JP2003168934A - Bias circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bias circuit which can improve the properties of a high frequency circuit and also can lower the supply voltage. <P>SOLUTION: An inductance LT is connected between a voltage source VB and the base of a transistor QA, and a capacitor CT is connected between one end on transistor QA side of the inductance LT and ground. Then, it is arranged so that the synchronized frequency being derived from the composite capacitance value of a floating capacitance CS existing equivalently between a wiring terminal 2 and ground, a capacitor CT, and the inductance value of the inductance LT, may be roughly equal to the signal frequency of a high frequency signal Si. It becomes hard for a leak current iL to flow by the floating capacitance CS constituting a part of a synchronizing circuit, which can sharply eliminate the influence of the floating capacitance to high frequency Si. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for preventing a decrease in the input level of a high frequency signal in a circuit such as a communication device that handles a high frequency signal.

[0002]

2. Description of the Related Art In a communication device or an electronic device using communication technology, a signal level (electric power) is usually amplified before feeding a high frequency signal to an antenna. At this time, a bias for setting an operating point is supplied together with the high frequency signal to the transistors forming the high frequency amplifier circuit. In Figure 4,
An example of the configuration of a part of a conventional high frequency amplifier circuit and a bias circuit for setting an operating point of a transistor is shown.

In FIG. 4, QA is a transistor forming an amplifier circuit, 1 is a signal input terminal for supplying a high frequency signal Si, and 2 is a wiring terminal as a bias supply point. Here, the base of the transistor QA is connected to the signal input terminal 1 via the coupling capacitor CC, and the base of the transistor QA is the current limiting resistor R.
It is connected to the wiring terminal 2 via B. And wiring terminal 2
Between the ground and the ground, a voltage source VB having an internal resistance r
Is connected to the bias circuit BC. The capacitive element (CS) existing between the wiring terminal 2 and the ground in FIG. 4 is distributed-constantly generated in the circuit portion from the coupling capacitor CC to the bias circuit BC, inside the bias circuit BC, and the like. The stray capacitance is equivalently shown as one lumped constant stray capacitance.

[0004]

The high frequency signal Si handled by the circuit of FIG. 4 is basically an AC signal. Therefore, the high frequency signal S to be supplied to the base of the transistor QA
Part of i leaks through stray capacitance CS in the form of leakage current i _L. Here, when the signal frequency fi of the high frequency signal Si is high, specifically, when the high frequency is several hundred MHz, the impedance of the stray capacitance CS with respect to the high frequency signal Si becomes low, and the leakage current i passing through the stray capacitance CS is reduced. _L
Grows. Then, a phenomenon occurs in which the signal level of the high frequency signal Si supplied to the base of the transistor QA decreases, and the characteristics of the amplifier circuit including the transistor QA in the high frequency region deteriorate.

Conventionally, in order to solve such a problem, each element is arranged so that the wiring distance between one end of the current limiting resistor RB and the base of the transistor QA becomes the shortest in the actual circuit, and the current limiting resistor RB is still present. The measure was taken to increase the resistance value of. However, such a measure cannot sufficiently eliminate the adverse effect caused by the stray capacitance CS, and thus the reduction of the signal level of the high frequency signal Si and the improvement of the characteristics of the amplifier circuit in the high frequency region are limited. In addition, if the resistance value of the current limiting resistor RB is increased, another problem such as the disadvantage that the supply voltage of the voltage source VB has to be set to a higher value than the originally required voltage value has occurred. Therefore, an object of the present invention is to provide a bias circuit capable of improving the characteristics of a high frequency circuit and reducing the supply voltage.

[0006]

A bias circuit according to the present invention for solving the above-mentioned problems is a bias circuit for supplying a predetermined bias to an active element to which a high frequency signal is input. A voltage source, an inductance element connected between the voltage source and an input terminal for high frequency signals of the active element, and a capacitor connected between one end of the inductance element on the active element side and the ground, A tuning circuit in which the tuning frequency derived from the combined capacitance value of the stray capacitance and the capacitor appearing between the input terminal of the active element and the ground and the inductance value of the inductance element is substantially equal to the frequency of the high-frequency signal; It is characterized by

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION An inductance element and a current limiting resistor are connected in series between a voltage source and an input terminal to which a high frequency signal of an active element is applied. Connect a capacitor between the active element side of the inductance element and the ground,
A bypass capacitor is connected between the other end of the inductance element and the ground. Where the bypass capacitor is
It is assumed that the stray capacitance appearing between the input terminal of the active element and the ground has a capacitance value much larger than the combined capacitance value obtained by combining the capacitances of the capacitors. Further, the tuning frequency derived from the combined electrostatic capacitance value of the stray capacitance and the capacitor and the inductance value of the inductance element is made substantially equal to the frequency of the high frequency signal input to the active element.

