Design Of A Low-temperature-drift Low-offset Amplifier

Usually, operational amplifier （op amp） is one of the key modules in theintegrated circuit. The offset indicator and temperature of drift indicator play animportant role in the performances of the entire chip. Therefore the study on theamplifier of low temperature drift and low offset voltage is important.In this paper, an amplifier of low offset and low temperature drift which cantreat signal continuous in high supply voltage （15V¹8V） has been designed. Threeenlarged structure is used in the op amp, which makes the amplifier has a higheropen-loop gain and low-offset low-temperature-drift characteristics. PMOS is usedfor input differential pair to reduce input noise in the first stage and second stage ofthe main circuit amplification. The differential structure and folded cascode structure,which can not only decrease the offset voltage and temperature drift coefficients butalso obtain the larger gain, are used in these stages. The basic common sourcestructure is used to increase the output amplitude of the whole amp circuit in the thirdstage. Nested Miller compensation is used as compensation circuit, which makes theop amp has a good closed-loop stability. In order to reduce temperature driftcoefficient of the bias source, a bandgap reference structure of three collector tandemgrounded has been chosen to provide bias voltage for the op amp, the structure hasultra-low temperature drift coefficient and a higher accuracy of the output.The transistor-level design of the operational amplifier has been completedbased on the0.5μm high voltage CMOS process, and the low offset and lowtemperature drift operational amplifier is simulated by the Cadence. The recults showthat the open-loop gain is109dB, phase margin is79°, the effect of temperature is114nV/℃, offset voltage is11.087μV, input equivalent noise is9.037nV/（Hz）^1/2. Thechange amplitude of the output voltage of bandgap reference is1.25mV in the rangeof-55℃¹25℃, temperature coefficient is18.1ppm/℃, the power supply rejection ratio is70dB at low frequencies. So the amplifier of low offset and low temperaturedrift meets the design requirements.