is zero. In the circuit shown, common-mode gain is caused by mismatch in the resistor ratios and by the mismatch in common-mode gains of the two input op-amps. Obtaining very closely matched resistors is a significant difficulty in fabricating these circuits, as is optimizing the common-mode performance. An instrumentation amplifier can also be built with two op-amps to save on cost, but the gain must be higher than two (+6 dB). Instrumentation amplifiers can be built with individual op-amps and precision resistors, but are also available in integrated circuit form from several manufacturers (including Texas Instruments, Analog Devices, Linear Technology and Maxim Integrated Products). An IC instrumentation amplifier typically contains closely matched laser-trimmed resistors, and therefore offers excellent common-mode rejection. Examples include INA128, AD8221, LT1167 and MAX4194. Instrumentation amplifiers can also be designed using "indirect current-feedback architecture", which extend the operating range of these amplifiers to the negative power supply rail, and in some cases the positive power supply rail. This can be particularly useful in single-supply systems, where the negative power rail is simply the circuit ground (GND). Examples of parts utilizing this architecture are MAX4208/MAX4209 and AD8129/AD8130. Types Feedback-free instrumentation amplifier Feedback-free instrumentation amplifier is the high-input-impedance differential amplifier designed without the external feedback network. This allows reduction in the number of amplifiers (one instead of three), reduced noise (no thermal noise is brought on by the feedback resistors) and increased bandwidth (no frequency compensation is needed). Chopper-stabilized (or zero-drift) instrumentation amplifiers such as the LTC2053 use a switching-input frontend to eliminate DC offset errors and drift. See also Isolation amplifier Operational amplifier applications References External links Interactive analysis of the Instrumentation Amplifier Opamp Instrumentation Amplifier Lessons In Electric Circuits — Volume III — The instrumentation amplifier A Practical Review of Common Mode and Instrumentation Amplifiers Instrumentation Amplifier Solutions, Circuits and Applications Fixed-gain CMOS differential amplifiers with no external feedback for a wide temperature range (Cryogenics) Electronic amplifiers