The picture below is an excerpt of Figure 2 of a DC/DC boost converter (see PDF here). It is part of a capacitance multiplier. What is the purpose of this (anti-parallel) diode between emitter and base of the transistor? I couldn't find this in any literature for capacitance multiplying.

enter image description here


2 Answers 2


The diode protects the BJT (inverted BE voltage) assuming the input voltage disappears.
Output capacitors can then discharge through the diode ... and the 500 Ohm safely.

For completeness, I simulated another configuration like this one (big output capacitor).
Check the currents without or with an added diode (currents through BJT). enter image description here

enter image description here

  • \$\begingroup\$ Great insight and demonstrations about spiking backwards the Base-collector current. I haven’t thought about that and your answer also clarified that. \$\endgroup\$
    – EJE
    Commented Jan 27, 2023 at 21:03
  • \$\begingroup\$ It is a real pleasure. Thanks. \$\endgroup\$
    – Antonio51
    Commented Jan 28, 2023 at 0:06

Probably to protect the transistor. Most BJTs will only tolerate -5 or so volts VBE.


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