Linear variable gain amplifiers

An amplifier system for providing an output which amplifies the difference between a first input and a second input, the amplifier system comprising:

• a first transistor which is coupled to the first input and which provides current which is proportional to the first input;
• a second transistor which is coupled to the second input and provides current which is proportional to the second input;
• a common mode transistor coupled to the first and second transistors which receives the current from the first transistor and the second transistor and provides the amplifier system with a gain which is proportional to bias current flowing through the common mode transistor;
• and
• a source of current including a first field effect transistor which is selectively coupled to the first transistor and a second field effect transistor which is selectively coupled to the second transistor for providing additional current to improve the gain of the amplifier by providing a greater range of current levels before the first transistor and second transistor saturate, with each of the first and second field effect transistors being driven by a third field effect transistor to selectively apply current to the first and second transistors.

A method of controlling the operating range of a common-mode amplifier system having a common mode transistor coupled to a first transistor and a second transistor, each coupled to an input signal the method comprising:

• providing a bias current through the common mode transistor which determines the gain of the amplifier system;
• providing a first source of current through the common mode transistors through the first and second transistors coupled to the first and second input signals;
• and
• selectively coupling a first supplemental source of current to the first transistor and a second supplemental source of current to the second transistor, where said first and second supplemental sources of current are independent of the first and second input signals, to selectively increase the bias current through the common mode transistor without saturating the first and second transistors.

A method of controlling the operating range of a common-mode amplifier system having a common mode transistor coupled to a first transistor and a second transistor, each coupled to an input signal, the method comprising:

• providing a bias current through the common mode transistor which determines the gain of the amplifier system;
• providing a first source of current through the common mode transistors through the first and second transistors coupled to the first and second input signals;
• providing a second source of current independent of the first and second input signals to selectively increase the bias current through the common mode transistor without saturating the first and second input transistors;
• and
• reducing the capacitance seen at the output of the amplifier, said step of reducing the capacitance including the step of isolating the output from the second current source.

A linear variable gain amplifier system for providing an output which amplifies the difference between a first input and a second input, the amplifier system comprising:

• a first transistor which is coupled to the first input and which provides current which is proportional to the first input;
• a second transistor which is coupled to the second input and provides current which is proportional to the:
• second input;
• a common mode transistor coupled to the first and second transistors to receive the current from the first transistor and the second transistor providing the amplifier system with a gain which is proportional to bias current flowing through the common mode transistor;
• and
• a first field effect transistor selectively coupled to the first transistor and a second field effect transistor selectively coupled to the second transistor for providing additional current to improve the gain of the amplifier by providing a greater range of current levels before the first transistor and second transistor saturate, said first field effect transistor and said second field effect transistor are driven by a current source which selectively applies current to the first and second transistors.