Amplifier circuit having a variable bandwidth

An amplifier circuit comprising:

• a differential amplifier, said differential amplifier voltage-biased at a first reference voltage and a second reference voltage, said differential amplifier including a first input terminal, a second input terminal, a first output terminal, and a second output terminal;
• a first capacitive element having a first end and a second end, said first end coupled to said second output terminal;
• a second capacitive element having a first end and a second end, said first end coupled to said first output terminal;
• a first npn transistor having a base coupled to said first output terminal, a collector coupled to said first reference voltage, and an emitter coupled to said second end of said first capacitive element;
• a second npn transistor having a base coupled to said second output terminal, a collector coupled to said first reference voltage, and an emitter coupled to said second end of said second capacitive element;
• and
• a switch control circuit coupled to said emitters of said first and second npn transistors, said switch control circuit for switching on and off said first and said second npn transistors.

An integrated circuit comprising:

• a differential amplifier further including:
• a first npn transistor and a second npn transistor, said first npn transistor having a base, collector, and an emitter, said second npn transistor having a base, collector, and an emitter, said base of said first npn transistor and said base of said second npn transistor providing an input to said amplifier circuit, said collector of said first npn transistor and said collector of said second npn transistor providing an output from said amplifier circuit;
• a first resistive element having a first end and a second end, said first end coupled to said collector of said first npn transistor, said second end coupled to a first reference voltage;
• a second resistive element having a first end and a second end, said first end coupled to a collector of said second npn transistor, said second end coupled to said first reference voltage;
• a current source having a first end and a second end, said first end coupled to said emitter of said first npn transistor and said emitter of said second npn transistor, said second end coupled to a second reference voltage;
• a first capacitive element having a first end, a second end, and a first capacitance, said first end of said first capacitive element coupled to said collector of said second npn transistor;
• a second capacitive element having a first end, a second end, and a second capacitance, said first end of said second capacitive element coupled to said collector of said first npn transistor;
• a third npn transistor having a base, a collector, and an emitter, said base coupled to said collector of said first npn transistor, said collector coupled to said first reference voltage, said emitter coupled to said second end of said first capacitive element;
• a fourth npn transistor having a base, a collector, and an emitter, said base coupled to said collector of said second npn transistor, said collector coupled to said first reference voltage, said emitter coupled to said second end of said second capacitive element;
• and
• a switch control circuit including a first controllable current source and a second controllable current source, said first controllable current source having a first end coupled to said emitter of said third npn transistor and a second end coupled to said second reference voltage, said second controllable current source having a first end coupled to said emitter of said fourth npn transistor and a second end coupled to said second reference voltage, said first and said second controllable current sources for switching on and off said third and said fourth npn transistors.

A mobile station comprising:

• a receiver;
• a transmitter including an integrated circuit, said integrated circuit further including:
• a differential amplifier, said differential amplifier voltage-biased at a first reference voltage and a second reference voltage, said differential amplifier having an input and an output, said output including a first output terminal and a second output terminal;
• a first capacitive element having a first end and a second end, said first end coupled to said second output terminal;
• a second capacitive element having a first end and a second end, said first end coupled to said first output terminal;
• a first npn transistor having a base coupled to said first output terminal, a collector coupled to said first reference voltage, and an emitter coupled to said second end of said first capacitive element;
• a second npn transistor having a base coupled to said second output terminal, a collector coupled to said first reference voltage, and an emitter coupled to said second end of said second capacitive element;
• a switch control circuit coupled to said emitters of said first and second npn transistors, said switch control circuit for effectively coupling and decoupling said first and second capacitive elements from said first and second output terminals;
• and
• a controller coupled to said receiver and said transmitter, said controller providing a control signal to said switch control circuit for effectively coupling and decoupling said first and second capacitive elements.