JP3216808B2 - Variable attenuator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable attenuator using an FET, and more particularly, to a variable attenuator that compensates for temperature drift and manufacturing variations when an integrated circuit is formed.

[0002]

2. Description of the Related Art A variable attenuator is used as a circuit portion for adjusting the transmission power of a device such as a portable telephone, and is particularly used as a variable gain amplifier combined with an amplifier for a high frequency signal of 500 MHz or more. In general, in variable attenuators, measures have been taken for temperature compensation at the time of manufacture before, but for variations in attenuation characteristics due to manufacturing variations,
No measures have been taken, and this has been addressed by adjusting the control voltage during the manufacture of the set of equipment.
However, there is a demand for a reduction in the number of working steps from the viewpoint of recent cost reduction and the like. Therefore, if the characteristic fluctuation due to temperature drift and manufacturing variation can be compensated at the stage of IC integration, this demand can be met.

A conventional variable attenuator having a temperature compensation function will be described with reference to FIG. This variable attenuator
It comprises a variable attenuator body 1 and a reference voltage generation circuit 2. The variable attenuator body 1 connects the control voltage Vcnt to the gate of the MOS transistor (GaAs FET) Q6, and outputs the reference voltage Vre generated by the reference voltage generation circuit 2.
By connecting f to the source and drain of the transistor Q6, the signal input from the input terminal IN is attenuated and output from the output terminal OUT. The reference voltage generation circuit 2
This is a temperature compensation circuit using a temperature compensation diode Di, and the temperature compensation diode Di adjusts the reference voltage Vref so as to suppress a shift in the attenuation characteristic of the MOS transistor caused by a temperature drift.

Japanese Unexamined Patent Publication No. 62-273787 discloses a technique relating to temperature compensation in which a diode prevents current fluctuation due to temperature drift of a light emitting element drive circuit. FIG. 8 is a circuit diagram of a light emitting element drive circuit described in the publication. This light emitting element driving circuit includes a light emitting element driving circuit main body and a temperature compensation circuit.

The light emitting element driving circuit body is made of GaAs-FE
It comprises TQg1, Qg2, a silicon transistor Qg3, and a semiconductor laser LD. Transistor Qg
1 to Qg2 are current changeover switches for driving the semiconductor laser LD, and the transistor Qg3 includes Qg1 and Qg2.
2 is supplied with a constant current. The temperature compensation circuit includes a temperature compensation diode Di and a variable resistor VR1 connected in series with each other. The temperature compensation circuit includes a variable resistor VR1
Is applied to the base of the transistor Qg3. The temperature characteristic of the temperature compensation circuit operates so as to suppress the temperature characteristic of the light emitting element drive circuit body. This light emitting element drive circuit can prevent the temperature change of the current with a simple temperature compensation circuit.

[0006]

In the temperature compensation circuit disclosed in the above publication, when a diode is used for temperature compensation to form an IC, characteristic variations due to variations in the device manufacturing of the GaAs FET and the diode of the variable attenuator occur independently. I do.
For this reason, the characteristic variation due to the temperature drift can be compensated, but the characteristic variation due to the manufacturing variation of the threshold voltage of the FET cannot be compensated.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems of the prior art,
It is an object of the present invention to provide a variable attenuator capable of compensating both a temperature drift and a characteristic variation due to a manufacturing variation at a stage of forming.

[0008]

To achieve the above object, a variable attenuator according to the present invention has a gate connected to a control terminal via a first resistor, and a drain connected to an input terminal via a first capacitance. And a source connected to the reference voltage source via a second resistor, a source connected to the reference voltage source via a third resistor, and connected to an output terminal via a second capacitance. A variable attenuator body having an FET and a reference voltage generating circuit for outputting the reference voltage source, wherein the reference voltage generating circuit has a drain connected to a first power supply, a gate and a drain. Is connected via a fourth resistor, and the source is connected to a second power supply via a fifth resistor.
A second FET having the same structure as that of the first embodiment, wherein the reference voltage source is output from the source of the second FET.

