Patent ID: 7705680

Claim:
A gain switching low-noise amplifier circuit, comprising: an input terminal of an RF signal; a transistor C having a base to which a signal from the input terminal is inputted; a first bias circuit connected to the base of the transistor C via a resistor and turning ON the transistor C; a first transistor group including transistors A 1 to AN (where N is a natural number of 2 or more) and a second transistor group including transistors B 1 to BN, the transistors having same size, same shape, and same characteristics; a first capacitor and a second capacitor connected to each base of the transistors of the first transistor group and the second transistor group, respectively, and short-circuited to ground at a frequency band of the RF signal in an AC manner; a first resistor and a second resistor connected to each base of the transistors of the first transistor group and the second resistor group, respectively; a second bias circuit which turns ON the first transistor group and the second transistor group via the first resistor and the second resistor, respectively; an inductor connected between collectors of N pieces of transistors from A 1 to AN of the first transistor group and X pieces of transistors from B 1 to BX (wherein X is a natural number smaller than N) of the second transistor group and a DC power source; a first switch which short-circuits between bases and emitters of the transistors of the first transistor group via the first resistor or connects the first transistor group to the second bias circuit; a second switch which short-circuits between bases and emitters of the transistors of the second transistor group via the second resistor or connects the second transistor group to the second bias circuit; a gain switching control terminal which exclusively turns ON/OFF the first switch and the second switch; and an output terminal connected to collectors of the transistors of the first transistor group, wherein collectors of N−X pieces of transistors from BX+1 to BN of the second transistor group are connected to the DC power source, wherein emitters of the transistors A 1 to AN of the first transistor group are short-circuited and connected to a collector of the transistor C, wherein emitters of X pieces of transistors from B 1 to BX of the second transistor group are short-circuited and connected, and wherein emitters of N−X pieces of transistors from BX+1 to BN of the second transistor group are short-circuited and connected, the gain switching low-noise amplifier circuit further comprising: a resistor B 1 connected between emitters of X pieces of transistors from B 1 to BX of the second transistor group and the collector of the transistor C; and a resistor B 2 connected between emitters of N−X pieces of transistors from BX+1 to BN of the second transistor group and the collector of the transistor C and having a resistance X/(N−X) times as high as that of the resistor B 1 .