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Re: gEDA-user: LED in reverse
On Nov 28, 2006, at 1:32 AM, Karel Kulhavy wrote:
Does the gold increase only 1/f noise or also the broadband (white)
noise?
Broadband. The main effect is to reduce beta by promoting
recombination. That increases base current shot noise: every
additional recombination event is one electron's worth of charge
noise variance.
Is this base current shot noise amplified by the transistor like any
other current that is applied to the base?
Depends on the impedance the base sees. You really need to use the
Ebers-Moll model or its elaborations to understand what's going on
here. In these models, the base-emitter *voltage* drives both base
and collector current. If the base-emitter voltage is fixed, the
collector current noise is just shot noise due to the collector
current itself (assuming Johnson noise from parasitic resistances may
be neglected). If the external circuit has a high impedance between
base and emitter, the base current shot noise will generate a voltage
through it in parallel with the transistor's base-emitter impedance.
If this is an important contributer to system noise, the designer
should consider using a FET instead of a bipolar transistor.
John Doty Noqsi Aerospace, Ltd.
jpd@xxxxxxxxx
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