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I designed an RF amplifier that should amplify signals in the FM band of 88 to 108MHz. The circuit is as shown below: enter image description here

The input to the circuit is connected to an USRP through an SMA cable. The output is connected to an half-wavelength (1.5m) dipole antenna. The input power is in the order of 50 to 100mW. The output which is close to 2W in my simulations is about the same power as the input in the real world. What is causing this?

Edit: I am actually using RD06HVF1 transistor. I am using the spice model for RD06HHF1 as the specs are identical.

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  • \$\begingroup\$ Well, you have a lowpass filter on the input (C6, R6, R4), and your FET has a Vth range of 1.9-4.9V, try different biasing and remove C6. \$\endgroup\$ – Captainj2001 May 28 '18 at 12:44
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    \$\begingroup\$ I added the 100n inductor to provide ac impedence and prevent the input signal to be grouned. Would it still act as a low pass filter with L4 inductor? 100n proivides 62 ohms of ac impedence to the input signal. That together with the gate capacitance of the mosfet should oscillate to provide high impedence at the input and it works well in my simulations. \$\endgroup\$ – Kevin Selva Prasanna May 28 '18 at 12:54
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    \$\begingroup\$ How is this wired up on the PCB? When you are dealing with such small values of inductance and capacitance, parasitics matter, and could cause your matching to be thrown of. What band are you measuring over? And how? \$\endgroup\$ – Joren Vaes May 28 '18 at 12:54
  • \$\begingroup\$ You're right, should have seen that, I'd still try adjusting the gate bias, best of luck. \$\endgroup\$ – Captainj2001 May 28 '18 at 12:57
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    \$\begingroup\$ Dipole antennas are intended for a balanced RF feed. Try using a monopole whip antenna and a ground. \$\endgroup\$ – Andy aka May 28 '18 at 14:40
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the RD06HHF1? I could see HVF1 maybe at band III, but the RD06HHF1 is only specified to 30MHz!

You may want to have a look at the gate capacitance of that RF Mosfet, they are usually not small and need some input matching (Usually at least an L network, sometimes two stages if you want broadband gain).

Take a look at some of the designs for the 2M band, what you have will probably be unstable if you do manage to get meaningful gain out of it, layout is also everything in RF power amps, and again the ham 2M stuff will show you how it is done (Solid planes and the like).

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  • \$\begingroup\$ Sorry about that. I am actually using RD06HVF1. I am using th e Spice model of RD06HHF1 as the specs are identical. Thanks for your answer. \$\endgroup\$ – Kevin Selva Prasanna May 28 '18 at 15:25
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    \$\begingroup\$ The specs are not identical, for all that the datasheet is woefully short on real specs. Annoyingly the data only gives the S parameters at a single frequency... And for RF it is these that you want (Granted S parameters are SMALL SIGNAL scattering values so less then ideal for power stages). \$\endgroup\$ – Dan Mills May 28 '18 at 17:24
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The output which is close to 2W in my simulations is about the same power as the input in the real world. What is causing this?

The number one suspect would be miller capacitance. I would guess if your circuit would have input and output tuning circuit simular to this it would give you the results you are looking for:

schematic

simulate this circuit – Schematic created using CircuitLab

EDIT Note: You might even need to stick two or more output devices in parallel to get the proper interelectrode capacitance for this tuning

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