1. Field of the Invention
This invention relates to an attenuator circuit for electrically varying the amount of attenuation of an input signal.
2. Related Art
In general, an attenuator circuit used for a transmitter on the side of the head end (or antenna site) in a two-way community antenna television (CATV) or for a small signal transmitting module such as a communication unit is a .pi. type resistance attenuator which, as shown in FIG. 4(a), is made up of resistors R1, R2 and R3, or a circuit as shown in FIG. 4(b) in which a signal detector circuit 3 detects the output signal of an amplifier 2, and an automatic gain control (AGC) amplifier 1 controls the output gain according to the detection output of the signal detector circuit 3.
In the case of the circuit shown in FIG. 4(a), the amount of attenuation is fixed. Hence, the circuit suffers from a problem that the amount of attenuation cannot be changed as desired.
In the case of the circuit shown in FIG. 4(b), the amount of attenuation can be changed. However, the circuit is still disadvantageous in that it is high in manufacturing cost because it needs the signal detector circuit made up of a number of circuit elements.
Accordingly, an object of this invention is to eliminate the above-described difficulties accompanying a conventional attenuator circuit. More specifically, an object of the invention is to provide an attenuator circuit which is able to vary the amount of attenuation, and which is low in manufacturing cost.
FIG. 7 shows another example of the attenuator circuit of this type, which is made up of a plurality of .pi. type resistance circuits each comprising resistors R1, R2 and R3, and transistor-transistor logic (TTL) drive circuits for controlling the resistance circuits, respectively. For instance, when it is required to attenuate a signal as much as 30 dB in steps of 1 dB, the attenuator circuit should be so designed that the attenuation is made in five steps, 1 dB, 2 dB, 4 dB, 8 dB and 15 dB, as shown in FIG. 7. The attenuator circuit is high in manufacturing cost.