Transistor amplifier won't couple power, Any thoughts why?

[ElectroTactical]

I've calculated out a class C amplifier while working with RF Power Amplifiers 2nd Ed. by Marian Kazimierczuk and the Spice model works of my design works wonderfully, but when I build up the circuit I get a large amount of current draw ~100-200 mA but hardly any output at all.

I notice maybe a ~10-20mA increase in power draw when connecting my 4.7Ohm load. The Xc of the capacitor I'm using is ~2Ohms at 7.2MHz so it's not the coupling cap. I must be doing something wrong.

I do realize that for the sake of clarity I'm going to point out that this is on a breadboard, but even if I adjust the frequency down a lot and redesign for ~1MHz I still get a problem with coupling of powers. Whenever I try to design an amplifier I always end up with funky coupling problems like this and I have no idea why since this design even has the matching network.

I'm using a function generator to bias the transistor with a DC offset on top of the signal, which is the only thing I've been able to come up with as an issue, as maybe it needs significant amounts of bias current but I thought this should be fine since I'm using an IRF510 MOSFET.

Any ideas? Feedback is much appreciated.

 

[Colin]

You need to show values. Consider the whole circuit as a voltage divider.

 

[Mikebits]

Sounds like your amp is oscillating.

 

[JimB]

ElectroTactical, you have a link to some picture or other (schematic ?), my system refuses to display that link, can you please copy the picture/schematic/?? directly to the forum please?
Use the Upload a File button at the bottom right hand corner of the page.

JimB

 

[audioguru]

This image was posted as a JPG on another website:

 

[ronsimpson]

I can see both pictures but I can't read them. Too small.

 

[audioguru]

The original was a Jpg file type so when I enlarge it the Jpg fuzziness makes it hard to read.

 

[JimB]

Looking at the schematic (Thank you AG),

If I read the values correctly, L1 is 411nH, which gives an Xl of about 18 Ohms.
This is a bit low, if you want a load impedance of 4.7Ohm, make L1 have a reactance at least 47 Ohm.

What is that parallel tuned circuit doing?
I calculate that it has a resonant frequency of about 19.7 MHz.

As a general comment, you have no decoupling capacitors on the 12v supply to the MOSFET.

If you built this on a plug in type breadboard, it probably stands no chance of working.
Have a look here:
https://www.electro-tech-online.com/articles/breadboards-how-bad-are-they-at-rf.278/

JimB