1. Chapter Noise

2. Fundamental Noise Characteristics
Thermal Noise
Shot Noise in a pn Junction
Shot Noise in Bipolar Transistors

3. 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 V2/Hz.

4. 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

5. Shot Noise in Bipolar Transistors
For the same IC, a SiGe HBT has a lower IB than a Si BJT, and hence a lower than a Si BJT, because of the inherently higher . Furthermore, a SiGe HBT can be operated at a lower IC than a Si BJT for the same RF gain, because of the higher fT and fmax, further reducing

6. Noiseless Tow-port I1 I2
Two-port Network V1 V2

7. I1 I2
- +
+ -
NoiselessNetwork (Z)
Vn1
Vn2 V1 V2

8. I1 I2
NoiselessNetwork (Y) V1 V2
In1
In2
(a)

9. I2 I1
- +
Vn1
(Y) V1 V2
In1
(b)

10. From equation (a) (b):
(1)
(2)
(3)
(4)
From (4)-(2):
From (3)-(1):

11. I1 I2 B C
BJT V1 V2 ib ic E E

12. Now we can solve for the spectral densities of

13. I2 I1 B
- + C
4kTrb
BJT V1
2qIB
2qIC E E

14. 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.

16. Cbe consists of the EB depletion capacitance Cte and the EB diffusion capacitance gmﺡ ( ), with begin the transit time, and Ci is related to fT through

17. Noise Resistance
Optimum Source Admittance

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

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