A bipolar-junction-transistor input stage is biased at collector current with…

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Semiconductor devices and analogue circuits · Feedback, operational amplifiers and analogue signal processing

Inside the Instrument, Ch Noise sources and amplifier noise

Problem

A bipolar-junction-transistor input stage is biased at collector current IC=200 μAI_C = 200\,\mu\mathrm{A} with common-emitter current gain βF=150\beta_F = 150 and base spreading resistance rb=50 Ωr_b = 50\,\Omega, at T=300 KT = 300\,\mathrm{K} (thermal voltage VT=25.9 mVV_T = 25.9\,\mathrm{mV}). ((a)) Calculate the transistor's input voltage-noise density ene_n, using en2=2(kBT)2/(qIC)+4kBTrbe_n^2 = 2(k_BT)^2/(qI_C) + 4k_BTr_b. ((b)) Calculate its input current-noise density ini_n, using in2=2qIC/βFi_n^2 = 2qI_C/\beta_F. ((c)) Using the complete ene_n from part (a), calculate the source resistance Ropt=en/inR_{\rm opt} = e_n/i_n at which its noise figure is minimised, and compare it with the shot-noise-only value βF/gm\sqrt{\beta_F}/g_m, where gm=IC/VTg_m = I_C/V_T.

Browse the problem library