A bandwidth of 1MHz would be good.Do you need it to work only at 7MHz?
Do you have a bandwidth requirement?
Yes, it should be a linear amplifier receiving a sinewave signal and producing low distortion at the output.Do you want a linear amplifier?
Frank
The purpose for this is for me to have a better understanding of RF circuits. I want to see what can be achieved with easy to find transistors. The simulation is very important to me. I expect to see a low amount of harmonics (> 40dB) and to use a pure 50 ohm resistor as a load and still have the 10dBm at the output.No simulation necessary. At 7 MHz, any 100 mW RF transistor (and most audio transistors) will work. The only thing to adjust is an optimum base bias and a tuned LC output circuit in the collector. And a good impedance transformer (tap on the output coil) to achieve a good power match to 50 Ohm load or antenna.You can check old radio-amateur circuit design for 7 MHz band.
The purpose for this is for me to have a better understanding of RF circuits. I want to see what can be achieved with easy to find transistors. The simulation is very important to me. I expect to see a low amount of harmonics (> 40dB) and to use a pure 50 ohm resistor as a load and still have the 10dBm at the output.
I already built a few VFO's and the level after the JFET is very good (4Vpp - but on high impedance load). On one of the diagrams, there is a follower transistor (repeater on emitter) and next a standard RF amplifier but the level is too low. There is no tuning circuit in the collector. If I tried to increase the level, the distortion greatly increased.
Based on your recommendation (and limited by my experience), I made the following circuit. At least in theory, it provides a good gain (30dB) and has an output level of about -3dBV ( = 10dBm).
I know there is some analysis to check if the circuit is stable. Maybe somebody can have a look and advise what improvements can be done. My main concerns is that I have no idea what the input impedance seen at the base of the transistor is and most likely there is a mismatch of impedances there (it can be seen there are pretty high distortion levels at the output of 3dB PI attenuator - which, I believe indicates a mismatch of impedances)
Many of the values of inductors, capacitors, resistors from the diagram are selected by successive tests, to see what gives the best results for simulation and probably they are far away from a perfect design.
Regards,
Nicolae
View attachment 83742
Thank you Jiripolivka. I will build this circuit (maybe in two weeks) and inform you about the results. I will lower the resistors as you advised.
Best regards,
Nicolae
I'm tempted to be boring and just post up a classic circuit of a CATV based design using a 4:1 TLT and feedback to produce a broadband amp with very well controlled 50R input and output but here's a quick and dirty tuned version for 7MHz that should have +15dB gain and 50R input and output at 7MHz.
it should deliver +10dBm with low harmonic distortion.
The transistor will get quite warm and I'd recommend trying various types here to see how they affect linearity. eg 2N2222A, 2N3866 2N5109 2N3904 etc etc.
View attachment 83879
- - - Updated - - -
For completeness here's a quick CATV inspired design which gives about 15dB gain and should give a flat gain across a wide bandwidth (eg 1MHz to maybe 30MHz depending on the transformer design) It also should give good 50R input and output match across the entire frequency range because of the feedback techniques used.
This is based on a classic circuit and you can wind the 4:1 TLT on a FT37-43 ferrite core.
View attachment 83880
Hi,Your circuit show ideal conditions to be an oscillator around 7MHz as well. There is an old joke in RF design: To get an oscillator, try to build an amplifier. :wink:
Look at the gain v phase of your circuit. You have massive gain (>40dB) around 8MHz, together with a rapid phase shift from -180° to 180°
Any stray capacitance or bad circuit layout may bump your circuit into oscillation.
The stability plot shows K going well below zero in your operating band. K should always stay above 1 with some margin.
Your design is unstable between 2-20MHz
I'm tempted to be boring and just post up a classic circuit of a CATV based design using a 4:1 TLT and feedback to produce a broadband amp with very well controlled 50R input and output but here's a quick and dirty tuned version for 7MHz that should have +15dB gain and 50R input and output at 7MHz.
it should deliver +10dBm with low harmonic distortion.
The transistor will get quite warm and I'd recommend trying various types here to see how they affect linearity. eg 2N2222A, 2N3866 2N5109 2N3904 etc etc.
View attachment 83879
- - - Updated - - -
For completeness here's a quick CATV inspired design which gives about 15dB gain and should give a flat gain across a wide bandwidth (eg 1MHz to maybe 30MHz depending on the transformer design) It also should give good 50R input and output match across the entire frequency range because of the feedback techniques used.
This is based on a classic circuit and you can wind the 4:1 TLT on a FT37-43 ferrite core.
View attachment 83880
Hi G0HZU,Here's a screenshot of the CATV amplifier design.
You can see it has about 15dB gain across 2MHz to 30MHz and K>1 and B1>0 for stability.
You can also see it has good input and output match for a 50R system and should also produce +10dBm with harmonics much lower than -40dBc. But this is just a simulation.
It achieves the flat gain and the controlled I/O impedance through negative feedback. eg R5 lowers the gain as it get bigger but raises the input impedance as it gets bigger.
R3 lowers the input impedance as it gets smaller and also reduces gain through negative feedback. So if you get the balance of R3 and R5 correct you can get 15dB gain and also good device matching.
You should get good stability with this amp if you use a good PCB layout.
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