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~ Common Collector Amplifier ~ Emitter Follower Noise Calculator ~

Calculates the root mean square value of the Noise Voltage VN(rms) at the output of an Emitter follower ~ Are emitter followers noisy when used as pre-amplifiers ? ~ If the output impedance presented to the next stage is lower than RS but the SNR is not affected very much is this an adavantage ?

Calculation does not take into account flicker noise and assumes: Perfect resistive loads and a modern silicon transistor with low noise bias and the transistor gain is constant within the measurment bandwidth defined

The circled nodes B C and E represent the external connections of the transistor so rbb and re are internal ~ The output resistance ro is included but if not known assume its value is >2MΩ

Boltzmann Constant — k
1.3806x10-23
 J/K ( joules/kelvin )
Charge on a single Electron — q
1.6022x10-19
C ( coulomb or amperes/sec)
Base bias Resistor — RB
Ω ( Base bias total resistance )
Emitter Resistor —RE
Ω 
Follower Load Resistor — RL
Ω ( affects input resistance Rin )
Source Resistor — RS
Ω
Internal output Resistance — ro
Ω
Intrinsic Base Resistance — rbb
Ω ( resistance of the base material )
Transistor d.c. current gain —β
   ( also hFE or 'beta' in some texts )
Collector Current — IC
mA    
Device Temperature — T
˚C ( calculation uses ˚K )
Noise Bandwidth — Bn
Hz
Calculated results using the values entered above
Signal   VS — SNR
dB = 20 log (GVVS/VNtot)     (SNRout)
Maximum SNR due to RS alone
dB = 20 log (VS/√4 k T Bn RS  )   (SNRin)
Noise Figure — NF
dB = SNRout –SNRin
Emitter Internal Resistance — re
Ω = VT/IC       VT = kT/q = mV
Common base current gain —α
= IC/(IC + IB)   IB = IC/β = µA
Amplifier input impedance — Rin
=  RB ‖ [ (rbb + (1 + β) (re + RE ‖ RL ‖ ro ) ]
Amplifier output impedance — Ro
Ω = (RE ‖ ro) ‖ [ (RS ‖ RB+rbb)/(β + 1) + re ]
Voltage Gain — Av
V/V = (RE ‖ RL ‖ ro )/(re + RE ‖ RL ‖ ro )
Amplifier Voltage Gain — Gv
V/V = AV Rin /(RS + Rin)
Noise due to R‖ R— VNRS
nVrms = √4 k T Bn RS‖R• G nV/√Hz
Noise due to rbb — VNrbb
nVrms = √4 k T Bn rbb • Gv       nV/√Hz
Noise due to R‖ Ro — VNRo
nVrms = √4 k T Bn R‖ Ro        nV/√Hz
Noise due to IB through re— VNIB
nVrms = √2 q IB Bn • (Rs+rbb –re) • Gv
Noise due to IC through re — VNIC
nVrms = √2 q IC Bn • re(RE ‖ RL ‖ ro )/(RE ‖ RL ‖ ro + re)
Total Noise V at Output — VNtot
µVrms = √VNRS2+VNrbb2+VNRo2+VNIB2+VNIC2

 

See these references:

The Art of Electronics 3rd edition chapter 8 ~ By Paul Horowitz and Winfield Hill ~ fellow cynics
Noise in Transistor Circuits ~ By P. J. Baxandall ~ Wireless World November 1968 ~ often referenced
Designing low–noise audio amplifiers ~ By Wilfried Adam ~ Wireless World June 1989
Introduction to low–noise amplifier design ~ By A. Foord ~ Wireless World April 1981
The design of Low-noise audio frequency amplifiers ~ By E. A. Faulkner ~ The Radio and Electronic Engineer July 1968 ~ This article along with the book 'Low-Noise Electronic System Design' By C. D. Motchenbacher and J. A. Connelly should answer most questions about electronic noise


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