Help with Bajoie BJ-300 Chinese amplifier

[SuperLid]

Wow! 30 MHz and six legs. New one on me.

73

Just about every Icom HF radio PA uses them.

 

[VA7JHI]

Negative feedback is all about reducing gain. Maybe that will tame a transistor that has ten times too much gain? Never had to try.

R10/R11 and R13/R12 provide negative feedback.

It appears to be set for unity voltage gain (1k for both resistors).

I imagine the input transformer supplies needed voltage gain, and the push-pull output only supplies current gain.

Regarding having the "wrong frequency" for the transistors, all transistors have a gaussian fall-off, with "ft" defined as the unity-gain frequency. I do not understand how having too high an ft would make them inappropriate for HF, especially since the voltage gain is set by negative feedback.

Personally, as an RF engineer, I don't really have a problem with the schematic or the components.

 

[nomadradio]

too high an ft would make them inappropriate

Not so much inapropriate as unstable. An amplifier circuit appropriate for a 30 MHz transistor may present an oscillator circuit for a part meant to run at a frequency ten or twenty times higher.

73

 

[VA7JHI]

Not so much inapropriate as unstable. An amplifier circuit appropriate for a 30 MHz transistor may present an oscillator circuit for a part meant to run at a frequency ten or twenty times higher.

That's what bypass caps are for. I see several of them in the circuit. They need to be physically close to the high ft transistor.

I'd rather have too much ft than too little!

 

[SuperLid]

That's what bypass caps are for. I see several of them in the circuit. They need to be physically close to the high ft transistor.

I'd rather have too much ft than too little!

Then build it. I wish you luck!

 

[VA7JHI]

Then build it. I wish you luck!

As an RF Engineer, I've done dozens of such designs.

This one might work a bit better with high ft transistors with some PCB layout tweaks, but I see nothing obviously wrong with it.

 

[nomadradio]

PCB layout tweaks

No doubt.

73

 

[SuperLid]

As an RF Engineer, I've done dozens of such designs.

This one might work a bit better with high ft transistors with some PCB layout tweaks, but I see nothing obviously wrong with it.

Rule of thumb here, is fT x 2 of design frequency.

The amp is $100. Buy one and make it work, then share your mods and results.

SL

 

[VA7JHI]

Rule of thumb here, is fT x 2 of design frequency.

That's only possible for a gain of two or less. Unless you want an attenuator!

fT is defined as the point of unity gain in a gaussian (6db/octave) roll-off.

The amp is $100. Buy one and make it work, then share your mods and results.

Oh yea. Like I really need one more project…

 

[VA7JHI]

Not so much inapropriate as unstable. An amplifier circuit appropriate for a 30 MHz transistor may present an oscillator circuit for a part meant to run at a frequency ten or twenty times higher.

But with the given feedback resistors, it's running at unity gain.

By your logic, just about any op-amp design should be an oscillator.

 

[SuperLid]

That's only possible for a gain of two or less. Unless you want an attenuator!

fT is defined as the point of unity gain in a gaussian (6db/octave) roll-off.

Oh yea. Like I really need one more project…

Talk is cheap.