High frequency amplifier

Abstract

To provide a high frequency amplifier capable of satisfying both stability and operation characteristics.SOLUTION: A high frequency amplifier is configured with a cascode circuit in which the drain of a source ground transistor and the source of a gate ground transistor are connected, an impedance circuit is inserted between the gate of the gate ground transistor and the ground. The impedance circuit includes a stabilizing resistor which is adjusted to be low impedance in using frequency band so that the gate of the gate ground transistor is grounded in high frequency and to be high impedance in a high frequency band in which stability degrades. Thus, stability is assured by avoiding K-factor not to be below 1 in the high frequency band in which stability degrades, and operation characteristics are made excellent by avoiding the effect of the stabilizing resistor in the using frequency.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cascode-connected high frequency power amplifier, and more particularly to an amplifier that can achieve both excellent operating characteristics and stability over a wide band. [Background technology]

[0002] High-frequency amplifiers, such as those used in high-frequency low-noise amplifiers (LNAs) in receivers, are known to have a cascode-connected configuration of transistors to increase power gain. This circuit topology is called a cascode circuit, in which one main electrode of a first transistor receives an input signal at its control electrode, and one electrode of a second transistor is connected to that transistor, and output is taken from the other electrode of the second transistor. Such cascode circuits can suffer from stability loss due to unwanted feedback components. For this reason, a high-frequency amplifier using a stabilizing resistor, as disclosed in Patent Document 1, has been proposed.

[0003] A conventional high-frequency amplifier of this type will be described below with reference to the drawings. Figure 10 is a circuit diagram showing a cascode circuit equipped with a stabilizing resistor. In this diagram, the drain of a first transistor 1, whose source is grounded, is connected to the source of a second transistor 2, whose gate is grounded. The gate of the second transistor 2 is grounded via a DC-blocking capacitor 4 and a stabilizing resistor 5 connected in series with it. An input terminal 7 is connected to the gate of the first transistor 1, and an output terminal 10 is connected to the drain of the second transistor via an output matching circuit 9. The stabilizing resistor is generally called a damping resistor. The transistors used in the cascode circuit shown in this diagram are GaAs-based high electron mobility transistors (HEMTs).

[0004] The cascode circuit of Figure 10 prevents oscillation in the high-frequency band because a stabilizing resistor 5 is inserted between the MIM capacitor 4 and ground. Figure 11 is a graph showing the stability of the cascode circuit of Figure 10 when the operating frequency is 1.5 GHz. The horizontal axis represents frequency, and the vertical axis represents the amplifier's K-factor. In this figure, the thick solid line represents the characteristics when the stabilizing resistor 5 is inserted, and the thin dotted line represents the characteristics when the stabilizing resistor 5 is not inserted (the gate of the second transistor 2 is grounded only via the MIM capacitor 4). Note that oscillation is not expected when the K-factor exceeds 1, and as shown in the figure, there is no oscillation when the stabilizing resistor 5 is inserted. Conversely, when the stabilizing resistor 5 is not inserted, oscillation occurs at approximately 22 to 22.5 GHz. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 9,825,603 Summary of the Invention [Problem to be solved by the invention]

[0006] However, there is a trade-off between the operating characteristics and stability of high-frequency power amplifiers, and operating characteristics are sacrificed to improve stability. Figure 12 is a graph showing the operating characteristics of the cascode circuit of Figure 10, with the horizontal axis representing frequency and the vertical axis representing noise figure (NF). In this graph, the thick solid line represents the characteristics when a stabilizing resistor 5 is inserted, and the thin dotted line represents the characteristics when a stabilizing resistor 5 is not inserted (the gate of the second transistor 2 is grounded only via capacitor 4). As shown in the graph, the bandwidth becomes narrow at 1.5 GHz, the operating frequency of the cascode circuit of Figure 10, and the NF also deteriorates. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a high frequency amplifier that can solve the above problems and achieve both stability and operating characteristics. [Means for solving the problem]

