A CMOS Wideband Low Noise Amplifier Using Bandwidth Extension Technique

In this paper, a wideband LNA implemented in a 0.18μm standard CMOS process is presented. We modified the cascode amplifier which is typically composed of common-source and common-gate amplifiers and improve the bandwidth by adding the active inductor to the gate of common-gate stage. It can be found that a series gate inductor to a common-gate amplifier introduces zeros into the transfer function of the modified cascode amplifier, just like shunt-peaking method. The bandwidth of the modified cascode amplifier can be compensated because of the zeros introduced to high frequency, but it also results in the unstable problem because of the negative resistance looking into the source of the common gate around the resonant frequency of , as shown in Fig. 3.11. To keep the amplifier stable and extend bandwidth, we use the variable inductance of active inductor, as Lg, which can be designed to keep the self-resonant frequency of the active inductor below the resonant frequency of . This method can provide very flat power gain at high frequency to extend the bandwidth and also avoid the negative resistance causing the amplifier unstable. And the regular methods to design LNA will require inductors for the impedance and noise matching. On-chip passive inductors exhibit poor quality-factor and require large silicon die area. Here, we use the active inductor with resistance feedback which can achieve higher inductance and quality-factor at GHz range, shown in Fig. 4.12. Following up this idea, it is used to design a wideband low noise amplifier by using thermal noise canceling technique. Finally, we applied this modified cascode amplifier to the thermal noise canceling structure to achieve low noise figure and impedance matching.
This CMOS wideband low noise amplifier using bandwidth extension technique can achieve a measured flat bandwidth (±0.4 dB) from 50 MHz to 900 MHz and 3-dB bandwidth from 40 to 1200 MHz. The wideband LNA provides a very flat power gain (S21) about 16.4 dB with a noise figure less than 3 dB while drawing 8 mA from a 1.8 V supply. Both the S11 and S22 are below -10 dB, and the IIP3 is measured about -1 dBm.