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Old 8th February 2006, 09:49 AM   #1
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Default two transistors voltage regulator using IRFBC30


I built a two transistors voltage regulator but it oscilated.

Pass element is IRFBC30 and error amp. is BUH51. I chose them because the Vout is high, 405V. Vin is 450V.

Operation current is about 40 mA.

It is a simple two transistor voltage regulator. Upper resistor is 81K and lower resistor is 120K for the voltage divider in the output. Voltage reference is 235V, using 3 zenor diodes in series connected to emittor of BUH51.

hfe of BUH51 is about 27, based on measurement.

A 100 ohm resistor is connected between G of IRFBC30 and collector of BUH51.

A 10K3 resistor is connected between Vin (that is D of IRFBC30) and collector of BUH51.

This regulator is for a tube power amp.

When I powered it up without preamp, it is stable, Vout is about 407V.

But as I connected it with preamp, it oscillated and Vout is varied.

Could someone help me out how to make it stable?
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Old 8th February 2006, 09:57 AM   #2
lineup is offline lineup  Sweden
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Put one small cap
between Base and Collector of BUH51.

Can be 470pF or 1nF.
Ceramic or small plastic does not matter.
Make sure it have correct Voltage rating.
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Old 13th February 2006, 08:33 AM   #3
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Thanks a lot!

The Vout is very stable after adding a small cap.

Would you share the reason why? And how to calculate the suitable value of cap?
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