In the bias circuit having such a structure,
When a high-frequency signal is input, the bypass capacitor having a large capacitance value can be considered to be in a state equivalent to a short circuit. Then, when viewed in terms of alternating current, the equivalent circuit of the bias circuit including the stray capacitance is substantially the configuration of the tuning circuit of the LC parallel circuit. The stray capacitance forming a part of the tuning circuit exhibits a high impedance for a high frequency signal having a frequency substantially the same as the tuning frequency. Therefore, it becomes difficult for leakage current to flow, and it is possible to improve the characteristics of the high frequency circuit and lower the supply voltage.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the configuration of a first embodiment of a bias circuit according to the present invention capable of improving the characteristics of a high frequency amplifier circuit in the high frequency region. In FIG. 1, the bias circuit BC according to the present invention has the following configuration. A voltage source VB having an inductance LT, a current limiting resistance RB and an internal resistance r is connected in series between the wiring terminal 2 and the ground. A capacitor CT is connected between one end of the inductance LT on the wiring terminal 2 side and the ground, and a bypass capacitor CP is connected between the other end of the inductance LT and the ground.

The circuit shown in FIG. 1 has substantially the same configuration as that of FIG. 4 except that the configuration of the bias circuit BC and the current limiting resistor RB are provided between the inductance LT of the bias circuit BC and the voltage source VB. Has become. The stray capacitance CS in FIG. 1 is slightly different from the stray capacitance CS in FIG.
The stray capacitance generated in a distributed constant manner in the circuit portion from the coupling capacitor CC to the inductance LT is equivalently shown as one lumped constant stray capacitance.
It can be considered that the stray capacitance CS appears between the base of the transistor QA and the ground due to the circuit configuration.

[0011] Here, when configuring the bypass circuit BC, the capacitance value C _P of the bypass capacitor CP is combined capacitance value of the stray capacitance CS and a capacitor CT (C _S +
The value is much larger than C _T ). Further, the resonance frequency (tuning frequency) derived from the combined electrostatic capacitance value (C _S + C _T ) of the stray capacitance CS and the capacitor CT and the inductance value L _{T of the} inductance LT becomes substantially the same as the signal frequency fi of the high frequency signal Si. To do so. In other words, the bypass capacitor CP, the capacitor CT, and the inductance LT are selected such that the following conditional expressions are satisfied.

[0012]

[Equation 1]

The bias circuit BC and the stray capacitance CS, which are constructed so as to satisfy the above connection relationship and characteristic values, form an equivalent circuit as shown in FIG. That is, it is an LC circuit network including a series circuit of the inductance LT and the bypass capacitor CP, and a combined capacitance element (CS + CT) connected in parallel to the series circuit. Note that the combined capacitive element (CS + CT) is a unit in which a capacitor CT and a stray capacitance CS, which are in a parallel connection relationship, are integrated for convenience.

When an AC signal having a signal frequency fi is supplied to the equivalent circuit of FIG. 2, the combined capacitive element (CS + CT)
The bypass capacitor CP having a much larger capacitance value is in a state equivalent to a short circuit. Therefore, it can be considered that the circuit of FIG. 2 substantially forms a tuning circuit of the LC parallel circuit. As is well known, a tuning circuit using an LC parallel circuit raises impedance by a parallel resonance operation for a signal having the same frequency as the tuning frequency. Therefore, the high frequency signal S having the signal frequency fi
For i, stray capacitance CS that forms part of the tuning circuit
The impedance of is apparently high. Then, the leakage current i _L becomes difficult to flow, and the decrease in the signal level of the high frequency signal Si is prevented. As a result, the influence of the stray capacitance can be largely eliminated, and the characteristics of the high frequency amplifier circuit can be improved.

Since the stray capacitance CS is part of the tuning circuit to prevent the leakage current i _L from flowing,
In the bias circuit BC of FIG. 1, it is not necessary to increase the resistance value of the current limiting resistance RB. Voltage source VB and transistor Q
Although an inductance LT exists between the base of A and the base of A, as is well known, the impedance of the inductance element with respect to a DC signal is low. Therefore, according to the bias circuit of the present invention, it is possible to lower the supply voltage of the voltage source.

