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I have made a simple digital amplifier using 2N4401 Philips switching transistor.

schematic

simulate this circuit – Schematic created using CircuitLab

I have tested this amplifier on 5 VDC and it has worked (though the effect was very weak). However, if I plug 12 volts from the power supply, the 2N4401 lets all the current through (even when base is disconnected). It doesn't seem to be damaged though, since 5V behaves normal.

I must say that I'm a little bit confused by all the tables in the datasheet. I'm not sure what should the resistors be - I can't see what is the maximum current allowed for the transistor.

The pull-down resistor is here to make the transistor close faster (so I was taught). But again, I don't know how to calculate the resistance so that the transistor never gets more than necessary current.

It might be that the transistor was somewhat damaged. But it's still closes and opens as expected if there's 5V over collector-emiter.

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  • \$\begingroup\$ Base disconnected is undefined because it's then at whatever voltage is determined by capacitance. Short the base to ground and determine if there's current flow. If there is, then you have a busted NPN. Also, please follow convention and make your diagrams/schematics have positive on top, ground in middle or bottom, and have the logic flow left to right. \$\endgroup\$
    – horta
    Jun 24, 2014 at 14:00
  • \$\begingroup\$ The base is always conencted to the ground using the pull-down resistor (which is of small resistance). It's never a floating value. \$\endgroup\$
    – Tomáš Zato
    Jun 24, 2014 at 14:04
  • \$\begingroup\$ What's a small resistance? What's R1 and R2? Busted NPN means your transistor (NPN) is not functioning according to specification (dead). \$\endgroup\$
    – horta
    Jun 24, 2014 at 14:05
  • \$\begingroup\$ I can't find out what busted NPN means. Small resistance is less than 1k. \$\endgroup\$
    – Tomáš Zato
    Jun 24, 2014 at 14:05
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    \$\begingroup\$ The other issue here is that a bipolar driven in and out of saturation is not going to want to switch at 150MHz. If you want an amplifier, you should bias the transistor so that it remains in the linear region, and scale and couple the input voltage to maintain operation there. The 2N4401 has a storage time of over 200ns, an your switching period is 6ns. \$\endgroup\$
    – John D
    Jun 24, 2014 at 14:35

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You've probably mixed up emitter and collector.

If the transistor is connected backward, it will sort-of work (with very low hfe), but Vce(br) will be about 8-9VDC, so you'll get the LED lighting without an input.

Breaking down the B-E junction is not good for the transistor and can permanently reduce forward gain, so if this turns out to be true you may as well splurge and use a new 2-cent transistor.

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