1. Field of the Invention
This invention relates to an attenuator circuit apparatus, and more particularly to an improvement of step attenuator circuit for attenuating in stages a signal such as a high frequency signal of a micro-wave band.
2. Description of the Related Art
Up to the present time, various step attenuators have been proposed to obtain an attenuated signal of plural bits with respect to a high frequency signal. Especially, when it is applied to a consumer appliance (for instance, a portable telephone), an essential condition is that it can be manufactured at a low price and in a small size. To reach these objectives an MMIC (Monolithic Microwave IC) attenuator which utilizes a GaAs FET (field effect transistor) is being developed.
These step attenuators can be divided roughly into two categories. One of them is the step attenuator in which the plural stages of the .pi.-type attenuator circuit are connected in series with respect to the signal path as shown in FIG. 1 ("FAST GaAs MMIC ATTENUATOR HAS 5-b RESOLUTION", MICROWAVE & RF OCTOBER 1991 pp.71-76). The other is the step attenuator in which the plural stages of the T-type attenuator circuit are connected in series with respect to the signal path as shown in FIG. 2 ("DC-12[GHz] 4-Bit GaAs Monolithic Digital Attenuator", Applied Microwave Winter 91/92 pp.60-67).
Thus, the step attenuator is usually composed of the .pi.-type attenuator circuits or the T-type attenuator circuits. In the case of .pi.-type attenuator circuit, there is a disadvantage that the insertion loss of it is larger in some degree (4 dB or more, at DC-10 GHz). This is mainly due to the increasing of the insertion loss which is caused by the electric power which leaks from these FETs, in view of the fact that there are two FETs connected in the shunt position with respect to the signal path. Therefore, this type is not suited for the realization of a step attenuator of plural bits, which requires a lower insertion loss.
On the other hand, in the case of the T-type attenuator circuit, the number of the FET which is connected in the shunt position with respect to the signal path is one. It is less than the number of the case of the .pi.-type attenuator circuit by one, therefore, the insertion loss can be restrained to a considerably small value (2 dB or less, at DC-2 GHz). Consequently, this type is suited for the realization of the step attenuator of plural bits, which requires the lower insertion loss.
However, in the case of the T-type attenuator circuit, there was a problem that the more the attenuation quantity increases the more the attenuation quantity is affected by the dispersion of the element precision. That is, if an attempt was made to attenuate the high frequency signal which is inputted to the 50 .OMEGA. terminated signal path by 16 dB with the T-type step attenuator 5, which is comprised of the three stages connected, the resistance value of the resistor r6a, which is connected in the shunt position with respect to the signal path, of the resistors composing the T-type attenuator circuit 5C which is connected to the third stage becomes a very little value, that is about 10 .OMEGA..
Therefore, when the ON resistance value of the FET16 located in the shunt position or the resistance value of the resistor r6a has fluctuated for some reason, the influence caused by the fluctuated quantity becomes large. For instance, when the resistance value has fluctuated by only 2 to 3 .OMEGA., this fluctuated quantity corresponds to 20 to 30% toward the resistance value of the resistor r6a. Hence, the attenuation quantity fluctuates extensively.
On the other hand, there is a limit to control very precisely the resistor and the FET which are located in the shunt position. Consequently, it is very difficult, in effect, to obtain a large attenuation quantity with a satisfactory precision by the T-type attenuator circuit.