1. Field of the Invention
The present invention generally relates to an attenuator unit and a step attenuator, and more particularly, to an attenuator unit and a step attenuator unit which attenuate a high-frequency signal in radio equipment. The present invention is also directed to an electronic apparatus including the step attenuator.
2. Description of the Related Art
A step attenuator is an attenuator whose attenuation value can flexibly be selected at a digital value. This step attenuator is widely used for controlling transmission power of radio equipment such as a portable telephone.
FIG. 1 shows a block diagram of the radio equipment to which the step attenuator is applied. A step attenuator 6 is connected between a transmission circuit 4 and a transmission power amplifier 8. For example, when extremely large output power is transmitted from the radio equipment, the large output power may saturate a reception amplifier in radio equipment at a transmission destination, and may interfere with other radio equipment. In such a case, an attenuation value of the step attenuator 6 is selected to be large to reduce the output power of the radio equipment.
Since the step attenuator is widely used for portable apparatuses, miniaturization of the step attenuator is required. Further, for applying the step attenuator to the radio equipment, the step attenuator needs to have wide-band frequency performance.
FIG. 2 shows a typical configuration of the step attenuator. A step attenuator 20 is constructed with a plurality of (3 in this case) attenuator units 22a, 22b, 22c connected in series.
The attenuator units 22a, 22b, 22c respectively include two single-pole double-through (SPDT) switches 24a-1 and 24a-2, 24b-1 and 24b-2, 24c-1 and 24c-2, and fixed attenuators 26a, 26b, 26c. In the SPDT switches, by selecting whether passing a supplied signal through the fixed attenuator or passing the supplied signal through the other path, an attenuation value of the attenuator 22a, 22b, 22c unit can be digitally controlled.
In a typical step attenuator, when attenuation values of the fixed attenuators are properly selected, by properly switching the SPDT switches of the attenuator units, a desired attenuation value can be digitally selected. In the step attenuator 20 shown in FIG. 2, the fixed attenuator 26a of the attenuator unit 22a has an attenuation value 1 dB, the fixed attenuator 26b of the attenuator unit 22b has an attenuation value 2 dB, and the fixed attenuator 26c of the attenuator unit 22c has an attenuation value 4 dB. Therefore, a total attenuation value of the step attenuator 20 can be varied from 0 to 7 dB by a 1-dB step by switching the SPDT switches of the attenuator units.
In each attenuator unit, for the fixed attenuator, a T-type attenuator and a .pi.-type attenuator are commonly used.
FIG. 3 shows a schematic diagram of a prior-art attenuator unit using the T-type attenuator. FIG. 4 shows a schematic diagram of a prior-art attenuator unit using the .pi.-type attenuator.
An attenuator unit 30 shown in FIG. 3 includes three resistors R31, R32, R33 constituting the T-type attenuator, and two field-effect transistors (FETs) 32, 34 operating as switches. When the FET 32 is non-conductive and the FET 34 is conductive, the attenuator unit 30 operates as the T-type attenuator and generates a large attenuation value. On the other hand, when the FET 32 is conductive and the FET 34 is non-conductive, the attenuation value of the attenuator unit 30 becomes small.
An attenuator unit 40 shown in FIG. 4 includes three resistors R41, R42, R43 constituting the .pi.-type attenuator, and three FETs 42, 44, 46 operating as switches. When the FET 42 is non-conductive and the FETs 44, 46 are conductive, the attenuator unit 40 operates as the .pi.-type attenuator and generates a large attenuation value. On the other hand, when the FET 42 is conductive and the FETs 44, 46 are non-conductive, the attenuation value of the attenuator unit 40 becomes small.
In a shunt side of the attenuator unit 30 shown in FIG. 3, the FET 34 is connected, while in a shunt side of the attenuator unit 40 shown in FIG. 4, the FETs 44, 46 are connected. In this way, since the shunt side of the T-type attenuator is constructed with a single FET and is in different from the .pi.-type attenuator, the degree to which dispersion of frequency performance of resistance in a conductive condition of the FET has an influence on the attenuator unit 30 may be smaller than that in which the dispersion has an influence on the attenuator unit 40.
However, when designing the attenuator unit 30 having the T-type attenuator to generate a large attenuation value, the resistance value in the shunt side needs to be extremely small. Such an extremely-small resistance needs a wide area and makes the design complex.
On the contrary, the attenuator unit 40 having the .pi.-type attenuator can overcome the above-discussed problem, and, thus, the attenuator unit 40 is suitable for constructing the step attenuator.
However, the above-discussed prior-art step attenuator using the .pi.-type attenuator has the following problems.
Since the attenuator unit 40 has two current paths in the shunt side, two FETs are required, and, thus, it is difficult to produce the step attenuator with small size and high density. As a result, the size of the step attenuator using the .pi.-type attenuator is larger than that using the T-type attenuator.
Furthermore, the SPDT switches in the attenuator units need to be controlled to turn on and off. Namely, for the attenuator units, two signals of a control signal and an inverted control signal are required. As shown in FIG. 2, the inverted control signals can be generated by inverting the control signals in inverter circuits 28a, 28b, 28c.
FIG. 5 shows a schematic diagram of a prior-art inverter circuit. An inverter circuit 50 shown in FIG. 5 is constructed with a depletion-type FET (D-FET) 52 and an enhancement-type FET (E-FET) 54. A signal supplied to a gate of the E-FET 54 is inverted and is produced from a drain of the E-FET 54.
Since the inverter circuit 50 includes two FETs, electrical performance of the inverter circuit 50 is easily varied by dispersion in a process. Therefore, the inverter circuit 50 needs to be designed so as to absorb influences due to the dispersion.
Further, size of the two FETs is not negligible as compared with a circuit scale of the step attenuator, and, thus, the step attenuator size becomes large.