(Transistor based) Capacitor Multiplier (Circuit Pattern)

About

A (transistor-based) capacitor multiplier is technique for implementing an RC (low-pass) filter.

The transistor multiplies the effect of the capacitor, which allows you to achieve the same performance as an RC low-pass filter while using a much smaller capacitor.

This circuit pattern is useful for reducing voltage ripple / noise in power supply circuits.

In particular, it complements a linear voltage regulator (e.g. an LM7805) well. A 7805 is good at regulating and cleaning up low frequency noise, but can't deal well with high-frequency noise. A capacitor multiplier can't regulate voltage, but cleans up the higher frequency noise which the 7805 can't.

The only downside is that it uses up some voltage headroom, and you'll have to account for this in your design (e.g., by using a 12 volt wall-wart instead of a 9 volt wall-wart).

It is hobbyist-friendly because it requires only a few cheap, common components, for example:

• a 2N3904 transistor
• a 0.1uF ceramic capacitor
• a 1k resistor

When should you I this circuit pattern?

Use this circuit when you have a noisy power supply which is affecting the performance of your circuit.

Examples of noisy power supplies which might benefit from this circuit:

• Flying capacitor chips, like the ICL7660.
• Switch-mode power supplies (including most modern wall warts)

Examples of clean supplies which probably don't need this circuit:

• Batteries
• The traditional transformer / linear regulator power supply (including old-school, heavy wall warts)

Examples of circuits which need a clean power supply:

• Circuits which perform analog measurements. Anything using an ADC or an op amp will benefit from clean supply rails.
• Audio circuits

Examples of circuits which don't need a clean power supply:

• Entirely digital circuits, e.g. an Arduino project with buttons which drives a 16x2 LCD display.

Resources:

• http://en.wikipedia.org/wiki/Capacitance_multiplier