The load is 50 ohms but the collector resistor value is much too high at 600 ohms which reduces the output level.

 

How about this one?

 

If its voltage gain is 18dB from its input to its 75 ohm load then it is 8 times.

 

Ylli, Please send your asc file.

 

Ylli, Please send your asc file.

 

How about this one?
View attachment 162068 View attachment 162069

Hey Ylli! Could you explain me how you did the input and output matching network?
I have tried to understand but I do not get the same results as you

 

Hey Ylli! Could you explain me how you did the input and output matching network?

I'm a technician not an engineer, so my approach is more trial and error than analytic.

Using LTSpice and Elsie (http://tonnesoftware.com/elsie.html), started out with a bare-bones cascode arrangement. Using LTSpice, I connected an input source through a 75 ohm resistor to the base of the lower transistor. Still using LTSpice, I measured the voltage at the transistor base and the current through the 75 ohm resistor. I could then see the phase relationship between the voltage and current.

I expected the input to look like a parallel RC. For matching, I felt it was easier to look at it as a series RC. So I added an inductance in series with the input in an attempt to tune out the equivalent input series capacitance. I varied this inductance until the voltage at the input and the current though the 75 ohm resistor were in phase.

Now I measured the voltage at the input and the voltage across the 75 ohm resistor and could calculate the equivalent series input resistance.

This equivalent input R was around 10 ohms, so I needed an impedance step up from the base to 75 ohms. Using Elsie, I came up with the rough values of a simple LC network (shunt C, series L) to do that. Put those values in front of the amp, combining the series L I just determined with the series L needed to turn out the equivalent input C.

I knew I needed to AC couple the input, so I added a C in series with the L. Bumped the L up to the next standard value and tweaked the C I just added for a centered response.

On the output, I knew I needed a impedance transformation that would match a 75 ohm load to 1K or so at the collector. Used Elsie to come up with some starting values. Plugged them in to the circuit and then used LTSpice to tweak all the added L's and C's to get the response I wanted with reasonable gain.

Probably not optimum, but it works (at least in the sim).

 

Sorry ..but this transistor is not very good for VHF amplifier for FM band
2N2222 is a switch transistor and is let say good as oscillator but not for VHF preamp.
So if you can find or use something else like:
BFR91A
BF199
BFG65
C3355
S9018

 

Here is schematic:

 

I'm a technician not an engineer, so my approach is more trial and error than analytic.

Using LTSpice and Elsie (http://tonnesoftware.com/elsie.html), started out with a bare-bones cascode arrangement. Using LTSpice, I connected an input source through a 75 ohm resistor to the base of the lower transistor. Still using LTSpice, I measured the voltage at the transistor base and the current through the 75 ohm resistor. I could then see the phase relationship between the voltage and current.

I expected the input to look like a parallel RC. For matching, I felt it was easier to look at it as a series RC. So I added an inductance in series with the input in an attempt to tune out the equivalent input series capacitance. I varied this inductance until the voltage at the input and the current though the 75 ohm resistor were in phase.

Now I measured the voltage at the input and the voltage across the 75 ohm resistor and could calculate the equivalent series input resistance.

This equivalent input R was around 10 ohms, so I needed an impedance step up from the base to 75 ohms. Using Elsie, I came up with the rough values of a simple LC network (shunt C, series L) to do that. Put those values in front of the amp, combining the series L I just determined with the series L needed to turn out the equivalent input C.

I knew I needed to AC couple the input, so I added a C in series with the L. Bumped the L up to the next standard value and tweaked the C I just added for a centered response.

On the output, I knew I needed a impedance transformation that would match a 75 ohm load to 1K or so at the collector. Used Elsie to come up with some starting values. Plugged them in to the circuit and then used LTSpice to tweak all the added L's and C's to get the response I wanted with reasonable gain.

Probably not optimum, but it works (at least in the sim).

I find this workflow pretty amazing. There are still some parts that elude me, such as the initial approximation of 1k at the collector. But, regardless, I ask - are there any books, videos, articles, or websites that cover your workflow in greater detail or that inspired or refined your workflow in some way? I am comfortable with engineering maths, Bowick, Smith charts, Miller effect, and the basic amplifier designs, including the cascode. Thank you.

 

 

Bordodynov, thanks for the example.

I was familiar with the .step command of LTspice, but it was still good to be reminded to stick to some sort of standard value scheme so as not to overload the options.
It appears that this iterative, quasi-monte-carlo technique is all about carefully choosing certain constant conditions - such as dc collector current, Vce, etc (likely from data sheet), and limiting the amount of variable degrees of freedom you have in your chosen topology in order to keep options manageable. Having a nanoVNA or something similar to characterize an antenna also helps, for this particular application. However, the real trick to this technique appears to have the experience to know what topology areas to initially set as constants, and which to iterate upon.
There is a also a .net command in LTspice that you can use with an ac analysis to get Zin, Zout, S-params, Y-params, etc... This command is a bit confusing to use, but Fesz Electronics covers it on youtube.

 

Great results from Bordodynov and Ylli

Here is a circuit using a 2N3904... 2N3904BU (from ONSemi) is ok for VHF. Biased at 10mA and 7Vce
I'm guessing, same as 2N2222 ?

It might not be very easy to get a proper grounded emitter with the capacitor
so I think it is good to leave the emitter to ground directly then bias the base with a potentiometer.


I've done this amplifier in the past.
If it helps, I can build it and show the result on a VNA?. I think, will most likely get a better S21