JP2015023390A - Differential amplifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a differential amplifier that can expect to perform a favorable differential amplification operation even in a high frequency band.SOLUTION: As a current source of the differential amplifier comprising two field effect transistors TR1, TR2, a grounded stub comprising a length of transmission line corresponding to a quarter wavelength at the frequency of input differential signals is used. This can expect to perform a favorable differential amplification operation even in a high frequency band above 100 GHz. A resistance component of the transmission line constituting the grounded stub 30 can be lower than a parasitic resistance of a general transistor to implement a higher gain than the case of using the transistor as the current source.

Description

  The present invention relates to a differential amplifier that inputs a high-frequency differential signal.

  The differential amplifier receives two signals having opposite phases (hereinafter referred to as “differential signal”) and outputs a signal obtained by amplifying the difference between the two signals as a differential signal again. Further, when two signals having the same phase are input, they are not amplified. A circuit configuration example of a conventional differential amplifier is shown in FIG. An input terminal is connected to each gate terminal of the two field effect transistors TR1 and TR2, an output terminal is connected to each drain terminal, and a source terminal is connected to a common current source. The current source is a circuit that maintains a constant current regardless of the voltage of the signal that passes through it, and is connected to an impedance having an infinite magnitude when viewed from the two field effect transistors TR1 and TR2.

  FIG. 7 shows an example of a circuit that realizes a current source. The field effect transistor has a characteristic that the current is constant when the drain voltage applied to the drain terminal is within a predetermined range. Operation as a current source can be obtained by connecting the field effect transistor TR3 as shown in FIG. 7 and adjusting the drive voltage so that a predetermined voltage is applied to the drain terminal.

  When signals having opposite phases to each other are input to the two input terminals of the differential amplifier, when one of the transistors is on, the other is off, and therefore flows through the two field effect transistors TR1 and TR2. An amplification operation can be performed while keeping the total current constant. However, when in-phase signals are input to the two input terminals, the two field effect transistors TR1 and TR2 cannot be turned on / off at the same time, so that the amplification operation is not performed. In addition, when a signal is input only to one input terminal and the other input terminal is terminated, the current sum is kept constant as the field effect transistor on the signal input side is turned on / off. Thus, the other field effect transistor operates in conjunction with each other, and as a result, signals having opposite phases are output from the two output terminals. That is, by inputting a signal to only one input terminal, signals having opposite phases can be obtained from the two output terminals.

  Differential amplifiers having the above characteristics are used in analog circuits of various systems.

Yasuyuki Ito and Naoki Takagi, “Basics and Applications of MMIC Technology”, Realize Inc., May 31, 1996, p. 65 Masayoshi Aikawa, Takashi Ohira, Tsuneo Tokuman, Tetsuo Hirota, Masahiro Muraguchi, “Monolithic Microwave Integrated Circuit (MMIC)”, The Institute of Electrical, Information and Communication Engineers, January 15, 1997, p. 54-55

  However, when a high-frequency differential signal exceeding 100 GHz is input, various parasitic components of the transistors constituting the current source affect the transmission characteristics, and the current source is ideal for the field effect transistors TR1 and TR2. Therefore, there is a problem that a desired differential amplification operation cannot be obtained in the high frequency band.

  An equivalent circuit of the field effect transistor used as a current source in FIG. 7 is shown in FIG. 8 (Non-Patent Document 1). In addition to the current source, the equivalent circuit of the field-effect transistor includes resistances Rg, Rd, Rs of the gate terminal, drain terminal, and source terminal, parasitic capacitance Cdg between the gate and drain, parasitic capacitance Cgs between the gate and source, source It can be described using parasitic components such as parasitic capacitance Cds between drains, channel resistance Rds, and resistance Ri immediately under the depletion layer. When the operating frequency band is low, the influence of the parasitic capacitances Cdg, Cgs, Cds can be ignored. Moreover, since the gain and response speed of the transistor itself are sufficient, the influence of the parasitic resistance component is small. However, when the frequency band used is high, the influence of the parasitic capacitances Cdg and Cgs cannot be ignored, and the input impedance as a current source becomes low. Further, since the gain obtained when the signal is amplified by the field effect transistors TR1 and TR2 is reduced, the influence of the gain reduction due to the parasitic resistance of the field effect transistor TR3 used as the current source is large. When the influence of these parasitic components exists, as a differential amplifier, the phase relationship of the output signal deviates from the reverse phase, the in-phase signal is also amplified, the gain is insufficient, and the signal is attenuated. Adverse effects occur. The adverse effects of these parasitic components are the same even when bipolar transistors are used as transistors.

  On the other hand, in order to obtain a differential signal in a frequency band exceeding 100 GHz, there is a technique of designing a circuit called a rat race coupler by combining transmission lines and obtaining signals having opposite phases (non-patent document 2). A rat race coupler is a circuit in which signals having opposite phases are obtained when a signal is input from a predetermined port by combining a 1/4 wavelength line and a 3/4 wavelength line. Since the rat race coupler uses a transmission line, it is advantageous in that the influence of parasitic components is small. However, since it is a passive circuit, the effect of signal amplification cannot be obtained, and conversely, a large insertion loss is a problem. In addition, the rat race coupler has a problem that, in principle, an accurate anti-phase signal can be obtained only at one frequency, and the phase error between the anti-phase signals increases with distance from the frequency.

