Chapter 7 Noise 1 Fundamental Noise Characteristics Thermal
- Slides: 19
Chapter 7 Noise 1
Fundamental Noise Characteristics Ø Thermal Noise Ø Shot Noise in a pn Junction Ø Shot Noise in Bipolar Transistors 2
Thermal Noise We cannot use the average value of the voltage as a figure-of-merit, because the time-averaged value is always zero. Instead, we used its mean square value The ratio of this mean square value with respect to , as approaches zero, gives the “power spectral density” (PSD) of the voltage noise, denoted by Sv(f), where Sv(f) has units of V 2/Hz. 3
Shot Noise in a pn Junction • “Shot noise” refers to the fluctuations associated with the dc current flow across a potential barrier, which naturally occurs, for instance, under forward bias in a pn junction. • The mean square of the current fluctuations is proportional to the average diode current Idc 4
Shot Noise in Bipolar Transistors For the same IC, a Si. Ge HBT has a lower IB than a Si BJT, and hence a lower than a Si BJT, because of the inherently higher. Furthermore, a Si. Ge HBT can be operated at a lower IC than a Si BJT for the same RF gain, because of the higher f. T and fmax, further reducing 5
Noiseless Tow-port I 1 V 1 I 2 Two-port Network V 2 6
I 1 -+ Vn 1 V 1 +- Noiseless Network (Z) I 2 Vn 2 V 2 7
I 1 V 1 I 2 In 1 Noiseless Network (Y) In 2 V 2 (a) 8
I 1 I 2 -+ Vn 1 V 1 (Y) V 2 In 1 (b) 9
From equation (a) (b): (1) (2) (3) (4) From (4)-(2): From (3)-(1): 10
I 1 I 2 C B V 1 E ib BJT ic V 2 E 11
Now we can solve for the spectral densities of 12
B I 1 I 2 4 k. Trb V 1 2 q. IB E C -+ BJT 2 q. IC E 13
• Noise Figure=(Si/Ni)/(So/No) • The noise figure of a transistor circuit is determined by the source termination admittance , and the noise parameters of the circuit, including the minimum noise figure NFmin, the optimum source Ys, opt, and the noise resistance Rn. 14
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Cbe consists of the EB depletion capacitance Cte and the EB diffusion capacitance gm ( ﺡ ), with begin the transit time, and Ci is related to f. T through 16
Noise Resistance Optimum Source Admittance 17
Minimum Noise Figure NFmin decreases with increasing β, decreasing Ci (transit time), and decreasing rb. 18
Associated Gain • GA increases with • • Increase IC Decrease Ci Decrease rb Decrease Ccb • GA decreases with • Increase β • Increase frequency 19
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