Variable gain amplifier having linear-in-dB gain characteristic

1. A variable gain amplifier (VGA) with a linear-in-dB gain, the VGA comprising:

a control signal converter which converts an input gain control signal V C , which is input so that the VGA has a linear-in-dB gain characteristic to the maximum gain, into an output gain control signal V x =V T ln((1/m)exp(−V C /V T )−1) (m is a constant, V T =kT/q); and

a variable gain amplifier which receives and converts the output gain control signal V X output from the control signal converter so that the gain has a linear-in-dB characteristic,

wherein a shape of a gain curve is externally controlled.

2. The VGA of claim 1 , wherein the control signal converter comprises:

a first bipolar transistor which includes a base connected to a (−) terminal of the input gain control signal and an emitter connected to a current source connected to the ground;

a second bipolar transistor which includes a base connected to a (+) terminal of the input gain control signal and an emitter connected to the current source connected to the ground;

a first MOS transistor which includes gate and drain commonly connected to a collector of the first bipolar transistor and a source connected to a power source;

a first gain control transistor which includes second and third terminals commonly connected to a collector of the second bipolar transistor and a first terminal connected to the power source;

a first differential amplifier that is constructed with the first bipolar transistor, the second bipolar transistor, the first MOS transistor, and the first gain controlled transistor, in which the base of the first bipolar transistor is applied with the (−) terminal of the input gain control signal, the base of the second bipolar transistor is applied with the (+) terminal of the input gain control signal, the emitters of the first and second bipolar transistors are connected to each other and connected to the current source connected to the ground;

a second MOS transistor which includes a gate connected to the gate of the first MOS transistor and a source connected to the power source;

a third MOS transistor which includes drain and gate commonly connected to the drain of the second MOS transistor and a source connected to the ground;

a fourth MOS transistor which includes a gate connected to the gate of the third MOS transistor and a source connected to the ground;

a fifth MOS transistor which includes a gate connected to the second input terminal of the first gain control transistor and a source connected to the power source;

a sixth MOS transistor which includes gate and drain commonly connected to the drain of the fifth MOS transistor and a grounded source;

a seventh MOS transistor which includes a gate connected to the gate of the sixth MOS transistor and a source is connected to the ground;

an eighth MOS transistor which includes a gate connected to the second terminal of the first gain control transistor, a source connected to the power source, and a drain connected to the fourth MOS transistor;

a third bipolar transistor which includes a collector connected to the drain of the eighth MOS transistor and the drain of the fourth MOS transistor; and

a fourth bipolar transistor which includes a collector connected to the drain of the seventh MOS transistor,

wherein the emitters and bases of the third and fourth bipolar transistors are connected to the power source, the collector of the third bipolar transistor is the (+) terminal of the output gain control signal, the collector of the fourth bipolar transistor is the (−) terminal of the output gain control signal, and accordingly, the gain of the current-controlled VGA is controlled.

3. The VGA of claim 1 , wherein the current-controlled VGA comprises:

a fifth bipolar transistor which includes a first terminal connected to the power source, a base connected to the (−) terminal of the output gain control signal, and a second terminal connected to a signal input terminal (+);

a sixth bipolar transistor which includes a first terminal connected to a signal output terminal (+), a base connected to the (+) terminal of the output gain control signal, and a second terminal connected to the signal input terminal (+);

a seventh bipolar transistor which includes a first terminal connected to the signal output terminal (−), a base connected to the (+) terminal of the output gain control signal, and a second terminal connected to the signal input terminal (−); and

an eighth bipolar transistor which includes a first terminal connected to the power source, a base connected to the (−) terminal of the output gain control signal, and a second terminal connected to the signal input terminal (−).

4. The VGA of claim 2 , wherein the first gain control transistor comprises:

first terminals constructed with sources of commonly connected PMOS transistors;

second terminals constructed with gates of the commonly connected PMOS transistors;

third terminals constructed with drains of the commonly connected PMOS transistors; and

switches which include first terminals connected to the third terminals of the first gain control transistor and second terminals connected to one another,

wherein the switches are externally controllable so as to be opened or closed.

5. The VGA of claim 4 , wherein the first terminals of the switches are sources of PMOS transistors, the second terminals of the switches are drains of the PMOS transistors, and the switches are easily controlled by adjusting voltages of gates of the PMOS transistors so as to be opened or closed.