[0007] According to one aspect of the present invention, there is provided a high-frequency amplifier comprising a cascode circuit including a first transistor having a control electrode to which a high-frequency signal is input and one of its main electrodes grounded, and a second transistor having one main electrode connected to the other main electrode of the first transistor, the other main electrode for outputting the amplified high-frequency signal, and a control electrode connected to ground via an impedance circuit, wherein the impedance circuit includes a stabilizing resistor and is adjusted to have low impedance in the frequency band used to ground the control electrode of the second transistor at high frequencies, and to have high impedance in the high-frequency band where stability deteriorates. The impedance circuit may include a capacitor having one end connected to the control electrode of the second transistor, a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground, and a low-pass filter connected in parallel with the stabilizing resistor, and the low-pass filter may have a pass band that is the operating frequency.

[0008] The impedance circuit may include a capacitor having one end connected to the control electrode of the second transistor, a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground, and an inductor connected in parallel with the stabilizing resistor, and the inductor may be adjusted to have a lower impedance than the stabilizing resistor at the operating frequency and a higher impedance than the stabilizing resistor in a high frequency band where stability deteriorates. The impedance circuit may include a capacitor having one end connected to the control electrode of the second transistor, a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground, and an LC filter consisting of a series circuit of an inductor and a capacitor connected in parallel to the stabilizing resistor, and the LC filter may be adjusted to have a lower impedance than the stabilizing resistor at the operating frequency and a higher impedance than the stabilizing resistor in the high frequency band where stability deteriorates. The impedance circuit may also include a first series circuit consisting of a capacitor having one end connected to the control electrode of the second transistor and the stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground, and a second series circuit consisting of a series circuit of an inductor and a capacitor connected in parallel to the first series circuit, wherein the second series circuit forms an LC filter and is adjusted to have a lower impedance than the first series circuit at the operating frequency and to have a higher impedance than the first series circuit in the high frequency band where stability deteriorates.

[0009] The impedance circuit may include a capacitor having one end connected to the control electrode of the second transistor, the stabilizing resistor having one end connected to the other end of the capacitor and the other end grounded, and an inductor having one end connected to a connection node between the capacitor and the stabilizing resistor and the other end grounded, wherein all circuit elements except the inductor are monolithically formed on an IC chip, the inductor is formed from a component outside the IC chip, and is adjusted to have a lower impedance than the stabilizing resistor at a used frequency and a higher impedance than the stabilizing resistor in a high frequency band where stability deteriorates. In this case, the inductor may be formed on a circuit board, and the IC chip may be mounted on the circuit board. [Effects of the Invention]

[0010] According to one aspect of the present invention, it is possible to provide a high-frequency amplifier that can achieve both stability and operating characteristics. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a diagram illustrating the basic concept of the present invention. [Figure 2] FIG. 1 is a diagram illustrating a cascode circuit according to a first embodiment of the present invention. [Figure 3] 3 is a graph showing the stability of the cascode circuit shown in FIG. 2; [Figure 4] 3 is a graph showing the operating characteristics of the cascode circuit shown in FIG. 2; [Figure 5] FIG. 10 is a diagram illustrating a cascode circuit according to a second embodiment of the present invention. [Figure 6] FIG. 10 is a diagram illustrating a cascode circuit according to a third embodiment of the present invention. [Figure 7] FIG. 10 is a diagram illustrating a cascode circuit according to a fourth embodiment of the present invention. [Figure 8] FIG. 10 is a diagram illustrating a cascode circuit according to a fifth embodiment of the present invention. [Figure 9] FIG. 10 is a diagram illustrating a cascode circuit according to a sixth embodiment of the present invention. [Figure 10] FIG. 1 is a diagram illustrating a cascode circuit according to a conventional example. [Figure 11] 10 is a graph showing stability of a cascode circuit according to a conventional example. [Figure 12] 10 is a graph showing the operating characteristics of a conventional cascode circuit. DETAILED DESCRIPTION OF THE INVENTION

[0012] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In the following embodiments, identical or equivalent parts will be denoted by the same reference numerals.

