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BFP420 is not working the way it should on ADS, why?

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Full Member level 3
Jan 24, 2018
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what could I possibly be doing wrong with this circuit? I have the correct Vce ~ 2V and Ic ~ 20 mA and I have the blocking capacitors to keep DC from interfering with the terminations. Shouldn't I get the same result for maximum available gain at the same frequencies shown on the datasheet? Thanks

Biasing is wrong.
Vce=51.6mV !!
R4 is too low, R5 is not appropriate. R6 is not necessary.
Use RFC instead of R6, delete all biasing resistors and use Constant Current Source to drive Base..
And learn first how to bias a transistor.
Download the latest DK for ADS from

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R4 seems like a typo error. 68 makes no sense. 68k works with 10k down for a bias ratio but the bias current is insufficient to drive a 50-ohm termination.
Using a collector choke of sufficient impedance but not excessive is often used when the output is expected within a decade of the frequency range. But without an Emitter resistor to linearize the Vbe to Ic current modulation, the result expected harmonic distortion or asymmetric peak amplitudes unless feedback is used to reduce the variation in Vbe.

One important thing when using these SiGe transistors is they have low absolute collector-emitter voltage.
Initial post supply the circuit at 9V which is totally inapropriate. A DC voltage between 2V and 3.3V should be used.

BFP420 datasheet doesn't suggest a biasing method taking temperature and type variations into acount. The specified measurement circuit, also applied in the test setup by BigBoss isn't well suited for practical applications. You can refer to Infineon application notes, e.g.

It seems the OP in the original question and my previous response proved that I did not read :confused:the datasheet.
But the Design Guide confirms my suggestion :cool:to use negative feedback for linear bias on Grounded (common) Emitter amplifiers to reduce distortion.


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