(Part 3) How to Design, Build, and Test an RF Linear Amplifier (Input Board)

This multi part video focuses on the critical design aspects of an RF Push-Pull amplifier. The example shown uses an IRF510 MOSFET transistor to explain impedance matching, biasing, amplifier classification, circuit stability, feedback, tuning, power output, harmonic distortion, and other important design characteristics. It also introduces LTSpice as a simulation tool. From this video, the viewer will gain an in-depth understanding of how to design, build, and test an RF power amplifier. Please visit my web site for the RF amplifiers and other products I offer. rflinear-amplifiers.com

[@emailuser3869]

RF Man, the removal of the resistor and replacement while watching the effect of the SWR.... that was very instructive. Thanks.

[@tawfiqulalam8928]

Enjoyed a lot.

[@kasperdasradio]

This is gold Sir.

[@pilobond]

Again, I really appreciate this series. At one point, could you show your LT Spice simulation for IRF510 input impedance? Are you measuring the input impedance at the primary side of the input transformer? VA3GPJ

[@rfmanchannel6915]

I will show how to use LTSpice for impedance measurements in part 4 and yes the measurement is made on the input side of the transformer. I show the basics in Part 2.

[@johnjames378]

Fantastic, I'm learning a lot. Very thorough. Waiting for the next one.
I am a bit confused. Question: The secondary is matched to 50 ohms, is this because the input Z of the IRF510 along with the input resistor is 50 ohms? Or are we just isolating the output of the radio from the input stage of the MOSFET and 50 ohms just happens to be the typical output Z of most radios and the MOSFET doesn't care about input ? In my mind I want to match the output of the transformer to the input Z of the MOSFET. Could you explain this please? Please correct my thinking.

[@rfmanchannel6915]

The standard for all Radio Transceivers and most RF test equipment is 50 ohms (Output impedance). You need to match the input impedance of your RF Amplifiers to 50 ohms. The turns ratio of the input transformer is used to transform the impedance to achieve a 50 ohm match. I hope this helps. RF Man

[@johnjames378]

@@rfmanchannel6915 Right, I got that part. My question is that do we have to then match the input Z of the MOSFET?

[@geirha75]

Thanks for interesting and really educational information on building amplifier. I have a question on the resistor across the transformer. Would be an idea to have a variable resistor there in order to fine tune the Q?

[@karwan6995]

❤❤

[@listings958]

Are the pages you display available for download?

[@tahanprahara]

Can it design for 144mhz sir? Thankyou

[@rfmanchannel6915]

The IRF510 will not operate at this frequency. I have a video for a 2 meter amplifier which may of interested in. czcams.com/video/UxdR1KoKc9I/video.html

[@tahanprahara]

@@rfmanchannel6915 to much power for me... would you like you teach me how to make booster for Hand Talkie 2m 20-60W Sir?

[@dimitrijejakov3767]

Where did you find the formula for calculating the minimum inductance of the primary winding of the RF transformer? That formula is incorrect. The reactive resistance or reactance of the primary winding of the RF transformer at the lowest operating frequency, in this case, 10MHz, should be 5 to 10 times greater than the input impedance, which is 50 ohms in this case. An inductance of 11.2uH for the two windings is acceptable because its reactance is approximately 7 times greater than 50 ohms. You are unclear about the input impedance of the IRF510 transistors. You've placed a 60-ohm resistor in parallel with the input impedance of both transistors, thus lowering the input impedance to 50 ohms and making it somewhat variable with frequency changes. The cost of this is increased drive RF power. The concept of adapting input power from this perspective is not good because it results in increased drive power. The correct way to maintain relatively constant input impedance over the range of 1.8 to 30MHz is through properly designed negative feedback drain-gate. The whole point is for such an amplifier to operate throughout the mentioned HF range and provide approximately uniform output RF power at the output. I have more criticisms, but for now, let's stick to these.