Further, in the variable attenuator of the present invention, the gate is connected to the reference voltage source via the first resistor, the drain is connected to the input terminal via the first capacitance, and the second capacitor is connected via the second capacitance. A variable attenuator body having a first FET connected to an output terminal, a source connected to a control terminal via a second resistor, and connected to a first power supply via a third capacitance; A variable attenuator having a reference voltage generator that outputs a reference voltage source, wherein the reference voltage generator has a drain connected to a second power supply, a gate and a drain connected via a third resistor, A first source connected to the first power source;
A second FET having the same structure as that of the first FET, and outputting the reference voltage source from a drain of the second FET.

Further, the variable attenuator of the present invention functions as a variable resistor to which a control signal is input, attenuates an input signal input from an input terminal based on the control signal, and outputs the signal from an output terminal. In a variable attenuator comprising a junction type transistor, one of the power supplies of the variable attenuator,
It is output as a source or drain voltage of a second junction type transistor having the same structure as the first junction type transistor, and the voltage fluctuates with the same temperature characteristics as a threshold voltage of the first junction type transistor. .

In the variable attenuator of the present invention, since the FET having the same structure as that of the main body of the variable attenuator is used for the reference voltage generating circuit, it is necessary to compensate for both the temperature drift and the characteristic variation due to the manufacturing variation at the stage of making the IC. Can be. That is, in the variable attenuator of the present invention, when the attenuation of the variable attenuator main body fluctuates due to temperature drift or process conditions, the reference voltage generation circuit changes the reference voltage in a direction to suppress the fluctuation, so that the fluctuation is compensated. can do.

Here, the fact that the FETs have the same structure is as follows.
It is formed under the same process conditions on the same substrate and has a common size, concentration, film thickness, and model.

[0013] The variable attenuator of the present invention may use an enhancement type transistor.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a variable attenuator according to an embodiment of the present invention will be described with reference to the drawings. In each embodiment, an example in which an enhancement-type FET using a compound semiconductor (GaAs) is employed as the FET will be described. However, it is not limited to this example.

FIG. 1 is a circuit diagram showing a variable attenuator according to a first embodiment of the present invention. The variable attenuator includes a variable attenuator main body 1 and a reference voltage generation circuit 2.

The variable attenuator main body 1 includes an FET Q1, a resistor R
1 to R3 and capacitors C1 and C2.
The drain of the FET Q1 is connected to the input terminal IN via the capacitor C1, and the reference voltage V is connected via the resistor R2.
ref is connected to a reference voltage line that supplies ref.
Is connected to the output terminal OUT via the capacitor C2 and to the reference voltage Vref via the resistor R3. The control voltage Vcnt is input to the gate of the FET Q1 via the resistor R1.

The reference voltage generating circuit 2 comprises an FET Q2 and resistors R4 and R5. The drain of the FET Q2 is
It is connected to the gate via the resistor R4 and directly to the power supply line VR. The source of the FET Q2 is a resistor R5
And outputs a reference voltage Vref at a connection node with the resistor R5.

Here, in order to facilitate understanding, prior to the description of the function of the variable attenuator main body 1, electric characteristics of a general enhancement type FET will be described. FIG.
Enhancement type FET with gate-source voltage V
FIG. 9 is a characteristic diagram of the on-resistance Ron when gs is changed. When the gate-source voltage Vgs is increased from zero, the resistance of the on-resistance Ron, which has been high, rapidly decreases from around the threshold voltage Vth, and thereafter converges to a constant value. Since the threshold voltage Vth of an FET generally tends to decrease as the temperature rises, the ON-resistance Ron characteristic shifts to the left.

The variable attenuator main body 1 forms a series type attenuator, and includes a reference voltage Vref and a control voltage Vc.
The on-resistance Ron of the FET Q1 is determined by the gate-source voltage Vgs of the FET Q1 determined by nt.
The variable attenuator body 1 attenuates the signal input from the input terminal IN with the on-resistance Ron and outputs the signal from the output terminal OUT.

