4.8 Base Shot Noise Discussion 85 2bf1mCiKIf α= (4.24)where m is between 0.5 and 2, a is about equal to 1, and K is a process constant.The 1/f noise is dominant at low frequencies as shown in Figure 4.10. However, beyond the corner frequency (shown as 10 kHz), thermal noise dominates. The ef-fect of 1/f noise on RF circuits can usually be ignored. An exception is in the design of oscillators, where 1/f noise can modulate the oscillator output signal, producing or increasing phase noise. The 1/f noise is also important in direct down-conversion receivers, as the output signal is close to dc. Note also that 1/f noise is much worse for MOS transistors, where it can be significant up to 1 MHz. 4.8 Base Shot Noise DiscussionIt is interesting that base shot noise can be related to noise in the resistor rp by not-ing that the base shot noise current is in parallel with rp as shown in Figure 4.11. As shown in (4.25), base shot noise can be related to resistor thermal noise, except that it has a value of 2kTR instead of the expected 4kTR, making use of (4.3), (4.5), (4.7), and (4.22). bnbn22222CCCBCmIIIvirqIrqrqrqrkTrI qg rrkTππππππππβ=×=×=× =×=×= (4.25)Thus, base shot noise can be related to thermal noise in the resistor rp (but is off by a factor of 2). This is sometimes expressed by stating that the diffusion resistance Figure 4.10 Illustration of noise power spectral density.