  The present invention has been made in view of the above, and an object of the present invention is to provide a differential amplifier that can be expected to have a good differential amplification operation even in a high frequency band.

  The differential amplifier according to the present invention is supplied with two field effect transistors, two input terminals connected to the gate terminals of the field effect transistors, to which a differential signal having a frequency of wavelength λ is input, and a driving voltage. Transmission having a length corresponding to a quarter wavelength of the wavelength λ connected to two output terminals connected to the drain terminals of each of the field effect transistors and a source terminal of the two field effect transistors And a ground stub composed of a track.

  In the differential amplifier, a matching circuit for matching an impedance between the gate terminal and the input terminal in the vicinity of the frequency is provided between the gate terminal and the input terminal, and a control voltage is supplied to the gate terminal. And an input signal matching circuit having a control voltage supply circuit.

  In the differential amplifier, a matching circuit for matching an impedance between the drain terminal and the output terminal in the vicinity of the frequency between the drain terminal and the output terminal, and a drive voltage to the drain terminal And an output signal matching circuit having a driving voltage supply circuit.

  In the differential amplifier, at least one ground stub is included, a plurality of ground stubs, open stubs, and ground capacitors are connected in parallel, and a circuit having the maximum input impedance at the frequency is a source terminal of the two field effect transistors. It is characterized by being connected to.

  In the differential amplifier, the field effect transistor is replaced with a bipolar transistor, the gate terminal is replaced with a base terminal, the drain terminal is replaced with a collector terminal, and the source terminal is replaced with an emitter terminal.

  According to the present invention, by using a ground stub with a transmission line having a length corresponding to a quarter wavelength of the differential amplification frequency as a current source of the differential amplifier, good differential amplification even in a high frequency band. A differential amplifier that can be expected to operate can be provided.

It is a functional block diagram which shows the structure of the differential amplifier in 1st Embodiment. It is a figure which shows the circuit structural example of the input side matching circuit of the said differential amplifier. It is a figure which shows the circuit structural example of the output side matching circuit of the said differential amplifier. It is a Smith chart which showed the input impedance of the ground stub of the differential amplifier. It is a functional block diagram which shows the structure of the differential amplifier in 2nd Embodiment. It is a figure which shows the circuit structural example of the conventional differential amplifier. It is a figure which shows an example of the circuit which implement | achieves the current source used with the conventional differential amplifier. It is a figure which shows the equivalent circuit of a field effect transistor.

[First Embodiment]
FIG. 1 is a functional block diagram showing the configuration of the differential amplifier according to the first embodiment.

  The differential amplifier shown in FIG. 1 is an amplifier that inputs two high-frequency signals having opposite phases and amplifies and outputs a difference. Each of the two field effect transistors TR1 and TR2 and each of the field effect transistors TR1 and TR2 Input-side matching circuits 10A and 10B connected to the gate terminals, output-side matching circuits 20A and 20B connected to the drain terminals of the field effect transistors TR1 and TR2, and ground stubs connected to the source terminals of the field effect transistors TR1 and TR2. 30.

  Input-side matching circuits 10A and 10B match the impedances between the gate terminals and input terminals of field effect transistors TR1 and TR2 in the vicinity of the operating frequency. The input side matching circuits 10A and 10B also include a voltage circuit for applying an appropriate control voltage to each gate terminal of the field effect transistors TR1 and TR2. FIG. 2 shows a circuit configuration example of the input side matching circuits 10A and 10B. Input side matching circuits 10A and 10B shown in the figure are distributed constant circuits composed of transmission lines, and utilize that the transmission lines have inductive and capacitive characteristics depending on their lengths. The input side matching circuits 10A and 10B may be matching circuits using resistors or the like. Further, when it is not necessary to apply a control voltage to the transistor, the input side matching circuits 10A and 10B may not include a voltage circuit.

  The output side matching circuits 20A and 20B match impedances between the drain terminals and the output terminals of the field effect transistors TR1 and TR2 in the vicinity of the operating frequency. The output side matching circuits 20A and 20B also include voltage circuits for applying appropriate drive voltages to the drain terminals of the field effect transistors TR1 and TR2. FIG. 3 shows a circuit configuration example of the output side matching circuits 20A and 20B. The output side matching circuits 20A and 20B shown in the figure are distributed constant circuits made of transmission lines. The output side matching circuits 20A and 20B may be matching circuits using resistors or the like.

  The ground stub 30 is a transmission line having a length corresponding to a quarter wavelength of the frequency to be differentially amplified, and operates as a high impedance load as a resonator in the vicinity of the operating frequency.

  Next, the operation of the ground stub in the present embodiment will be described.

Ideally, the source terminals of the transistors TR1 and TR2 of the differential amplifier are connected to a current source whose input impedance looks infinite. The impedance Z of the ground stub 30 can be expressed by the following equation (1).