[0013] (Basic concept of the invention) The basic concept of the present invention will be described with reference to FIG. 1. FIG. 1 shows a cascode circuit, which is identical to the circuit shown in FIG. 10 except for the circuit portion extending from the gate of the grounded-gate transistor (second transistor 2) to ground. Specifically, the gate of first transistor 1 is connected to input terminal 7 and one end of bias resistor 8, its source is grounded, and its drain is connected to the source of second transistor 2. The drain of second transistor 2 is connected to output terminal 10 via output matching circuit 9 and to a power supply (not shown) that applies bias. The gate is connected to one end of bias resistor 3 and is grounded via impedance circuit 6. This impedance circuit 6, together with bias resistor 3, forms part of the bias circuit. Impedance circuit 6 is adjusted to have low impedance in the operating frequency band (e.g., 1.5 GHz) and high impedance in the high-frequency band (e.g., 22 GHz) where stability deteriorates. A stabilizing resistor is incorporated in part of the circuit. Therefore, the gate of second transistor 2 is grounded at high frequencies in the operating frequency band, improving noise figure characteristics, while the stabilizing resistor suppresses deterioration of stability in the high-frequency band.

[0014] (First embodiment) A first embodiment of the present invention will be described with reference to Figures 2 to 4. Figure 2 shows a cascode circuit, illustrating the impedance circuit 6 of Figure 1 in more detail. That is, the impedance circuit 6 of this embodiment is configured by inserting a series circuit of a DC blocking capacitor 61 and a stabilizing resistor 62, and a low-pass filter (LPF) 63 connected in parallel to the stabilizing resistor 62, between the gate of the second transistor 2 and ground. This LPF 63 has the usable frequency as its pass band. The circuit operates as follows: A high-frequency signal is input to input terminal 7 via an input matching circuit (not shown). The input signal is biased to a predetermined voltage value via bias resistor 8 and then input to the gate of first transistor 1. There are various other ways to bias first transistor 1, but these are outside the scope of the present invention and will not be described here. A bias voltage is supplied to the drain and gate of second transistor 2, so the signal input to the source is amplified by a predetermined gain. In other words, the input high-frequency signal is amplified by first transistor 1, then further amplified by second transistor 2, and output from the drain of second transistor 2.

[0015] Figure 3 is a graph showing the stability of the cascode circuit of Figure 2 when the operating frequency is 1.5 GHz, with the horizontal axis representing frequency and the vertical axis representing the amplifier's stability index (K factor). In this graph, the characteristics of the circuit of Figure 2 are shown by a dashed line. For comparison of circuit characteristics, the characteristics of the conventional example (the circuit of Figure 10) are shown by a solid line, and the characteristics when no stabilizing resistor 5 is inserted (the gate of the second transistor 2 is grounded only via capacitor 4) are shown by a dotted line. As shown in the graph, although there are some differences, the circuit of Figure 2 maintains stability comparable to that of the circuit of Figure 10, and the K factor does not fall below 1 even at approximately 22 to 22.5 GHz.

[0016] FIG. 4 is a graph showing the operating characteristics of the cascode circuit of FIG. 2, with the horizontal axis representing frequency and the vertical axis representing NF. In this graph, the characteristics of the circuit of FIG. 2 are shown by a dashed line, and for comparison, the characteristics of the conventional example (circuit of FIG. 10) are shown by a solid line. As shown in the graph, at 1.5 GHz, the operating frequency of the cascode circuit of FIG. 2, the bandwidth is wider than that of the conventional example, and the NF is also improved. This is comparable to the characteristics when no stabilizing resistor is used, as shown by the dotted line in FIG. 12. As such, in this embodiment, even when a stabilizing resistor is used, the operating characteristics do not deteriorate at the operating frequency.

