Re: Searching for IMD

Henning Weddig


please compare the schematics of the Wes Hayward BiDI amp and Ashars version.

As Wes states his version is designed for 15 dB gain, resulting that he has a series feedback (680 ohms + cap) from collector to base. The biasing is done with two extra resistors.

Ashar uses only the biasing and feedback with the two resistors.

The result is that Ashars amp has about 20 dB gain, worse freqeuncy response (due to the limited ft of the 2N3904) and worse input reflection (S11). Due to the higher gain IMD must be worse at the same input level. If the textbook curve applies (3 dB for every 1 dB of input power change) the IMD will be 15 dB worse.


Henning Weddig


Am 15.09.2018 um 06:44 schrieb Glenn:

Post 59439 is partly incorrect.  Just after I posted mentioning the visible clipping I realised, stupid me, was measuring gain during clipping which of course compresses the result.

I deleted that post. And have attached the new plots where you can see the  gain difference between uBITX and Hayward version. This obviously partly accounts for the difference in input levels before output clipping occurs. The biasing arrangement gives different Iq also.  Gain is reduced about  4dB in the Hayward version I built over uBITX. It now accords more closely also with  his test results of 15.5dB I get 16dB at 30MHz.

Haywards paper gives some values for varying the gain to other values also.

deleted this:-
On Sat, Sep 15, 2018 at 12:16 PM, Glenn wrote:
Hi Ashhur,
I just built up a proto of both versions on vero board.

As a rough check of IP3, since I am not set up to do proper measurement of IP3, I found that the 'visible distortion' of the Hayward version occurs about 10dB higher than the uBITX version.  ie the point of clipping (negative side in this case)  
~-22dBm for uBITX
~ -13dBm for Hayward.   This would then imply IMD is better.

In my set-up I found gain to be similar and pretty flat. I used junk-box parts of mixed tolerances. I built only one side of the Bi-Amps.
uBITX:-   ~ 12dB gain, flat over the range 3-45MHz flat within 0.8dB
Hayward:- ~ 10db gain, flat over same range within 0.6db


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