Design Of A Low Noise Operational Amplifier Using Bipolar Process

Nowadays, operational amplifier has become one of the most important unit circuit in circuit system. In high sensitivity application environment, the op amp will be impacted by some irregular voltage outside and inside easily,when noise equal to input signal,the noise will impacted output signal. the noise of the system becomes very important, thus reducing noise of op amps has considerable significance. While compared to MOS process, bipolar process has smaller offset, stronger current driving capability and better noise characteristics and so on, so most of the highest precision op amp using bipolar process.The paper introduces the design of a low-noise op amps using bipolar process. Reduced niose in four perspective, including process, device, circuit structure, circuit parameters; in order to get a low-noise, low input bias current, high gain, high load capacity,high output swing, the input stage using a resistive load differential pair and bias current cancellation circuit, the middle class using folding cascode structure, the output stage using double-buffered structure; design appropriate reference current source and a bias circuit to set the appropriate quiescent point; using a typical Miller compensation and feedforward compensation to improve the stability of the op amp; derived major parameters of op amp such as equivalent input noise voltage, input offset voltage, open-loop gain, slew rate.The design used 3 μm bipolar process of ASMC, and HSPICE software to simulate, it indicated that when the supply voltage is ±15 V, equivalent input noise voltage is 2.465 nV/ Hz at 1 kHz, input offset voltage is 0.0146 mV, open-loop gain is 131.3 dB,CMRR is 170 dB, unity gain bandwidth is 6.06 MHz, the phase margin is 51.7°, and the slew rate is 2.61 V/us. Using only one metal in layout,it passed the layout verification, and the layout area is 4100 μm × 2590 μm. Therefore, this article designed an operational amplifier not only with very low noise, but also performance excellently in other ways, it meets the design specifications basically.