Moderm RF transistors in SMD packages.

[1608cc]

One commercial transmitter had VCO semirigid coax between two 5mm thick pieces of Armaflex like foam.

Will try with this - 3mm ID and RG316.

obraz_2025-02-03_004515164.png

[pukje62]

Albert:

Is the push-push setup for 2 x BFG135 something like this:
Can we also use the BFG591 in the same setup ?

[Albert H]

That's pretty much the same!

[Krakatoa]

BFQ19 is the same silicon as BFR96 but in SOT89 package, if it is of any help.
In fact, there are many transistors of the old through hole times that went SMD but was never stated in the datasheets. For example, the BF199 is now the BFS20, the BF324 is the BF824, all same silicon, so, in some way, they never went obsolete, only the package!

[EFR]

https://www.eevblog.com/forum/rf-microw ... mplifiers/

[Shedbuilt]

I was going to mention MRF4427, but I see that's also listed as obsolete now.

[jvok]

Some of the no-name chinese fabs make SMD equivalents of the ss9018 labelled as s9018 (only one s). No idea if they're any good

https://www.lcsc.com/mobile/products/Bi ... _z=n_S9018

[EFR]

Some of the no-name chinese fabs make SMD equivalents of the ss9018 labelled as s9018 (only one s). No idea if they're any good

https://www.lcsc.com/mobile/products/Bi ... _z=n_S9018

I have those, almost same as BC547

[reverend]

A couple of 2SC3357 in parallel should get you a Watt or so.

[nrgkits.nz]

Use the LMX2541 VCO, it's what I used in my last SMD design, very low phase noise and not microphonic. I had one running on the bench connected to a 300W PA with no shielding or anything and there was no hum, it was rock solid. This chip operates in the GHz range, and the output is divided down to your intended output frequency, this eliminates RF feedback and hum. I use a 470uF capacitor in the PLL line and modulate the MPX directly onto this, no overmod with a 30Hz square wave.

[zulu53]

Hi reveverend,
Would you please share a 12V application circuit using a couple of 2SC3357 as suggested?

[reverend]

As long as you use a bit of emitter degeneration (i.e. place a resistor in the emitter of each transistor to ground) to stop one transistor from becoming thermally overloaded, you can pretty much replace any output transistor with more than one. Doesn't work for higher power circuits as you typically need to ground the emitter directly. For lower power, it's no biggy.

The circuit below is for a 1 Watt HF amplifier but the princples are pretty much the same. See the separate emitter resistors (and bypass capacitors) on each output transistor?

hf_amp.gif

Note that the input matching will be different to a single transistor as you have two base/emitter junctions in parallel so the impedance will be lower.

[zulu53]

Thank you for the circuit and explanations.