VHF RF Amplifier Design with 2N2222 - I have some issues

Ylli

Joined Nov 13, 2015
1,061
Hey Ylli! Could you explain me how you did the input and output matching network? :D
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).
 

ccfly

Joined Jun 22, 2008
4
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
 

HenryLH

Joined Jun 18, 2022
2
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.
 

HenryLH

Joined Jun 18, 2022
2
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.
 
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