[0017] (Second embodiment) A second embodiment of the present invention will be described with reference to FIG. 5. FIG. 5 shows a cascode circuit, illustrating the impedance circuit 6 of FIG. 1 in more detail. That is, the impedance circuit 6 of this embodiment is configured by inserting a series circuit of a DC blocking capacitor 61 and a stabilizing resistor 62, and an inductor 64 connected in parallel to the stabilizing resistor 62 between the gate of the second transistor 2 and ground. The inductor 64 has frequency characteristics and is adjusted to have a lower impedance than the stabilizing resistor 62 at the operating frequency and a higher impedance than the stabilizing resistor 62 in the high-frequency band where stability deteriorates. In this embodiment, the stabilizing resistor ensures stability in the high-frequency band, and the operating characteristics at the operating frequency do not deteriorate.

[0018] (Third embodiment) A third embodiment of the present invention will be described with reference to FIG. 6. FIG. 6 shows a cascode circuit, illustrating the impedance circuit 6 of FIG. 1 in more detail. The impedance circuit 6 of this embodiment is configured by inserting a series circuit of a DC-blocking capacitor 61 and a stabilizing resistor 62, and a series circuit of an inductor 64 and a capacitor 65 connected in parallel to the stabilizing resistor 62, between the gate of the second transistor 2 and ground. The inductor 64 and the capacitor 65 form an LC filter, and the constants of the individual elements are adjusted so that the resonant frequency of the inductor 64 is the operating frequency, resulting in a lower impedance than the stabilizing resistor 62 at the operating frequency. At the same time, the constants of the individual elements are also adjusted so that the impedance is higher than the stabilizing resistor 62 in the high-frequency band where stability deteriorates. Therefore, the influence of the stabilizing resistor 62 is either absent or very small at the operating frequency, preventing degradation of the operating characteristics. While the second embodiment uses only an inductor, adding a capacitor to form an LC filter as in this embodiment increases the Q factor, allowing for pinpoint matching to the operating frequency. This is advantageous when the high-frequency band where stability deteriorates is close to the operating frequency. Also in this embodiment, stability in the high frequency band is ensured by the stabilizing resistor.

[0019] (Fourth embodiment) A fourth embodiment of the present invention will be described with reference to FIG. 7. FIG. 7 shows a cascode circuit, illustrating the impedance circuit 6 of FIG. 1 in more detail. Specifically, the impedance circuit 6 of this embodiment is configured by inserting a series circuit of a DC-blocking capacitor 61 and a stabilizing resistor 62, and a series circuit of an inductor 64 and a capacitor 65 connected in parallel to the series circuit, between the gate of the second transistor 2 and ground. The inductor 64 and the capacitor 65 form an LC filter, and the constants of the individual elements are adjusted so that the resonant frequency of the inductor 64 is the operating frequency. The inductor 64 has a lower impedance than the stabilizing resistor 62 at the operating frequency. At the same time, the constants of the individual elements are adjusted so that the impedance is higher than the stabilizing resistor 62 at high frequencies where stability deteriorates. In this embodiment, the capacitor 65 also functions as a DC-blocking filter. If the same characteristics as those of the second embodiment shown in FIG. 5 are required, this can be achieved by adjusting the constant of the capacitor 65 to the same as that of the capacitor 61. In this embodiment, the influence of the stabilizing resistor 62 is either absent or very slight at the operating frequency, so that the operating characteristics are not degraded and stability in high frequencies is ensured.

[0020] (Fifth embodiment) A fifth embodiment of the present invention will be described with reference to FIG. 8. FIG. 8 shows a cascode circuit, illustrating the impedance circuit 6 of FIG. 1 in more detail. Specifically, the impedance circuit 6 of this embodiment is configured by inserting a series circuit of a DC-blocking capacitor 61 and a stabilizing resistor 62, and an inductor 66 connected in parallel to the stabilizing resistor 62, between the gate of the second transistor 2 and ground. While the inductor 64 in the second to fourth embodiments is a spiral inductor that can be incorporated into the IC, the inductor 66 in this embodiment utilizes a parasitic inductor of a component external to the IC chip. A bonding wire, for example, may be used as the external component, and its parasitic inductor may be utilized. As in the second embodiment, the inductor 66 is adjusted to have a lower impedance than the stabilizing resistor 62 at the operating frequency and a higher impedance than the stabilizing resistor 62 in the high-frequency band where stability deteriorates. The wire bonding can be performed from a pad on the chip to a pad at ground potential, or from a pad on the chip to a portion of the lead frame at ground potential (such as a tab or part of an inner lead). This allows the wire length to be adjusted appropriately to adjust the impedance. The impedance can also be adjusted by changing the number of bonding wires. In this embodiment, the impedance can be adjusted in a post-process. In this embodiment, the influence of the stabilizing resistor 62 is either nonexistent or very small at the operating frequency, so the operating characteristics do not deteriorate and stability in the high frequency band can be ensured.

