Audio Signal Injector

[vu2nan-nandu]

Another useful test gadget!

http://nandustips.blogspot.in/2012/0...-injector.html

Regards,

Nandu.

[unitaudio]

Thanks, very useful indeed and beautifully simple.

Regards,
Paul

[paulsherwin]

If you use silicon transistors it will produce higher frequency harmonics, better for RF/IF work.

[jimmc101]

This circuit relies on the leakage current of the germanium transistors to work. (It probably makes a good audio thermometer.)

With the much lower leakage current of silicon devices, resistors from base to -ve supply for each transistor would be required for it to function (220k?)

Jim

[paulsherwin]

Yes, you're quite right.

[G8HQP Dave]

Could I perhaps suggest that VU2NAN should just post one link to his home page, instead of starting lots of threads here and elswhere (e.g. DIYaudio) to promote individual pages on his website? If we all did what he is doing then forums would rapidly fill up with personal adverts instead of discussion.

[vu2nan-nandu]

Originally Posted by G8HQP Dave

Could I perhaps suggest that VU2NAN should just post one link to his home page, instead of starting lots of threads here and elswhere (e.g. DIYaudio) to promote individual pages on his website? If we all did what he is doing then forums would rapidly fill up with personal adverts instead of discussion.

Hi OM Dave,

I agree with you.

Thank you.

73,

Nandu.

[vu2nan-nandu]

Originally Posted by unitaudio

Thanks, very useful indeed and beautifully simple.

Many thanks, Paul!
Regards,
Nandu.

[Ian - G4JQT]

Am I missing something, or is the circuit in the OP just a PNP astable multi-vibrator? Not sure it would even work without any proper bias on the bases. Must be relying on leakage through the capacitors!

Here's a more reliable design using any cheap NPN silicon transistors. Change C1/C2 to change frequency.

As Paul says in post 3, modern silicon transistors will also be better for IF/RF work.

Ian

[vu2nan-nandu]

Hi OM Ian,

It's just that. Base bias is not required on account of Germanium transistor junction leakages.

That's what makes the circuits using Germanium transistors simpler.

73,

Nandu.

[Ian - G4JQT]

Originally Posted by vu2nan-nandu

It's just that. Base bias is not required on account of Germanium transistor junction leakages.

Yes, perhaps, but the leakage is likely to be unreliable. It may work with some AC125 transistors, but not others.

I've never seen a germanium transistor multi-vibrator circuit without some base biasing resistors to ensure consistent and reliable operation.

I'm not sure that hoping a circuit will function on a variable (and not usually desirable) parameter like internal leakage is necessarily good design practice...

Ian

[Nickthedentist]

...but perfectly OK for simple homebrew circuits, surely.

[Oldcodger]

I've always used a Cmos gate oscillator giving at original frequency something on the audio range works well into the RF range. First one I built was to fault a radio board. I'd bought two non working boards to make one for a project. Worked a treat into the RF range.