# Let high frequencies pass through Transistor

I would like to know if there can be any problems when high frequencies(500MHz) pass from the emitter to the collector in a transistor. Example: 2N3904 (NPN).

I know there are a few limits with the frequency on the base, which switch the transistor, but just wanted to know if there is anything you have to keep in mind when letting high frequencies passing from the emitter to the collector.

• Check the datasheet, but 500 MHz sounds much too high for a 2N3904 to do anything predictable with. Feb 10, 2017 at 21:14
• @OlinLathrop Does it mean that the pass-through current in any transistor is generally limited? Feb 10, 2017 at 22:03
• There is an output capacitance of 4pF, which translates to 80Ohm resistance, so at least you got that to worry about Feb 10, 2017 at 22:17
• @Christian Thanks. Does it mean that the current coming out of the emitter would be reduced by the resistance of 800 Ohms at this configuration? I am a newbie in electronics and would really appreciate some support ;) Feb 10, 2017 at 22:52
• The datasheet says max frequency 300MHz, which means either that the output is down -3dB or more probably the the gain reduces to 1 at that frequency. Feb 11, 2017 at 0:31

To "pass through" from emitter to collector, the bipolar is running in common-base configuration which is the fastest way to use a bipolar.

Given the base is not very controlled (the rbb' sits between the base wiring and the actual base region between emitter and collector), I'd expect significant distortion of any high frequency sinwave current.

And you will drive the emitter with a voltage? or with a current?

Hang an inductor from emitter to "gnd" and inject lots of base current through a low value Rbase. The inductor lets the DC_Ibase become DC_Iemitter that exits through the inductor. Now inject your sinwave of current into the emitter, by way of 1nanoFarad cap. [this assumes your current-originator does not need a DC-path; better would be to simply let any DC_pedestal exit thru that Inductor]

• It seems that there are some limits when using a transistor that way. A relay should definitely work as the circuits aren't connected. What about a TRIAC? Feb 11, 2017 at 8:15