Boosting Performance with Low-Noise Cascode Structures

Designing ultra-low-noise circuits often means balancing trade-offs between gain, distortion, and complexity. One proven technique for minimizing noise in differential amplifier stages is the use of cascode structures. Below, we explore three cascode variations and how each impacts noise performance and distortion.

(a) Bipolar Cascode – Highest Noise

In this configuration, a bipolar transistor is used in the cascode stage. While effective in many designs, the high base current of bipolar devices introduces significant noise. This makes it the noisiest of the three structures.

(b) JFET Cascode – Lower Noise

Replacing the bipolar transistor with a JFET greatly reduces input current, thanks to the JFET's extremely low gate current. The result? Lower overall noise compared to the bipolar version. This structure is ideal for applications requiring improved signal integrity.

(c) Driven Cascode – Lowest Distortion

The final circuit goes a step further. Known as a driven cascode, this configuration virtually eliminates input capacitance through a bootstrapping effect created by the JFETs. Not only does this help reduce noise, but it also dramatically lowers intermodulation distortion—making it the cleanest option for precision analog designs.

Whether you're working on sensitive instrumentation, high-end audio, or RF front ends, these cascode techniques can help you achieve cleaner signals and higher performance.

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