-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
Help with Bajoie BJ-300 Chinese amplifier
- Thread starter fastfreddie1269
- Start date
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.
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
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!
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
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…
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.
Talk is cheap.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…
How can 1k in series with a cap from collector to base give unity gain ?,
its not a single ended amp,
No feedback gives max gain & max impedance pertubation with different drive level & frequency,
1K is hardly any feedback at all & certainly will not give unity gain in that circuit, it would hardly effect gain at all,
That's why the baoji has 100 ohm in series with a cap, a much more common feedback value,
even lower resistance feedback would reduce gain more & an input resistive pad could also help with input vswr & stop overdriving with a typical cb output power,
The input transformer is a stepdown type as seen in all hf amplifiers of this type, it is not a stepuup with voltage gain,
It steps down voltage by the turns ratio & impedance by the square of the turns ratio,
The input transformer brings the low input impedance of the transistor closer to the 50ohm impedance the radio needs to see,
The circuit does not work how you imagine ! it just looks that way from your theoretical armchair.
its not a single ended amp,
No feedback gives max gain & max impedance pertubation with different drive level & frequency,
1K is hardly any feedback at all & certainly will not give unity gain in that circuit, it would hardly effect gain at all,
That's why the baoji has 100 ohm in series with a cap, a much more common feedback value,
even lower resistance feedback would reduce gain more & an input resistive pad could also help with input vswr & stop overdriving with a typical cb output power,
The input transformer is a stepdown type as seen in all hf amplifiers of this type, it is not a stepuup with voltage gain,
It steps down voltage by the turns ratio & impedance by the square of the turns ratio,
The input transformer brings the low input impedance of the transistor closer to the 50ohm impedance the radio needs to see,
The circuit does not work how you imagine ! it just looks that way from your theoretical armchair.
It doesn't.
It's the ratio of R10 to R11, both of which have the same value.
That sets the AC gain. The cap allows it to have a different DC gain. It self-biases at 0.6 volts DC.
You must be thinking of a common emitter single ended amp where resistors in that position do set the gain,
The base emitter resistors do not self bias @0.6v or set the stage gain of the baoji,
It is a class C amp with the transistor bases grounded through the single turn secondary of the input transformer,
If those resistors did set the gain then virtually all cb & hf amps would be unity gain.
The base emitter resistors do not self bias @0.6v or set the stage gain of the baoji,
It is a class C amp with the transistor bases grounded through the single turn secondary of the input transformer,
If those resistors did set the gain then virtually all cb & hf amps would be unity gain.