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    Building, troubleshooting and testing of these amplifiers should only be
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Question about rp and the AC load line

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hpupo

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Joined 2011
#1
I've been doing some studying, and as far as i understood, when drawing the AC load line for a stage, I should only use RL (load resistor) and Rg (resistance of the next stage's grid resistor) in parallel as a slope. But what about rp (plate resistance). shouldn't i include that on the slope of the AC load line. If not, could someone explain why? Plate resistance disappears under high frequency?

Thanks
 
#2
The plate resistance is not part of the load, it's part of the (equivalent) source. To first order, the tube looks like a generator (mu time Vgk) in series with the plate resistance. This forms a voltage divider with the load resistance (plate resistor and grid resistor in parallel). The output voltage is measured across the load, so the load line only has those resistances as components.
 
#4
hpupo said:
I've been doing some studying, and as far as i understood, when drawing the AC load line for a stage, I should only use RL (load resistor) and Rg (resistance of the next stage's grid resistor) in parallel as a slope. But what about rp (plate resistance). shouldn't i include that on the slope of the AC load line. If not, could someone explain why? Plate resistance disappears under high frequency?
Click to expand...

It isn't a bad idea to consider the impedance of the capacitive load that the following stage grid places as well. Figure the capacitance's impedance out at 20kHz and see how much further the AC load line gets cocked.
 
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