[keith-snook.info ~ Emmitter Follower Transistor Noise Figure ]

~ Emitter Follower Gain & Noise Calculator ~

 

Calculates the root mean square value of the Noise Voltage VN(rms) at the Output of an Emitter follower [Common Collector Amplifier] ~ Are emitter followers noisy when used as input to 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 advantage ?

 

The 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 measurement bandwidth defined

The circled nodes B C and E represent the external connections of the transistor and lower case rbb and re are internal resistances ~ The output resistance ro is included in calculation but if not known assume its value is >1MΩ although for some PNP transistors it may be much lower

Start by entering a Collector Current and change variables as required


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 [ resistive part 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 = SNRin – SNRout
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 kΩ = RB∥ [ (rbb'+(1 + β) (re+RE∥RL∥ro) ]
Amplifier output impedance — Ro Ω = (RE∥ro) ∥ [ (RS∥RB+rbb')/(1+β)+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 RS∥RB — VNRS nVrms = √4 k T BnRS∥RB · AV nV/√Hz
Noise due to rbb — VNrbb'

nVrms = √4 k T Bn rbb' · AV   nV/√Hz

Noise due to RL∥Ro — VNRo nVrms = √4 k T Bn RL∥Ro   nV/√Hz
Noise due to IB through re — VNIB nVrms = √2 q IB Bn · (RS∥RB+rbb'–re) · Av
Noise due to IC through re — VNIC nVrms = √2 q IC Bn · (RE∥RL∥ro∥re)
Total Noise V at Output — VNtot µVrms = √NRS+ V²Nrbb'+ V²NRo+ V²NIB+ V²NIC

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Keith Snook Transistor Noise references See these references for more about transistor noise :

The Art of Electronics 3rd edition chapter 8 ~ By Paul Horowitz and Winfield Hill
Noise in Transistor Circuits ~ By P. J. Baxandall ~ Wireless World November 1968
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 excellent 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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