# Why the noise from cascode transistors are negligible at low frequencies?

I am reading about noise of cascode amplifier in Razavi book, Design of Analog CMOS Integrated Circuits.

However, I couldn't understand the yellow highlighted sentence below. Could anyone explain it?

Why the noise from M3, M4, M5 and M6 are negligible at low frequencies?

Thank you.

• Is it due to lower channel operating conductance of the cascode current sources? – Tony Stewart EE75 Nov 2 '16 at 22:41
• Not authoritative : but I suspect it's because the amplifier works by acting as a current source to (whatever load impedance) and the cascode devices operate at unity current gain, so there's no amplification of their internal noise. – user_1818839 Nov 2 '16 at 23:04
• I think it is because M3 to M6 are biased nonlinear loads. Their nonlinearity increases with frequency as well as M1 and M2 input transistors. Frequency response of M1 and M2 delays with increasing frequency, depending on the response of M1 and M2, M3 and M4 also show a delayed response. As you see, nonlinearity actually increase by mutliply as number of the cascoded transistors. But for low frequency, frequency response of all transistors are fine. – Alper91 Nov 7 '16 at 6:48

You end up with the following equation for the noise: $$\frac{i_D}{1 + g_m r_o} \approx 0$$ Where $r_o$ is the degeneration. For high frequencies $C_{gs}$ provides an alternative current path, reducing the degeneration.