Like most things in electronic design there are trade offs. I wanted to be able to more fully understand the realtionship between Zo, the feedback ratio and operating current.
My thought was that it had to be be better than a 560 ohm resister in the drain circuit and amplifier stage with no feed back at all. The objective was to improve the signal handling capability and dynamic range without messing up the match by to much.
For the parameters with IC and N I need an equation that provides al least a ball park figure for Zo. Try as I may I can't see how to solve this.
Gain has its own problems of dynamic range with high Zo with a Vs of 10Volt as well as IP3 for the stage. Feedback has the advantage of added linearity reducing other undesired products of ajacent and large signals.
Sure but I'll bet IP3 and overall linearity are considerably worse.
1% resisters of such values are like hens teeth. ;-)I would be more tempted to run at 10mA ;-) and 2.5 ohms with RE of 470E. Then Rseries match of 560E. Should Zin/hFE not be added to this?
Sorry I have never been much of a fan of resistive matching for RF unless absolutely no other better way exists. One needs to achieve a good design balance for the full spectrum of requirements most times.
Retro fitting to existing circuits has it own problem of never enough space or remote mounting adding even more problems. But if the transformer feedback proved inferior I'll give it a try.
A perfect match is not the only requirement but one of a set involving noise, dynamic range, large signal handling, linearity.... It takes a considerable amount of time and often one has to think real hard to find a way of evaluating what you need to know. More frustrating is perhaps better because I know how but lack the equipment. For messing about there is no chance of recovering a large outlay on seldom used equipment.
But a sage with no feedback has no such protection or is the IC current drive to the resister match drain load sufficient to overcome the reflected impedences?
I am happy to reduce the gain down to 6 dB or whatever the noise figure is at the filter input plus 1 if needed because it is far easier to add a good low noise 50 ohm in/out stage at the antenna input ahead of the preselector or the preselector input which is 50E.
Ok but out of interest and because it has now become a mission I would still like to know how to calculate the approx Zo for the emitter feedback circuit given.
I am busy putting the final touches to ALC on my cheap and cheerful Chinese signal generator and that will allow me to more easily check the filter under both conditions. I gave up trying to control the tubes large output with junction fets or cathode degeneration and instead regulated the plate voltage. Pair of signal diodes + TL431 + 4N35 optocoupler and IRF830 as the series pass element feeding a 150E resister ahead of the smoothing capacitor. Rectified and regulated heater voltage was used to supply the TL431 and opto diode. The 4N35 CE shunts the gate and 15 volt protection zener driven by a 100K resister by the plate voltage connected to the drain. Crude but works well once the TL431 is stabilised. Then to modify my old AVO signal generator with 6J5 tube and figure out how to mount the stuff. ;-) A BF459 and J310 cascode works fine as a 6J5 replacement.
Only one problem on one range the plate supply current needs to be less than 1mA@20V and my PFET leakage Idss is that. A BU406 chosen for good gain does no better. VCE sat of the opto is 0.15V. A 200V darlington may do it but I don't have one handy.
Regards and thanks for your adviced and insight to the problem.
Peter