CMOS Operational Amplifiers

Performance parameters

• Gain :The open-loop gain of an op-amp determines the precision of the

feedback system employing the op-amp. Trading with such parameters as speed and output voltage swings, the minimum required gain must therefore be known. A high open-loop gain may also be necessary to suppress non-linearity.

• Small-signal Bandwidth :The high frequency behaviour of op-amps plays a critical role in

many applications. The fall of open-loop gain with frequency creates large errors in the feedback system. The unity-gain frequency fu specifies the small-signal bandwidth, which exceeds 1 GHz in today’s CMOS op-amps.

• Large-signal bandwidth :When op-amps operate with large-signal conditions, the non-

linear phenomena make it difficult to characterize the speed by merely small-signal properties such as open-loop response.

Gain boosting

Power supply rejection

Supply rejection of differential pair with active current mirrorPower Supply Rejection Ratio (PSRR) is defined as the gain from the input to the output divided by the gain from the supply to the output. At low frequencies, PSRR ≈ gmN( roP ǁ roN)

Comparison