Variable-gain amplifier

1. A variable-gain amplifier comprising:

n number/numbers of cascode amplifiers each including a plurality of cascode-connected field effect transistors, n being an integer not fewer than two; and

an attenuator;

wherein said n number/numbers of cascode amplifiers are mutually connected in parallel via said attenuator,

wherein a first-stage cascode amplifier among the n number/numbers of cascode amplifiers comprises a first field effect transistor among the plurality of cascode-connected field effect transistors, and

wherein an operating point of the first field effect transistor is transferred from a saturation region to a linear region during an intermediate stage of operation of the variable-gain amplifier.

2. The variable-gain amplifier according to claim 1 ,

wherein said field effect transistor is of a MOS type.

3. The variable-gain amplifier according to claim 1 ,

wherein a first cascode amplifier of said n number/numbers of cascode amplifiers is connected to an input terminal of said variable-gain amplifier;

a j-th cascode amplifier of said n number/numbers of cascode amplifiers is connected to said input terminal via (j-1) number/numbers of said attenuators, j being an integer not fewer than two and not greater than n.

4. The variable-gain amplifier according to claim 3 ,

wherein an n-th cascode amplifier of said n number/numbers of cascode amplifiers is an output current variable type amplifier.

5. The variable-gain amplifier according to claim 4 , further comprising a load to which an output terminal of each of said cascode amplifiers is connected;

wherein said n-th cascode amplifier includes a first and a second field effect transistor having respective sources thereof mutually connected; and

a drain of said first and said second field effect transistor is connected to said load and to a power source, respectively.

6. The variable-gain amplifier according to claim 1 ,

wherein said n is two.

7. The variable-gain amplifier according to claim 1 ,

wherein said cascode amplifier is of a differential type.

8. The variable-gain amplifier according to claim 1 , wherein the n number/numbers of cascode amplifiers comprises a second-stage cascode amplifier, and during the intermediate stage of operation of the variable-gain amplifier, current supplied to the first-stage cascode amplifier is decreasing and current supplied to the second-stage cascode amplifier is increasing.

9. The variable-gain amplifier according to claim 8 , wherein a gate bias control circuit supplies the current to the first-stage and second-stage cascode amplifiers in response to a control voltage.

10. The variable-gain amplifier according to claim 9 , wherein a range of the control voltage during the intermediate stage of operation is 1 to 1.5 Volts.