~ 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 and lower case rbb and re are internal ~ The output resistance ro is included but if not known assume its value is >2MΩ ~ Click here for other calculators


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 R‖ R— VNRS nVrms = √4 k T Bn RS‖R• A nV/√Hz
Noise due to rbb — VNrbb' nVrms = √4 k T Bn rbb' • Av        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 ‖ 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 = √V2NRS + V2Nrbb' + V2NRo + V2NIB + V2NIC

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See these references:

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 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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