[0021] (Sixth embodiment) A sixth embodiment of the present invention will be described with reference to FIG. 9. FIG. 9 shows an IC chip 100 on which a cascode circuit is formed, a circuit board 200 on which the IC chip 100 is mounted, and bonding wires 300 connecting them. The IC chip 100 is, for example, a monolithic microwave integrated circuit (MMIC), in which the circuit elements of the cascode circuit, except for inductor 67, are monolithically formed. Pads 11 to 13 are bonding pads formed on the IC chip 100, and pad 12 is electrically connected to the connection node between capacitor 61 and stabilizing resistor 62. The other pads 11 and 13 are electrically connected to the input and output terminals of the cascode circuit, respectively. The circuit board 200 is, for example, made of low-temperature co-fired ceramics (LTCC), and has multiple lands 21 formed thereon for electrical connection to the IC chip 100 or other components. Among these, lands 211 and 212 have inductor 67 formed between them. Through vias (not shown) are formed inside the circuit board 200, electrically connecting the lands on the front surface to external terminals such as bumps on the back surface. Pad 12 is electrically connected to land 211, and pads 11 and 13 are electrically connected to lands 21, 21, by bonding wires 300. The external terminals electrically connected to land 212 via through vias (not shown) are at ground potential.

[0022] Like the fifth embodiment, this embodiment also uses an inductor formed using components external to the IC chip. However, the inductor 67 in this embodiment is formed in a fixed shape on the surface of the circuit board 200 and is not intended to produce a parasitic effect. For example, if the circuit board is made of LTCC, the inductor in this embodiment would be formed by sputtering or vapor deposition. While a lead frame could be used instead of a circuit board, it is difficult to reproduce fine patterns with a lead frame created by stamping, and lead frames created by etching have poor productivity. Therefore, the above-mentioned LTCC is more effective in terms of both integration and productivity. Furthermore, if integration is not required to be high, ceramic substrates formed by sintering green sheets or organic substrates can also be used in addition to the above-mentioned lead frames. In addition to forming components external to the IC chip using patterns, chip components can also be mounted on the LTCC. For example, a chip inductor can be mounted on the LTCC together with the IC chip 100 and electrically connected via internally connected lands. There are two ways to form the inductor: using a pattern or using chip components. The choice is determined appropriately depending on the frequency band level of the frequency used and the desired integration level. In this embodiment, as in the second embodiment, inductor 67 is adjusted so that it has lower impedance than stabilizing resistor 62 at the operating frequency, and higher impedance than stabilizing resistor 62 in the high frequency band where stability deteriorates. Therefore, there is no or very little influence from stabilizing resistor 62 at the operating frequency, so the operating characteristics do not deteriorate and stability in the high frequency band can be ensured.

