Common mode rejection means for differential circuits

Claim 1

Electronic circuit apparatus for amplifying the difference between first and second input signals with high common mode rejection over a wide frequency range, said first and second input signals being representable as (A + CM) and (B + CM), where (A - B) is the differential signaL which it is desired be amplified by said apparatus, and where (CM) is the common mode signal contained in each of the input signals which it is desired be rejected by said apparatus, said apparatus comprising: a common mode cancellation circuit to which said input signals are applied for producing first and second common mode cancellation circuit output signals having a difference proportional to (A - B) and with no common mode signal (CM) being contained in either of said first and second cancellation circuit output signals, and a differential amplifier to which said first and second cancellation circuit output signals are applied for providing differential amplification thereof, said common mode cancellation circuit including a voltage divider to which said input signals are applied for producing a composite signal at a junction of said voltage divider containing predetermined porportions of said first and second input signals, said common mode cancellation circuit also including an inverting ampliifer coupled to said junction and responsive to said composite signal for producing a cancellation signal of the form (-K1A/2 - K2B/2 - CM) where K1 and K2 are constants, and said common mode cancellation circuit further including summing means to which said first and second input signals and said cancellation signal are applied for algebraically summing each of first and second input signals with said cancellation signal to produce said first and second cancellation circuit output signals.