Operational amplifier having selectable inputs

Claim 1

An operational amplifier comprising: an output; a first amplification stage including first and second voltage sensing inputs, a common input and an output, said first amplification stage comprising first and second amplification substages, each said substage including a first input, a second input and an output wherein said second substage inputs are coupled together to form said common input, wherein said first substage first input comprises said first amplification stage first voltage sensing input, and wherein said second substage first input comprises said first amplification stage second voltage sensing input; a second amplification stage including an input and an output, said second amplification stage output being coupled to said output; switch means disposed between said first amplification stage output and said second amplification stage input for selectively coupling said first substage output to said second amplification stage input or said second substage output to said second amplification stage input, said switch means comprising first and second field-effect transistors wherein said first field-effect transistor is coupled between said first substage output and said second amplification stage input and wherein said second field-effect transistor is coupled between and second substage output and said second amplification stage input; a control input and an inverting means, said control input being coupled directly to the gate of said first field-effect transistor and said control input being coupled to the gate of said second field-effect transistor through said inverting means; and a third amplification stage having an input and an output, said third amplification stage input being coupled to said second amplification stage output, and said third amplification stage output comprising the output of said operational amplifier and being coupled to said first amplification stage common input.