Accurate, wide-band, low-noise variable-gain amplifier structures and gain control methods

An amplifier which provides a signal gain between a differential input signal and a differential output signal, the amplifier comprising:

• a signal differential amplifier that has first and second signal loads and a signal current source that provides a signal current wherein said signal differential amplifier steers said signal current to said first and second signal loads in response to said differential input signal;
• a bias differential amplifier that has a bias differential input port, first and second bias loads and a bias current source that provides a bias current wherein one of said bias loads is coupled to one side of said bias differential input port and said bias differential amplifier steers said bias current to said first and second bias loads in response to a gain signal at said bias differential input port;
• a gain current source that provides a gain current;
• a gain load positioned across said bias differential input port to receive said gain current and provide said gain signal;
• and
• an error amplifier coupled to respond to a difference signal between said first and second bias loads and, in response, adjust said signal and bias currents to reduce said difference signal;
• said differential output signal thereby generated across said first and second signal loads with a signal gain controlled by a ratio of one of said bias loads to said gain load.

An amplifier which provides a signal gain between a differential input signal and a differential output signal that varies with a digital code, the amplifier comprising:

• a signal differential amplifier that has first and second signal loads and a signal current source that provides a signal current wherein said signal differential amplifier steers said signal current to said first and second signal loads in response to said differential input signal;
• a bias differential amplifier that has a bias differential input port, first and second bias loads and a bias current source that provides a bias current wherein said bias differential amplifier steers said bias current to said first and second bias loads in response to gain signals at said differential input port;
• a digital-to-analog converter that converts said digital code to a gain current;
• a gain resistor positioned across said bias differential input port to receive said gain current and provide said gain signals;
• a control current source that drives a control current through one of said first and second bias loads;
• and
• an error amplifier coupled to respond to a difference signal between said first and second bias loads and, in response, adjust said signal and bias currents to reduce said difference signal;
• said differential output signal thereby generated across said first and second signal loads with said signal gain controlled by said digital code.

A method of controlling a signal gain between a differential input signal and a differential output signal, the method comprising the steps of:

• providing a signal differential amplifier that steers a signal current between first and second signal loads in response to said differential input signal;
• providing a bias differential amplifier that has a bias differential input port and steers a bias current between first and second bias loads in response to a gain signal at said bias differential input port wherein one of said bias loads is coupled to one side of said bias differential input port;
• providing a gain current;
• receiving said gain current with a gain load positioned across said bias differential input port to generate a gain signal;
• adjusting said bias current by a different current that reduces a difference signal between said first and second bias loads;
• and
• adjusting said signal current by substantially said difference current;
• said differential output signal thereby generated across said first and second signal loads with said signal gain controlled by said gain signal.

A method of controlling a signal gain between a differential input signal and a differential output signal in response to a gain signal, the method comprising the steps of:

• providing a signal differential amplifier that steers a signal current between first and second signal loads in response to said differential input signal;
• providing a bias differential amplifier that steers a bias current between first and second bias loads in response to said gain signal;
• driving a control current through one of said first and second bias loads,
• adjusting said bias current by a different current that reduces a difference signal between said first and second bias loads;
• and
• adjusting said signal current by substantially said difference current;
• said differential output signal thereby generated across said first and second signal loads with said signal gain controlled by said gain signal.