Capacitance multiplier - do I need to protect the NPN using a diode?

[humbug11]

Hi,

I'm experimenting with using a capacitance multiplier as a noise filter. It seems to work rather well, but one thing that I'm not sure about is if it is necessary to protect the NPN transistor from negative Emitter-Collector voltages. According to the DS, the maximum Emitter-Base voltage is 6V, which will be exceeded when removing the battery or if there is a large negative "spike" on "+Batt". However, only very little current can flow in these situations, as the total capacitance on the "V_BATT_FILT" net is in the order of 10uF.

Will removing the battery or negative spikes damage the NPN transistor over time and is it necessary to protect it using a diode? If so, what diode would be best? Ideally, it should start conducting at a few volts in forward direction and the reverse voltage should be in the order of 40V.

[Paul Price]

Adding a series >40V rated schottky diode to the +Bat input would protect this circuit from negative spikes and add only a minimal voltage drop. Anode of diode to +BAT

[langwadt]

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An optional additional explanation is:

[iMo]

This seems to protect the npn transistor best.
The diodes and transistor types an example only.
Provided as-is, no warranties of any kind are provided

[David Hess]

The capacitance you are multiplying can only discharge through the base-collector junction if the input voltage is removed which has the characteristics of a diode with a rating of the Ic (collector current) and Vcbo (collector-base breakdown voltage) of the transistor so it is very unlikely to be damaged.  Collector-base junctions of transistors are sometimes used as high performance power diodes.

The base-emitter junction is a different matter and is often protected by an anti-parallel diode between the base and emitter leads or base and collector leads.  In a power transistor the base-emitter junction can actually sustain considerable reverse breakdown current without affecting the circuit however the high power resulting from the relatively high breakdown voltage can lead to thermal damage.