Differential output driver circuit and method for same

In a circuit having inputs, a method for amplifying differential signals, the method comprising:

• providing a final stage differential amplifier including an emitter-coupled transistor pair;
• providing a cascode including a pair of transistors cascoded with the final stage emitter-coupled transistors;
• receiving a pair of differential input signals at a corresponding pair of circuit inputs of the final stage differential amplifier including receiving a varying voltage input signal at each of the bases of the final stage emitter-coupled transistors;
• differentially amplifying the input signals;
• simultaneously with the amplification of the differential input signals, eliminating changes in the capacitance of the circuit inputs responsive to the amplification of the differential input signals, including eliminating the Miller-effect capacitance at the bases of the final stage emitter-coupled transistors as the differential input signals vary in voltage by, with the cascode transistor pair, maintaining a constant voltage at the collectors of the final stage emitter-coupled transistors and bleeding current from the collectors of the final stage emitter-coupled transistors;
• and
• providing a pair of differential output signals which are amplified replicas of the corresponding differential input signals.

An output driver circuit comprising:

• a final stage means for differentially amplifying a pair of input signals to generate a pair of output signals;
• a pre-driver stage means for differentially amplifying a pair of pre-driver input signals, to provide the input signals to the final stage differential amplifying means;
• a means for generating an output impedance of the final stage amplifying means;
• and
• a means for buffering the output signals of the final stage differential amplifying means from changes to the input signals from the output impedance of the final stage amplifying means;
• in which the final stage differential amplifying means includes a first and second emitter-coupled transistor;
• in which the output impedance means includes a first and second load resistor;
• in which the buffering means includes a pair of cascoded transistors, with a first cascode transistor having a collector connected to the first load resistor and an emitter connected to the collector of the final stage differential amplifier first transistor, and a second cascode transistor having a collector connected to the second load resistor and an emitter connected to the collector of the final stage differential amplifier second transistor;
• and
• in which the buffering means includes a first means for bleeding current connected to the collector of the final stage first transistor, and a second means for bleeding current connected to toe collector of the final stage second transistor.

An output driver circuit comprising:

• a final stage differential amplifier including emitter-coupled first and second transistors, a pair of differential outputs, and a pair of differential inputs;
• a pre-driver stage differential amplifier including emitter-coupled first and second transistors and a pair of differential outputs connected to the differential inputs of the final stage differential amplifier;
• a cascode including a pair of cascoded transistors connected to the final stage differential amplifier;
• and
• a first and second load resistor, the first load resistor having a first input connected to a final stage differential amplifier first transistor collector through the cascode, and the second load resistor having a first input connected to a final stage differential amplifier second transistor collector through the cascode;
• wherein a first cascode transistor has a collector connected to the first load resistor first input and an emitter connected to the collector of the final stage differential amplifier first transistor, a second cascode transistor has a collector connected to the second load resistor first input and an emitter connected to the collector of the final stage differential amplifier second transistor, and wherein the cascode includes a first current bleeder connected to the collector of the final stage differential amplifier first transistor, and a second current bleeder connected to the collector of the final stage differential amplifier second transistor.

A method for amplifying differential input signals in a circuit, the method comprising:

• providing a final stage differential amplifier including an emitter-coupled transistor pair;
• providing a cascode including a pair of transistors cascoded with the final stage emitter-coupled transistors;
• receiving a pair of differential input signals at a corresponding pair of circuit inputs of the final stage differential amplifier;
• differentially amplifying the input signals;
• eliminating changes in the capacitance of the circuit inputs responsive to the amplification of the differential input signals;
• bleeding current from the collectors of the final stage emitter-coupled transistors;
• and
• providing a pair of differential output signals which are amplified replicas of the corresponding differential input signals.