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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.

Tomáš Zato
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  • 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. – horta Jun 24 '14 at 14:00
  • The base is always conencted to the ground using the pull-down resistor (which is of small resistance). It's never a floating value. – Tomáš Zato Jun 24 '14 at 14:04
  • What's a small resistance? What's R1 and R2? Busted NPN means your transistor (NPN) is not functioning according to specification (dead). – horta Jun 24 '14 at 14:05
  • I can't find out what busted NPN means. Small resistance is less than 1k. – Tomáš Zato Jun 24 '14 at 14:05
  • Less than 1k is not anywhere near a short. 0.7mA would cause the transistor to turn on if you have a 1k there. – horta Jun 24 '14 at 14:08
  • I don't know how to calculate them properly. Instead of R1, there's a LED diode currently, to indicate current. When testing with 12V, I add a resistor in series with diode. R2 is to prevent too much current flowing through base. – Tomáš Zato Jun 24 '14 at 14:08
  • Let us [continue this discussion in chat](http://chat.stackexchange.com/rooms/15306/discussion-between-tomas-zato-and-horta). – Tomáš Zato Jun 24 '14 at 14:08
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    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. – John D Jun 24 '14 at 14:35
  • I was trying to prevent the saturation using the pull-down resistor. How do I "*bias the transistor so that it remains in the linear region*"? – Tomáš Zato Jun 24 '14 at 15:18
  • @horta So it appears that in fact, I've used 10k resistor for the pull-down. I have also updated current values of other resistors. – Tomáš Zato Jun 24 '14 at 15:27
  • Check your LED circuit with a new transistor and connect the base through a resistor to ground and Vdd. See if it still matches the off at 5V and dim at 12V. – horta Jun 24 '14 at 15:55
  • @TomášZato Here's an example of how to bias a BJT for linear operation: http://en.wikipedia.org/wiki/Bipolar_transistor_biasing – John D Jun 25 '14 at 13:07
  • You can add a Baker clamp (Schottky diode from base to collector.) That will help with the turn off time. But I still don't know if it will do 150 MHz. – George Herold Jul 24 '14 at 20:10

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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.

Spehro Pefhany
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