Patent ID: 7340229
Filing Date: 2008-03-04
Classification: H03G

Abstract:
1. A high frequency amplification circuit having a variable gain, comprising: a terminal group including a signal input terminal to which a high frequency signal to be amplified is to be input, a signal output terminal for outputting the amplified high frequency signal, a gain control terminal to which a control voltage is to be applied, and a reference voltage terminal to which a reference voltage is to be applied; an amplifier provided between the signal input terminal and the signal output terminal for amplifying the input high frequency signal; a gain control circuit, provided between the signal input terminal and the signal output terminal and connected in series to the amplifier, for changing an attenuation of the input high frequency signal in accordance with the control voltage applied to the gain control terminal; and a reference voltage circuit for generating an internal reference voltage from the reference voltage applied to the reference voltage terminal and supplying the generated internal reference voltage to the gain control circuit; wherein: the gain control circuit includes at least one first field effect transistor for receiving the control voltage at a gate terminal thereof, the gain control circuit having a resistance value which is changed in accordance with the given control voltage; the reference voltage circuit includes a second field effect transistor having a threshold voltage which is substantially equal to that of the at least one first field effect transistor, the second field effect transistor being provided for generating the internal reference voltage which is shifted with respect to the reference voltage by the threshold voltage; the gain control circuit continuously changes the attenuation of the input high frequency signal by the resistance value of the at least one first field effect transistor being continuously changed; and the reference voltage circuit supplies the internal reference voltage to a source terminal and/or a drain terminal of the at least one first field effect transistor, thereby counteracting an influence exerted by the threshold voltage of the at least one first field effect transistor on a change in the attenuation of the high frequency signal in the gain control circuit.