and a resistor are serially connected is connected to a first amplifier unit in a feedback manner. It thus allows a variable gain amplifier to perform an amplification operation at a wideband.

In addition, four amplifier units whose gain can be controlled are included. It is therefore possible to expand a gain control range of a variable gain amplifier.