The output rp approximates the load resistor Rl in parallel with the following grid resistor. If the load varies can always be damped with a paralleled resistor just as the ancients did with transformer coupling. 🙂
jhstewart9,
That is a good way to improve the frequency response of a circuit.
Just reduce the total load impedance.
Swamping works!
The lower the load resistance, the lower the gain.
The advantage of the cascode then becomes linearity, but not gain.
Tradeoffs.
That is a good way to improve the frequency response of a circuit.
Just reduce the total load impedance.
Swamping works!
The lower the load resistance, the lower the gain.
The advantage of the cascode then becomes linearity, but not gain.
Tradeoffs.
A low load Z keeps the cascode stage honest, but it's the stage below it that is difficult to get linear in the 1st place. Most tubes aren't linear into a near fixed voltage load or low Z diode function load. The beam deflection tubes are linear (current, at least for 6JH8) into a fixed V load.
An interesting topology might derive from the latest "new" series Schade (UnSet, CED) scheme (using cascode stages up top) combined with the beam deflection tube (on the bottom).
A set of R dividers from the cascode plates back to each of the cascode grids (then down to a ground ref.) would make sure the cascode output V's are proportional to the cascode cathode input V's. So producing a low output Z from the cascode(s).
The beam defl. tube performs the input V to cascode linear drive function. The "new" series Schade top acts as an impedance transformer for the BDT. The cascode input V is some fixed ratio of the cascode output V due to the grid N Fdbks, and with the same currents. Of course some small Mosfets could perform the cascode functions well here too, without heater voltage issues.
note:
The BDT outputs have a lowish Rp due to the Mu 6 of the BDT. The Mu factor comes from the dual plates acting as deflectors also, acting as N Fdbk to the beam deflection resulting from the input deflectors. Both produce transvers E fields to the beam direction, giving linear deflection results.
There was a pentode version of the BDT designed also, the 7763 tube. It had a screen grid between the deflectors and the plates. Only a few were made by GE before the tube era ended. It had high Z outputs.
An interesting topology might derive from the latest "new" series Schade (UnSet, CED) scheme (using cascode stages up top) combined with the beam deflection tube (on the bottom).
A set of R dividers from the cascode plates back to each of the cascode grids (then down to a ground ref.) would make sure the cascode output V's are proportional to the cascode cathode input V's. So producing a low output Z from the cascode(s).
The beam defl. tube performs the input V to cascode linear drive function. The "new" series Schade top acts as an impedance transformer for the BDT. The cascode input V is some fixed ratio of the cascode output V due to the grid N Fdbks, and with the same currents. Of course some small Mosfets could perform the cascode functions well here too, without heater voltage issues.
note:
The BDT outputs have a lowish Rp due to the Mu 6 of the BDT. The Mu factor comes from the dual plates acting as deflectors also, acting as N Fdbk to the beam deflection resulting from the input deflectors. Both produce transvers E fields to the beam direction, giving linear deflection results.
There was a pentode version of the BDT designed also, the 7763 tube. It had a screen grid between the deflectors and the plates. Only a few were made by GE before the tube era ended. It had high Z outputs.
Last edited: