The invention relates to auto-zeroing circuitry for correcting the effects of input offset voltage in the input stage of a differential amplifier.
Several techniques have been widely used to reduce or eliminate the effects of the DC input offset voltage in the input stage of a differential amplifier. The input offset voltage is due to a mismatch in input turn-on voltages of a pair of differentially connected input transistors in the input stage of the differential amplifier. If the two input signals are short-circuited together, the output voltage of the differential amplifier will be zero only if the offset voltage is zero. If the offset voltage is not zero, then when the input terminals are shorted together the amplifier output voltage will be equal to the offset voltage multiplied by the gain of the differential amplifier.
One common technique for compensating the input offset voltage of an amplifier is to laser trim thin film load resistors of the input stage enough to cause the output voltage to be zero when the differential input voltage is zero. However, laser trimming is an expensive, time consuming procedure which is to be avoided if possible. In another technique referred to as "chopper stabilization", the connections of the input signal are repeatedly reversed between the (+) and (-) input terminals of the differential input stage by operating chopper switches in response to a chopper clock signal, and the connections of the amplifier output signal to the (+) and (-) outputs of the output stage also are alternately reversed in synchronization with the chopper clock signal. Another known offset compensation technique, shown in U.S. Pat. No. 4,933,642 (Lee), U.S. Pat. No. 4,622,521 (Redfern), and U.S. Pat. No. 3,988,689 (Ochi et al.), includes sampling the output voltage of the differential input stage of the amplifier onto a capacitor and then applying the sampled voltage on the capacitor to the inputs of a second differential stage, the outputs of which are used to compensate the output signal of the differential amplifier so as to eliminate the effect of the offset voltage. The known chopper stabilization and auto-zeroing techniques typically result in an undesirable amount of switching noise being generated at the amplifier inputs and propagating to the amplifier output after being amplified by the gain of the amplifier. There is an unmet need for a technique for adjusting various parameters of an amplifying circuit, including an input offset voltage thereof, without the need for laser trimming or other mechanical trimming or circuit components.