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~ Triode Amplifier Un–Bypassed Cathode Calculator ~

Un bypassed cathode resistor amplifier

This Calculator was originally to support my Amplifier–Buffer project and can be used to calculate the a.c. gain and impedances but not d.c. operating point of a triode amplifier with a common cathode gain stage and a cathode follower output buffer

Results update as input variables are entered ~ gm ra and µ should be taken from the valve characteristics or manufacturers data ~ The default values are for an ECC81 with gain stage V1 Ia ≈1mA and Cathode follower V2 Ia ≈2.3mA ~ Values ra1' and gm1' for V1 show how un-bypassed Rk1 lowers µ and gm but may improve distortion

It includes anode load RL2 to calculate the gain of a concertina phase splitter where RL2 = Rk2 and the outputs are the same level but anti–phase

Variables for gain stage — V1 Output Significant Figures 
Mutual conductance — gm1 mA/V — See notes below
Internal anode resistance — ra1 kΩ
Amplification factor — µ1 Check — gm1×ra1 =
Anode load resistor — RL1 kΩ
Cathode resistor — Rk1 kΩ
Calculated results for V1 gain stage
ra1 modified by Rk1 — ra1' kΩ = ra1+ (µ1+1) × Rk1
gm1 modified by Rk1 — gm1'  mA/V =  µ1 / ( ra1+(µ1+1) Rk1)
Unloaded voltage gain — AV1 V/V = µ RL1 / ( RL1+ ra1+ (µ1+1) × Rk1)
Unloaded voltage gain — AV1 dB = 20log (Av1)
Output resistance at V1 anode kΩ = ra1∥RL
Variables for cathode follower — V2
Mutual conductance — gm2 mA/V — See notes below
Internal anode resistance — ra2 kΩ
Amplification factor — µ2 Check — gm2 × ra2 =
Anode resistor — RL2 kΩ
Cathode resistor — Rk2 kΩ
Calculated results for Cathode Follower V2
Voltage gain at V2 anode — ARL2 V/V = µ2 RL2 / ( RL2 + ra2 + (µ2+1 ) × Rk2)
Voltage gain at V2 cathode — ARk2 V/V = µ2 Rk2 / (RL2 + ra2 + (µ2+1) × Rk2)
Output resistance at V2 cathode — Rout Ω = (ra2+RL2)  / (µ2+1) ∥ Rk2
Overall voltage gain at V2 cathode V/V  dB = 20log (AV1 × ARk2)

Keith Snook triode amplifier calculator

The value entered for gm is not used to calculate results but is used to calculate the value µ = gm × ra which should be close to the entered value of µ for the results to be valid ~ As with most calculations with triodes gm and gm' could be used but ra and ra' would also be needed ~ The factor (µ+1) helps simplify things

It is assumed the  d.c. bias is optimised for both the gain stage and follower and that the separate values for gm ra and µ have been found and entered for both stages and that a sensible Ia and HT voltage have been chosen

Click here for other calculators ~ ΐ

 

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