FIG. 4 is a characteristic diagram of the amount of attenuation B when the reference voltage Vref is fixed and the control voltage Vcnt is changed in the variable attenuator body 1 of FIG. As the control voltage Vcnt increases, the attenuation B of the variable attenuator body 1 decreases uniformly to a certain stage, and thereafter converges to a constant value. Further, the characteristic of the attenuation B is such that the FET Q
As the threshold voltage Vth of 1 decreases, it shifts to the left.

The operation of a general enhancement-type FET in which the gate and the drain are short-circuited will now be described with reference to FIG. FIG. 3A shows a measurement circuit for measuring characteristics of an enhancement-type FET, in which a gate and a drain are connected by a resistor, and a drain-source voltage Vd is changed to measure a drain current Id. . The resistor is connected to protect the gate oxide film of the FET from static electricity or overvoltage. FIG. 3B is a characteristic diagram showing the measurement results. In the operation of the measurement circuit, when the voltage Vd is increased from zero, the initial increase in Id is small, but rapidly increases near the threshold voltage Vth. This operating characteristic shows a constant voltage characteristic similar to the forward characteristic of the diode, and shifts to the left as the temperature rises.

The reference voltage generation circuit 2 outputs a reference voltage Vref using a source potential constituting a source follower in accordance with the operation characteristics of the circuit shown in FIG. Therefore, the voltage characteristic shifts to the left according to the characteristic change of the reference voltage Vref due to the temperature rise.

In the variable attenuator of this embodiment, when the control voltage Vcnt is fixed and the variation of the attenuation amount B of the variable attenuator main body 1 is observed, as described above, the variable attenuator decreases as the temperature rises. The reference voltage Vref of the reference voltage generation circuit 2 increases, and acts to reduce the gate-source voltage Vgs of the FET Q1. Therefore, the reference voltage generation circuit 2 generates the reference voltage Vref so as to cancel the temperature drift of the variable attenuator body 1.

If the FET Q1 of the variable attenuator main body 1 and the FET Q2 of the reference voltage generating circuit 2 are formed into an IC on the same chip by a similar process, the fluctuation of the threshold voltage Vth due to temperature drift and manufacturing variation. Occurs in both FETs Q1 and Q2 in the same manner, so that both can be canceled.

According to the above embodiment, the monolithic I
By forming the variable attenuator in the same chip as in C, it is possible to compensate for both the temperature drift of the variable attenuator and the variation due to manufacturing variations.

FIG. 6 is a circuit diagram showing a variable attenuator according to a second embodiment of the present invention. The variable attenuator of the present embodiment differs from the first embodiment in that the variable attenuator body 1 has a shunt-type attenuator configuration and the reference voltage generating circuit 2 has a grounded source configuration.

The variable attenuator main body 1 includes an FET Q3, a resistor R
6 to R7 and capacitors C4 to C6.
The drain of the FET Q3 is connected to the input terminal IN via the capacitor C4 and to the output terminal O via the capacitor C5.
Connect to UT. The source of the FET Q3 inputs the control voltage Vcnt via the resistor R6, and is connected to GND via the capacitor C6. The reference voltage Vref is input to the gate of the FET Q3 via the resistor R7.

The reference voltage generating circuit 2 comprises an FET Q4 and resistors R9 and R10. The gate of the FET Q4 is
Connected to the drain via the resistor R10, and the source of the FET Q4 is connected to GND. The drain of the FET Q4 is
It is connected to the power supply line VR via the resistor R9, and outputs a reference voltage Vref to a connection node with the resistor R9.

FIG. 5 is a characteristic diagram of the amount of attenuation B when the reference voltage Vref is fixed and the control voltage Vcnt is changed in the variable attenuator body 1 of FIG. Variable attenuator body 1
Is basically the same as that of the first embodiment,
The characteristic of the amount of attenuation B shifts to the right as the temperature rises.

Since the reference voltage generating circuit 2 outputs the reference voltage Vref using the drain potential forming the common source, the characteristic of the reference voltage Vref shifts to the right due to a rise in temperature.

