Ylli, Please send your asc file.
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.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).