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I'm trying to build a capacitor multiplier to remove ripple from a power supply. The power comes from a dc-to-dc converter (MC34063). The actual load is a circuit that has multiple modes of operation and it takes 2mA to 20mA. Fortunately enough, it works from 12V to 26V so I do not need to regulate the voltage to a fixed value. I only need to remove the ripple.

schematic

simulate this circuit – Schematic created using CircuitLab

The idea of ripple reduction came from here:

https://www.edn.com/design/analog/4350857/Capacitor-amplifier-reduces-ripple-without-dc-loss

The RC filter is 220k Ohm 10uF with a cutoff frequency of 0.072Hz. C2 is a ceramic cap that has 20V DC offset so probably the real capacity is lower and the cutoff frequency is higher, but it should not matter. The 2N2222 has an absolute minimum hFe=35.

Here is what I have measured:

enter image description here

The original ripple of 1.42V was reduced to 1.25V.

Note: in my original question I zoomed in the time scale too much and I was seeing another ripple that was superimposed on the main signal, this was corrected to the above version.

As far as I understand, the transistor is in common collector configuration, and it inverts and amplifies the error between the actual and the "average" voltage. The above image shows that ripple at Vin and Vout are in phase. There is something fundamentally wrong with my design, I just don't see it.

UPDATE This diagram shows: Vin (yellow), V(C2)(blue) and Vout(purple).

enter image description here

The problem seems to be with the voltage at the transistor's base. It should not have 1.2V ripple. I'm still not sure what is happening here.

UPDATE: first of all, the transistor was faulty, it had a short between the collector and the base. That is why there was a "problem" with the voltage at the base. Replaced the transistor and changed the circuit to use the suggested voltage divider instead of a single resistor. R1 = 47k and R2 = 220k. If I remove the transistor then there is 16.8V at Vbase.

schematic

simulate this circuit

The basic ripple is now reduced from about 2V to 50mV. But a very high frequency oscillation appeared, as glen_geek had foreseen. This circuit is much more complicated than I tought.

enter image description here

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    \$\begingroup\$ I must be doing something very wrong, Dave from EEVBlog demonstrates the very same circuit here: youtu.be/wopmEyZKnYo?t=1094 and for him it is working. \$\endgroup\$ – nagylzs Dec 27 '18 at 11:35
  • \$\begingroup\$ Can you measure voltage directly across C2? , directly across C3? If you still see such large ripple, those capacitors are suspect, or else you have a ground loop involving your oscilloscope. \$\endgroup\$ – glen_geek Dec 27 '18 at 15:24
  • \$\begingroup\$ Measured voltage with a DMM across C1 C2 and C3. They are all 20.54V (but the DMM is not true rms). I can also post a screen that shows voltage across C2 on a third channel. The buck converter is powered from a single Li-Po cell, I think it is not possible to have a ground loop. \$\endgroup\$ – nagylzs Dec 27 '18 at 15:42
  • \$\begingroup\$ Added new screen that also shows the voltage across transistor base and GND \$\endgroup\$ – nagylzs Dec 27 '18 at 15:53
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    \$\begingroup\$ Which point on ground are you measuring from? There is a lot of current through C1 so the ground lead inductance and resistance will result in some voltage drop in the ground connection. \$\endgroup\$ – Spehro Pefhany Dec 27 '18 at 16:03
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The all important component that is missing is in the red box below: -

enter image description here

Taken from the EDN article linked by OP.

You have to have the resistor in the red box to bias the base at a few volts below the incoming supply voltage or it just won't work properly. So if your input supply is 20 volts, arrange for R2 and R1 to provide a DC voltage of about 16 volts at the base. This will means about 15.5 volts at the emitter on light loads and maybe 14 volts on the full load.

The parallel impedance of R1 and R2 should be 220 kohm to acheive the same cut-off as you quoted when C2 is 10 uF.

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  • \$\begingroup\$ I'm going to try that tomorrow. That resistor was not present in the eevblog version. I wonder how it worked for them. \$\endgroup\$ – nagylzs Dec 27 '18 at 17:39
  • \$\begingroup\$ @nagylzs If enough base current flows through \$ R_1 \$ , a DC collector-to-base voltage drop allows the transistor to work. Ideally, \$ R_2 \$ is very much larger than \$ R_1 \$. \$ R_2 \$ might be required when the minimum current flowing through \$ Q_1 \$ is too small. \$\endgroup\$ – glen_geek Dec 27 '18 at 19:22
  • \$\begingroup\$ I have noticed that this circuit often oscillates at some very high frequency (hundreds of MHz). This affects voltage regulation - when oscillations are stopped, load voltage drops a little. Oscillation amplitude is often too low in amplitude, and too high frequency to see on oscilloscope. Adding a probe can easily kill oscillations. A ferrite bead on transistor base or emitter is a sure cure. \$\endgroup\$ – glen_geek Dec 27 '18 at 19:27
  • \$\begingroup\$ Okay, so the transistor had a short between the collector and the base. After one day of practicing, it finally works, but the very high frequency noise appeared. This circuit is more complicated than I thought (and much more complicated than it was demonstrated in eevblog). I'm going try to build a voltage follower with a voltage reference instead, that seems to be easier. This was a very good lesson for me. Thank you! \$\endgroup\$ – nagylzs Dec 28 '18 at 14:07
  • \$\begingroup\$ Voltage follower with a TL431 could do 38mV max ripple plus 120mV high frequency oscillation. I'm helpless. :-) \$\endgroup\$ – nagylzs Dec 28 '18 at 19:13

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