Chapter 7 Noise 1 Fundamental Noise Characteristics Thermal

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Chapter 7 Noise 1

Chapter 7 Noise 1

Fundamental Noise Characteristics Ø Thermal Noise Ø Shot Noise in a pn Junction Ø

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,

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

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

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

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

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

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

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

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

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

Now we can solve for the spectral densities of 12

B I 1 I 2 4 k. Trb V 1 2 q. IB E

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

• 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

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

Noise Resistance Optimum Source Admittance 17

Minimum Noise Figure NFmin decreases with increasing β, decreasing Ci (transit time), and decreasing

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

Associated Gain • GA increases with • • Increase IC Decrease Ci Decrease rb Decrease Ccb • GA decreases with • Increase β • Increase frequency 19