Here, j is a complex unit, Z ₀ is the characteristic impedance of the transmission line constituting the ground stub 30, λ is the wavelength at the amplification frequency of the differential amplifier, and L is the stub length. For simplicity, the transmission line constituting the ground stub 30 is assumed to be lossless.

  From equation (1), when L is λ / 4, which is a quarter wavelength of the used frequency band, Z becomes infinite, and a sufficiently high input impedance can be obtained. That is, the ground stub 30 performs the same operation as the current source at a frequency having a wavelength of λ. Actually, the transmission line constituting the ground stub 30 is not lossless, and the impedance includes a resistance component. However, the resistance component of the transmission line is sufficiently smaller than the parasitic resistances Rg, Rd, Rs, Rds in the equivalent circuit of the current source configured using the field effect transistor, and the operation of obtaining a sufficiently high impedance is impaired. There is no.

FIG. 4 is a Smith chart showing the input impedance of the ground stub calculated using Equation (1). As shown in the figure, an input impedance more than 10 times the characteristic impedance Z ₀ is obtained in a frequency band of ± 5% centered on the frequency f0 having the wavelength λ, and the ground stub is about ± 5% of the frequency f0. It can be seen that the range behaves like a current source.

  As described above, according to the present embodiment, the current source of the differential amplifier composed of the two field effect transistors TR1 and TR2 has a length corresponding to a quarter wavelength of the frequency of the input differential signal. By using a ground stub by the transmission line, a good differential amplification operation can be expected even in a high frequency band exceeding 100 GHz. Further, since the resistance component of the transmission line constituting the ground stub 30 is generally smaller than the parasitic resistance of the transistor, a higher gain can be obtained as compared with the case where the transistor is used as a current source. With these features, the differential amplifier in the present embodiment has an advantage that it can generate a differential signal with higher phase accuracy than the prior art and can have a large gain.

[Second Embodiment]
FIG. 5 is a functional block diagram showing the configuration of the differential amplifier according to the second embodiment. In the differential amplifier according to the second embodiment, a plurality of ground stubs 30A including at least one ground stub at the source terminals of the field effect transistors TR1 and TR2 instead of the ground stub 30 according to the first embodiment. , 30B, an open stub 31A, and a grounded capacitor 32A are connected in parallel. In FIG. 5, only one open stub 31A and one grounded capacitor 32A are shown, but two or more of them may or may not exist.

The input impedances of the ground stubs 30A and 30B are as shown in Expression (1), and the input impedances of the open stub 31A and the ground capacitor 32A are expressed as Expressions (2) and (3), respectively.

  Here, ω is the angular frequency, and C is the capacitance of the grounding capacitor.

  The stub length L and the capacitance C are adjusted so that the input impedance obtained by synthesizing the impedances represented by the equations (1), (2), and (3) becomes infinite at the frequency λ. In the circuit designed in this way, the impedance change with respect to the frequency is faster than the ground stub of the first embodiment, and the frequency selectivity is increased. In addition, a differential amplifier that performs differential amplification only at a predetermined frequency can be provided.

  In the first and second embodiments, the field effect transistor is used. However, the field effect transistor is replaced with a bipolar transistor, the gate terminal is a base terminal, the drain terminal is a collector terminal, the source terminal is an emitter terminal, A differential amplifier may be configured by replacing them.

TR1, TR2, TR3 ... Field effect transistors 10A, 10B ... Input side matching circuit 20A, 20B ... Output side matching circuit 30, 30A, 30B ... Ground stub 31A ... Open stub 32A ... Ground capacitor

Claims (5)

Two field effect transistors;
Two input terminals connected to the gate terminals of each of the field effect transistors, to which a differential signal having a frequency of wavelength λ is input;
Two output terminals connected to the drain terminals of each of the field effect transistors to which a drive voltage is supplied;
A ground stub composed of a transmission line having a length corresponding to a quarter wavelength of the wavelength λ connected to the source terminals of the two field effect transistors;
A differential amplifier comprising:   A matching circuit for matching an impedance between the gate terminal and the input terminal in the vicinity of the frequency between the gate terminal and the input terminal; and a control voltage supply circuit for supplying a control voltage to the gate terminal; The differential amplifier according to claim 1, further comprising an input signal matching circuit including:   A matching circuit for matching an impedance between the drain terminal and the output terminal in the vicinity of the frequency between the drain terminal and the output terminal; and a drive voltage supply circuit for supplying a drive voltage to the drain terminal; The differential amplifier according to claim 1, further comprising an output signal matching circuit including:   A structure including at least one ground stub, a plurality of ground stubs, open stubs, and ground capacitors connected in parallel, and a circuit having the maximum input impedance at the frequency connected to the source terminals of the two field effect transistors. The differential amplifier according to claim 1.   5. The field effect transistor according to claim 1, wherein the field effect transistor is replaced with a bipolar transistor, a gate terminal is replaced with a base terminal, a drain terminal is replaced with a collector terminal, and a source terminal is replaced with an emitter terminal. Differential amplifier.

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