Operational Amplifier Fundamentals Unit 7 – The Difference Amplifier
A difference amplifier can combine dc and ac voltages. A difference amplifier exhibits
a characteristic called Common-mode rejection (CMR). CMR is the ability to cancel input
voltages that simultaneously appear at both circuit inputs.
In this circuit, the sine wave generator applies an unwanted common- mode input voltage
(VCM) to both circuit inputs. Due to the common-mode rejection of U1, the sine wave does not
appear at VO.
Common-mode rejection ratio (CMRR), usually given in dB, measures the ability to cancel
(attenuate or reduce) common-mode voltages. The amplitude of the unwanted signal appearing at
the circuit output (VOCM) equals (Av/CMRR) x VCM: CMRR must be converted from dB form
to numeric form. VOCM is the remaining common-mode input signal appearing at the amplifier
output.
NEW TERMS AND WORDS
difference amplifier - A circuit that amplifies the difference between two or more input voltages.
Common-mode rejection (CMR) - the ability of a difference amplifier to attenuate a signal
appearing simultaneously at both inputs of the circuit.
common- mode input voltage - a voltage appearing simultaneously at both inputs of an
amplifier.
Common-mode rejection ratio (CMRR) - a measure (usually in dB) of the amount of common-
mode input voltage that appears at the circuit output.