Capacitance Multiplier - Why are frequency and ripple not linked?

[katastic]

Hello all, hope you're doing well.

According to the eevblog video on Capacitance Multipliers



at around the 20:30 mark, Dave talks about how F,-3db doesn't change with beta when you add an emitter follower to the RC.

But if F,-3db marks the frequency of 3 db drop on a filter... then how is the voltage ripple improving further? Aren't those two directly related? If I used a second order filter, the F,-3db would change, right? Because the signal at the original F,3db point would be now reduced, so the new F,-3db point would be at a reduced in frequency.

So I'm having trouble grasping: How is F,-3db is separate from the actual ripple drop? Because if it's not, then we're not multiplying our RC constant just reducing the current through R at which point... what's the point except allowing you to use a very high value for R? That is, the circuit doesn't multiply at all but simply allows you to use a higher value for R? Is that what's going on?

I don't know. There's clearly something key I'm just "not getting" about this.

Thanks for any insight,
--Chris Katko

[T3sl4co1l]

Draw the equivalent circuit.  The transistor is equivalent to a resistance in parallel with the supply resistor.  It will have some effect depending on load, but typically the supply resistor is dominant (much smaller) so the effect is small.

The equivalent resistance is approximately hFE * RL.

Tim