Capacitance Multiplier

The blue node is the ideal voltage source (10V) that 1MHz ripple is being added onto (1Vpp). The red node is this source (1mohm output impedance) powering a test load (50ohms) without any filtering. The result is that ~1Vpp of ripple is seen by the load making it very noisy & imprecise. The orange node is the same test load with a 100kHz low pass RC filter in series to dampen the ripple. The result is ~0.1Vpp of ripple on its output; however the output has dropped to ~2.4V nominal making it unusable for many applications. The output voltage varies greatly with the load’s power draw getting worse and worse as the load’s impedance decreases. This is because the series resistance of the filter forms a voltage divider with the load and reduces the output. The green node is the same test load with a capacitance multiplier instead of just a passive filter. This also results in ~0.1Vpp of ripple on the output; however the output has only dropped to ~8.7V this time. This is because the NPN BJT configured as an emitter follower is able to vary its impedance to maintain a more stable voltage and higher current output. The same RC filter connects to the transistor’s base and is able to filter out the 1MHz ripple so the transistor passes significantly less of the ripple through to the load. The name comes from this effect: essentially the capacitance of the filter reducing the ripple as seen by the load is that of the capacitor times the transistor’s gain (100 typically for BJTs).

published 4 years ago