[SOLVED] RF amplifier with one or two transistors

[rohitkhanna]

Here's a popular tried & tested circuit --

https://kitsandparts.com/rfamp1.1.php

 

[G0HZU]

Yes, that's pretty much the same classic 4:1 TLT based circuit I posted up earlier but the circuit in your link is configured for much higher gain.

I'd expect it to show more than 20dB gain and presumably it will require a more careful layout to prevent possible instability up at VHF and UHF. I configured my version for 15dB gain but it's essentially the same type of amplifier

 

[rohitkhanna]

Yes, that's pretty much the same classic 4:1 TLT based circuit I posted up earlier but the circuit in your link is configured for much higher gain.

I'd expect it to show more than 20dB gain and presumably it will require a more careful layout to prevent possible instability up at VHF and UHF. I configured my version for 15dB gain but it's essentially the same type of amplifier

I had mine in mind on reading the initial few posts, and though I did see your posts with the same config/ minor variations, I thought i'd post mine anyway..... Haven't done any simulation analysis, but have done quick implementations for fixed lower frequencies (VHF). worked quite well.

 

[Nick_F]

Thanks again G0HZU,

So, 4:1 TLT has in fact two coils with equal number of turns, wounded at the same time. I made another transformer and I have 40uH on each winding. When they are put in series, both measure 152uH.
Now the gain has increased to 19dB at 7MHz.

Best regards,
Nicolae

The 4:1 TLT is 4:1 impedance

Wind it as per fig 4C in the link below (i.e. the 4:1 transmission line transformer)

You have to be careful how you strap the two windings together and the image in the link below shows it quite well.

The 4Zo connection goes to the collector of the transistor.

The strap connection is the low impedance tap point and is the amplifier output.

The other connection should go to your RF decoupled supply voltage. This connection is shown as ground in the link below but you obviously need to connect this point to your RF decoupled supply in order to feed the DC to the transistor via the windings.

http://www.raylab.com/antenna/Transmission.htm

Yes, I would use a 2N3866 but you should really run a heatsink on it as it will run quite hot if you run it at high current.