Quote:
Originally Posted by regenfreak
I am try to a get an idea of the order of magnitude for Miller effect input capacitance...
ECC85: Cin = Cg(k+f+s) + Cag (A+1) = 3 + 1.5(57+1)=90pF
Correct me if I am wrong, these are very high for VHF valves. I am surprised! I assume Cg(k+f+s) to be the grid to cathod capacitance.
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First, Cg(k+f+s) is capacitance between grid, and (cathode, heater, and intersection screen all strapped).
Since the cathode is RF earth (in common-cathode configuration), the screen is earthed, and the heater is also RF earth, this makes a useful capacitance to include on a data sheet.
Next, the formula you give is correct IF the voltage gain is 57 times. In practice, it won't be. The 57 is the maximum possible gain, with infinite load resistance (eg a constant-current load). Even if you were to use this valve as an AF amplifier, a stage gain of 30 would be more likely - in which case, replace the 57 by 30 in your calculation, giving Cin = 49.5pF.
At RF, you're unlikely to get a gain of 30 and a gain of 10 is more realistic. Now we get Cin = 19.5pF. However, at RF the load is unlikely to be a pure resistance - it's more likely to be capacitive (in which case the Miller effect makes the input look like Cg(k+f+s) in parallel with a
resistance) or inductive (in which case it will look like Cg(k+f+s) in parallel with a
negative resistance)
But if you use the ECC85 as a cascade, which gives the benefits that David Radio Wrangler has indicated, it turns out that the gain is around 1 from grid to anode (the anode being fed into the cathode of the second stage which looks pretty much resistive) and Cin is now 6pF.
And that is much more like what we want from an RF amplifier!