[0023] Although the above describes the embodiments of the present invention, various modifications are possible without departing from the spirit of the present invention. For example, while the transistors used in the cascode circuit are HEMTs, depending on the receiver, they may be junction field effect transistors (JFETs) or heterojunction bipolar transistors (HBTs). When HBTs are used, the cascode circuit is configured such that a common-base transistor is connected to the collector of a common-emitter transistor. The drain, gate, and source of a HEMT or JFET can be interpreted interchangeably with the collector, base, and emitter of an HBT. Generally speaking, the transistors in this invention are three-terminal devices with gain, consisting of one control electrode and two main electrodes. Although the transistor substrate is described as GaAs, other materials can also be used, such as GaN, SiC, and other compound semiconductor substrates, and SiGe, SOI, and other silicon substrates, which can be used in high frequency and wide bands, but which may lose stability in high frequency bands when configured as an amplifier, and the present invention is also applicable to such materials. Furthermore, it goes without saying that the present invention can be applied to modules that do not use ICs but have discrete devices mounted on a printed circuit. Furthermore, in the fifth and sixth embodiments, the frequency characteristics are provided only by an inductor formed from a component outside the IC chip, but a capacitor may also be added to this to form a circuit configuration similar to that of the third and fourth embodiments. If the capacitor is mounted on a circuit board together with the IC chip as a chip capacitor and connected in series with the inductor by wiring, adjustment can be made in a post-process by laser trimming. In addition, in the sixth embodiment, a meander-shaped inductor is used, but it is also possible to use a multilayer wiring board as the circuit board, form part of the inductor wiring on each layer, and connect these with through vias to create a spiral-shaped inductor or an inductor of other shapes. [Explanation of symbols]

[0024] 1. First transistor 2 Second transistor 3 Bias resistors 6 Impedance Circuit 61,65 Capacitor 62 Stabilization resistor 63 Low-pass filter 64, 66, 67 Inductors 7 Input terminals 8 bias resistors 9 Output matching circuit 10 Output terminal

Claims

1. a first transistor having a control electrode to which a high frequency signal is input and one of its main electrodes grounded; a cascode circuit including a second transistor having one main electrode connected to the other main electrode of the first transistor, the other main electrode for outputting the amplified high-frequency signal, and a control electrode connected to ground via an impedance circuit; the impedance circuit includes a stabilizing resistor, and is adjusted to have a low impedance in the frequency band used to ground the control electrode of the second transistor at high frequencies, and to have a high impedance in the high frequency band where stability deteriorates.

2. the impedance circuit includes a capacitor having one end connected to the control electrode of the second transistor; a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground; a low-pass filter connected in parallel with the stabilizing resistor; 2. The high-frequency amplifier according to claim 1, wherein the low-pass filter has a pass band equal to the frequency to be used.

3. the impedance circuit includes a capacitor having one end connected to the control electrode of the second transistor; a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground; an inductor connected in parallel with the stabilizing resistor; 2. The high-frequency amplifier according to claim 1, wherein the inductor is adjusted to have a lower impedance than the stabilizing resistor at the operating frequency and a higher impedance than the stabilizing resistor in the high-frequency band where stability deteriorates.

4. the impedance circuit includes a capacitor having one end connected to the control electrode of the second transistor; a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground; an LC filter including a series circuit of an inductor and a capacitor connected in parallel with the stabilizing resistor; 2. The high-frequency amplifier according to claim 1, wherein the LC filter is adjusted to have a lower impedance than the stabilizing resistor at the operating frequency and a higher impedance than the stabilizing resistor in a high-frequency band where stability deteriorates.

5. the impedance circuit includes a first series circuit including a capacitor having one end connected to the control electrode of the second transistor, and a stabilizing resistor having one end connected to the other end of the capacitor and the other end connected to the ground; a second series circuit consisting of a series circuit of an inductor and a capacitor connected in parallel with the first series circuit, 2. The high-frequency amplifier according to claim 1, wherein the second series circuit forms an LC filter and is adjusted to have a lower impedance than the first series circuit at an operating frequency and a higher impedance than the first series circuit in a high-frequency band where stability deteriorates.

6. the impedance circuit includes a capacitor having one end connected to the control electrode of the second transistor; a stabilizing resistor having one end connected to the other end of the capacitor and the other end grounded; an inductor having one end connected to a connection node between the capacitor and the stabilizing resistor and the other end grounded; The circuit elements except for the inductor are monolithically formed on an IC chip, 2. The high-frequency amplifier according to claim 1, wherein the inductor is configured as a component external to the IC chip and is adjusted to have a lower impedance than the stabilizing resistor at the operating frequency and a higher impedance than the stabilizing resistor in the high-frequency band where stability deteriorates.

7. 7. The high-frequency amplifier according to claim 6, wherein the inductor is formed on a circuit board, and the IC chip is mounted on the circuit board.

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