(a) Neglecting capacitive effects, calculate the noise figure in decibels of the circuit of Fig. 11.52 with R_S= 5 kΩ. Use data as in Problem 11.10.

(b) If the flicker noise corner frequency for each device is 1 kHz, calculate the low frequency where the spot noise figure is 3 dB above the value in (a).

Data from Prob. 11.10:

The ac schematic of a low-input-impedance common-base amplifier is shown in Fig. 11.52.

(a) Calculate the equivalent noise voltage and current generators of this circuit at the emitter of Q₁ using I_C1 = I_C2 = 1 mA, r_b1 = r_b2 = 0, β₁ = β₂ = 100, f_T1 = f_T2 = 400 MHz. Neglect flicker noise but include capacitive effects in the transistors. Use SPICE to check your result.

(b) If R_S = 5 kΩ, and later stages limit the bandwidth to a sharp cutoff at 150 MHz, calculate the value of i_S giving an output signal-to-noise ratio of 10 dB.

Fig. 11.52:

Q2 5 k2 Rs