A moving-coil sensor presents a source resistance of to its amplifier

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 moving-coil sensor presents a source resistance of

R_s = 500\,\Omega

to its amplifier. A bipolar transistor with

\beta_F = 150

(datasheet typical) is to be used as the input device, biased at

300\,\mathrm{K}

where

V_T = 25.9\,\mathrm{mV}

. The base spreading resistance

r_b

is small enough to neglect. ((a)) Find the collector bias current

I_C

that makes the transistor's optimum source resistance

R_{\rm opt} = \sqrt{\beta_F}/g_m

equal to

R_s

. ((b)) At this bias, compute the transistor's input voltage noise

e_n = \sqrt{2(k_BT)^2/(qI_C)}

and current noise

i_n = \sqrt{2qI_C/\beta_F}

, and confirm that

e_n/i_n

reproduces

R_{\rm opt}

from part (a). ((c)) In one line, state whether a bipolar or a field-effect input transistor is the better choice for this sensor, and why.

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