That is, in the variable attenuator of the present embodiment, the amount of attenuation B of the variable attenuator body 1 increases due to a rise in temperature, but this is compensated for by reducing the reference voltage Vref of the reference voltage generating circuit 2. ing.

The shunt-type attenuator employed in this embodiment can be designed to have a desired attenuation characteristic if designed based on the filter theory.

Although the present invention has been described based on the preferred embodiment, the variable attenuator of the present invention is not limited to the configuration of the above-described embodiment, but the configuration of the above-described embodiment. The variable attenuator obtained by making various modifications and alterations from the above is also included in the scope of the present invention.

[0034]

As described above, according to the variable attenuator of the present invention, the FE of the variable attenuator main body is added to the reference voltage generating circuit.
By using the FET having the same structure as T, it is possible to compensate for the variation of the attenuator due to both the temperature drift and the manufacturing variation at the manufacturing stage.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a variable attenuator according to a first embodiment of the present invention.

FIG. 2 is a graph showing Vgs vs. Ron characteristics of an enhancement type FET.

FIG. 3 is a Vd vs. Id characteristic diagram of an enhancement type FET.

FIG. 4 is a Vcnt vs. B characteristic diagram of the variable attenuator main body 1 of FIG.

FIG. 5 is a Vcnt vs. B characteristic diagram of the variable attenuator main body 1 of FIG.

FIG. 6 is a circuit diagram showing a variable attenuator according to a second embodiment of the present invention.

FIG. 7 is a circuit diagram showing a conventional variable attenuator.

FIG. 8 shows a light emitting element driving circuit described in Japanese Patent Application Laid-Open No. 62-27378.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 Variable attenuator main body 2 Reference voltage generation circuit Q1 to Q4 FET Di Temperature compensation diode R1 to R14 Resistance C1 to C7 Capacitor VR External power supply Vref Reference voltage Vcnt Control voltage IN Input terminal OUT Output terminal Qg1, Qg2, Q6 GaAs FET Qg3 Si Transistor LD Semiconductor laser

Claims (5)

(57) [Claims] A gate connected to a control terminal via a first resistor; a drain connected to an input terminal via a first capacitance; and a drain connected to a reference voltage source via a second resistor. A variable attenuator body having a first FET connected to the reference voltage source via a third resistor and connected to an output terminal via a second capacitance;
A variable attenuator having a reference voltage generator that outputs the reference voltage source, wherein the reference voltage generator has a drain connected to a first power supply, and a gate and a drain connected via a fourth resistor. A second FET having a source connected to a second power supply via a fifth resistor and having the same structure as the first FET, and outputting the reference voltage source from a source of the second FET. A variable attenuator characterized in that: 2. The variable attenuator according to claim 1, wherein said first and second FETs are enhancement junction transistors. A gate connected to the reference voltage source via the first resistor, a drain connected to the input terminal via the first capacitance, and a drain connected to the output terminal via the second capacitance; Is connected to the control terminal via a second resistor and the first terminal via a third capacitance.
A variable attenuator body having a first FET connected to a power supply and a reference voltage generation circuit for outputting the reference voltage source, wherein the reference voltage generation circuit has a drain connected to a second power supply. A second FET having the same structure as the first FET, wherein a gate and a drain are connected via a third resistor, and a source is connected to the first power supply. F
A variable attenuator for outputting the reference voltage source from a drain of ET. 4. The variable attenuator according to claim 3, wherein said first and second FETs are enhancement type junction transistors. 5. A variable attenuator comprising a first junction type transistor functioning as a variable resistor, to which a control signal is input, an input signal input from an input terminal is attenuated based on the control signal and output from an output terminal. In one embodiment, one of the power supplies of the variable attenuator is output as a source or drain voltage of a second junction transistor having the same structure as the first junction transistor, and a threshold voltage of the first junction transistor and A variable attenuator characterized in that a voltage varies with the same temperature characteristic.