The impedance of the tuning circuit is low for signals having a frequency different from the tuning frequency. Therefore, part of noise and harmonics having a frequency different from the signal frequency fi flows to the ground via the stray capacitance CS and the bias circuit BC. As a result, the high frequency signal Si
While the signal level of 1 is hardly reduced, the signal level of noise and the signal level of harmonics are reduced, which contributes to the improvement of the characteristics of the high frequency amplifier circuit. Further, since the stray capacitance CS is used as a part of the tuning circuit to eliminate its adverse effect, the inductance LT (current limiting resistor RB in the conventional example) does not have to be forcibly placed near the base of the transistor QA. . Therefore, there is an additional effect that the degree of freedom in circuit design is improved.

By the way, there may be a voltage source VB which can protect the circuit without depending on the current limiting resistor RB and has an extremely low internal resistance r in terms of AC. When using such a voltage source VB, the current limiting resistor RB and the bypass capacitor CP shown in FIG. 1 can be omitted from the circuit. Figure 3
Includes a current limiting resistor RB and a bypass capacitor CP.
A second embodiment of the bias circuit according to the present invention in which is omitted is shown. Also in the circuit of FIG. 3, the inductance LT, the capacitor CT, and the stray capacitance CS form a tuning circuit by an LC parallel circuit when viewed in terms of AC. Therefore, it goes without saying that the circuit of FIG. 3 can also obtain the same operational effect as the circuit of FIG.

[0018]

As described above, in the bias circuit according to the present invention, the inductance element is connected between the voltage source and the high-frequency signal input terminal of the active element, and one end of the inductance element on the active element side and the ground are connected. Connect a capacitor between. If necessary, a bypass capacitor is connected between the other end of the inductance element and the ground. The stray capacitance appearing between the input terminal of the active element and the ground and the tuning frequency derived from the combined capacitance value of the capacitances of the capacitors and the inductance value of the inductance element are The feature is that it is set to be almost equal to the frequency.

According to the present invention, the bias circuit and the stray capacitance form a tuning circuit. The stray capacitance forming a part of the tuning circuit has a high impedance for a high frequency signal having a frequency substantially the same as the tuning frequency and, on the contrary, a low impedance for noise and harmonics different from the tuning frequency. Then, the signal level of the high-frequency signal is prevented from being lowered, while the signal level of noise and harmonics is reduced. As a result, the characteristics of the high frequency amplifier circuit can be improved. Further, since the adverse effect of the stray capacitance can be eliminated regardless of the current limiting resistance, the resistance value of the current limiting resistance can be lowered and the supply voltage of the voltage source can be lowered. Therefore, according to the present invention, it is possible to provide a bias circuit capable of improving the characteristics of a high frequency circuit and reducing the supply voltage.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a part of a high frequency amplifier circuit and a first embodiment of a bias circuit according to the present invention.

FIG. 2 is an equivalent circuit of the bias circuit and the stray capacitance of FIG.

FIG. 3 is a circuit diagram of a part of a high frequency amplifier circuit and a bias circuit according to a second embodiment of the present invention.

FIG. 4 is a circuit diagram of a part of a high frequency amplifier circuit and an example of a bias circuit for setting an operating point of a conventional transistor. .

[Explanation of symbols]

1: Signal input terminal 2: Wiring terminal BC: Bias circuit CC: Coupling capacitor C
P: Bypass capacitor CS: Stray capacitance CT: Capacitor LT: Inductance Q
A: Transistor (active element) r: Internal resistance of voltage source RB: Current limiting resistance VB: Voltage source

Continued front page    (72) Inventor Yoshito Tomizuka             18 Tomi, Gomigaya, Tsurugashima City, Saitama Prefecture             Saitama Office Co., Ltd. F term (reference) 5J092 AA01 AA58 CA35 CA37 CA41                       FA20 HA02 HA25 HA29 HA32                       HA33 KA12 KA14 SA13 VL08                 5J500 AA01 AA58 AC35 AC37 AC41                       AF20 AH02 AH25 AH29 AH32                       AH33 AK12 AK14 AS13 LV08

Claims (3)

[Claims] 1. A bias circuit for supplying a predetermined bias to an active element to which a high frequency signal is input, the bias circuit comprising: a voltage source; and a voltage source and an input terminal of the high frequency signal of the active element. A connected inductance element and a capacitor connected between one end of the inductance element on the active element side and the ground, where a floating appearing between the input terminal of the active element and the ground A bias circuit comprising: a tuning circuit having a tuning frequency derived from a combined capacitance value of the capacitor and the capacitor and an inductance value of the inductance element and substantially equal to the frequency of the high frequency signal. 2. A current limiting resistor connected in series between the voltage source and the inductance element, and a capacitance value that is much larger than the combined capacitance value. And a bypass capacitor connected in parallel to the series circuit of the source, the bias circuit according to claim 1. 3. The bias circuit according to claim 1, wherein the active element is a transistor forming a high frequency amplifier circuit.