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Argus25 · 9th Jun 2018, 11:18 am

Everything RW says is correct. There is a very simple way to look at this too;

One limiting factor in a triode's frequency response is the grid-plate (Miller) capacitance. Excursions of plate voltage (which are out of phase with the grid) provide negative feedback to the grid which increases as the frequency increases, lowering the upper frequency response. One idea of fixing the plate voltage (as in a cathode follower or in the lower valve of a cascode circuit) kills this negative feedback.

If the plate voltage is allowed a tiny amount of wriggle room, with a small resistor, some frequency dependent negative feedback persists and this affects the signal regardless whether it it taken from the plate or the cathode.

And of course if reactive elements are in the plate wiring (for example the plate wiring inductance) any resonance of this with the plate capacitance is damped by the resistor.

So the stability of the cathode follower stage at very high frequencies is improved with the 100R plate resistor, but it is not large enough to significantly affect (roll off) the upper frequency response of the stage for the application, but it might be if the value was too high.

9th Jun 2018, 11:18 am	#6
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Cathode follower anode resistor. Everything RW says is correct. There is a very simple way to look at this too; One limiting factor in a triode's frequency response is the grid-plate (Miller) capacitance. Excursions of plate voltage (which are out of phase with the grid) provide negative feedback to the grid which increases as the frequency increases, lowering the upper frequency response. One idea of fixing the plate voltage (as in a cathode follower or in the lower valve of a cascode circuit) kills this negative feedback. If the plate voltage is allowed a tiny amount of wriggle room, with a small resistor, some frequency dependent negative feedback persists and this affects the signal regardless whether it it taken from the plate or the cathode. And of course if reactive elements are in the plate wiring (for example the plate wiring inductance) any resonance of this with the plate capacitance is damped by the resistor. So the stability of the cathode follower stage at very high frequencies is improved with the 100R plate resistor, but it is not large enough to significantly affect (roll off) the upper frequency response of the stage for the application, but it might be if the value was too high.