Gain variable amplifier
A gain variable amplifier according to an embodiment of the present invention includes: an amplifier circuit amplifying an input signal with a variable gain; and a gain control circuit controlling the gain of the amplifier circuit based on a gain control signal, in which the amplifier circuit includes: an amplifying element amplifying the input signal; an output element series-connected with the amplifying element and outputting a signal amplified with the amplifying element; and a bias circuit changing a potential at a node between the output element and the amplifying element based on the gain control of the gain control circuit.
1. A gain variable amplifier, comprising:
a plurality of amplifier circuits connected to a common input terminal and a common output terminal; and
a gain control circuit,
wherein each of the plurality of amplifier circuits includes:
a first amplifying element connected to the input terminal;
a first cascoded element connected to the first amplifying element;
a variable current source connected to an amplifying element; and
a bias circuit connected to the first cascoded element.
2. The gain variable amplifier according to claim 1 , wherein:
the first amplifying element comprises a first amplifying transistor,
the first cascoded element comprises a first cascoded transistor cascoded with the first amplifying transistor, and
a current-output-side terminal of the first amplifying transistor and a current-input-side terminal of the first cascoded transistor are connected mutually.
3. The gain variable amplifier according to claim 1 , wherein:
a gain of each of the plurality of amplifier circuits is different and
the gain control circuit controls the gains of the amplifier circuits by controlling a current of the variable current source.
4. The gain variable amplifier according to claim 3 , wherein the gain control circuit controls a sum current of the plurality of variable current sources included in the plurality of amplifier circuits respectively to be constant.
5. The gain variable amplifier according to claim 2 , wherein the first bias circuit is connected to a control terminal of the first cascoded transistor.
6. The gain variable amplifier according to claim 5 , wherein the first bias circuit included in at least one amplifier circuit is controlled to lower a potential at a node between the first amplifying transistor and the first cascoded transistor when the current from the variable current source is decreased by the gain control circuit.
7. The gain variable amplifier according to claim 1 , wherein the plurality of amplifier circuits include a common load element connected to the first cascoded element.
8. The gain variable amplifier according to claim 2 , wherein the amplifying circuit comprises a differential amplifier.
9. The gain variable amplifier according to claim 8 , wherein each of the plurality of amplifier circuits further includes:
a second amplifying transistor forming a pair with the first amplifying transistor;
a second cascaded transistor forming a pair with the first cascoded transistor; and
a second bias circuit forming a pair with the first bias circuit.
10. The gain variable amplifier according to claim 1 , wherein said first amplifying element of each of the plurality of amplifier circuits is biased so as to operate in a non-saturated region.