I'm trying to simulate a push pull amplifier in Spectrasys to replicate the results I see on the bench in a current design. My problem is that the gain shown in Spectrasys does not match measured results. I would expect to see a gain which is 3 dB higher than the single amp gain, minus the output balun losses. In practice with a pair of 19 dB amplifiers in this configuration I get a gain of 20.7 dB, Spectrasys is showing 18.7 dB. I initially assumed that the problem was caused by the transformer but I now wonder if it is a coherency problem. Does anyone have any experience with this type of model to help with the fault finding?

[file]test_push_pull_amp.wsx[/file]

[file]test_push_pull_amp.wsx[/file]

1. There is a units bug in the table if you use the up and down arrows with the measurement you added to the end of the table that has different units than the prior last measurement. This bug has been fixed internally and will go out in the next release of the software. The work around for now is to not use the up / down arrows in the table ... just add the measurements to the table in the order you want them.

2. I added the DCR measurement to see what impedances the RF amps were seeing â€¦ they were not seeing 50 ohms. From circuit theory for an ideal transformer Zin = (N1/N2)^2 Zload, where N1 is the primary turns ratio and N2 is the secondary turns ratio. If you change the transformer to the impedance ratio (instead of turns ratio) then basically you are removing the square from the above equation. We know that the load impedance seen by the transformer secondaries must be 100 ohms. â€¦ So there must be a 2 to 1 impedance ratio. Now, you will notice that each of the RF amplifiers see 150 ohms as their load impedanceâ€¦ this is ok â€¦ if you go through all the math the 50 output impedance is transformed to 100 ohms which is in series with the 50 ohms from the output of the other amplifier.

3. You can increased the reverse isolation really high (200 dB) so that you can just look at the primary effects â€¦ gain, impedance, etc â€¦ before examining all the effects of the push pull configuration.

4. You must be careful using measurements like ECGAIN â€¦ or any of the E measurements â€¦ because they are equation based â€¦ and they completely ignore any simulation results â€¦ they are strictly based on cascaded values based on what the user typed into the parameters. Since cascaded equations (E based measurements) ignore more than one path the EOP1dB will be the wrong answer â€¦ just like a spreadsheet â€¦ you will need to use OP1DB to get the true performance of this type of circuits since it has multiple paths.

5. I don't believe the gain of this configuration can increase above that of the individual amplifiers (unless slighty due to matching) â€¦ so you will only get 19 dB of gain. However, the power capability definitely will improve by 3 dB â€¦ as seen by the OP1dB measurement.

6. During the debugging process I turned off noise, intermods, and harmonics â€¦ to get quick answers. This makes it much easier to look at amplitudes and phases of individual components without the clutter of all the other components.

I have updated and attached the file.

There are also a few shipping examples you can look at in the directory:

C:\Program Files\GENESYS...\Examples\SPECTRASYS\Amplifiers\

Dual Hybrid Matrix Amp.wsx

Harmonic Suppressed Amp.wsx

[file]test_push_pull_amp_updated.wsx[/file]