Correct in case of series resonant tank but wrong with the parallel resonant tank
Yes it has but has its bias been understood so that it is used to advantage? The only place where I see its advantages exploited well is the circa 1960's circuit showing the front end of an Eddystone EC10 Communications Receiver (see above). The LTspice simulations given early on in this thread show that bias is critical and there is a "tipping point" the good properties of Common Base reach a maximum and then turn bad, the Eddystone EC10 circuit knows this because an operator can adjust the bias because he is turning an AGC (or RF gain) pot.
As regards your line level preamp example I agree that it is common base for the first and second transistor but is it? My understanding of Common Base is that the Base is connected to signal ground or shorted to signal ground for AC by a good size capacitor.
This so called Cascoode for tubes or transistors really is a terrible bad jargon thing and just guarantees that the two devices thus connected are not properly biased.
I just don't like Cascode much prefer the use of Cascade as it is a real word and explains.
For valves/tubes the Cascade is a compromise, if you go with setting the upper Triode Grid at 1/3rd of HT it means that the bias point for both Triodes is different and not ideal. If you set the voltage at the upper Triode Grid at a value that makes the bias point the same for both valves it pushes the HT up and then power dissipation is getting higher so what you gain by keeping Grid capacitance down you loose by higher HT voltage. They used the Cascade in Valve TV tuners to get VHF UHF performance at low cost.
I ran a simulation for a Cascade and copy the LTspice circuit below:
I'm just not getting excited about Cascade and any idea that (for solid state) a Cascade configuration is anywhere near correctly biased Common base.
As for Miller effect well hardly much justification for having to call it Miller effect because its glaringly obvious that the Capacitance between Grid and Anode will be static Grid Capacitance times the increased Anode voltages that increases as function of gain.
For valves/tubes the Cascade is a compromise, if you go with setting the upper Triode Grid at 1/3rd of HT it means that the bias point for both Triodes is different and not ideal. If you set the voltage at the upper Triode Grid at a value that makes the bias point the same for both valves it pushes the HT up and then power dissipation is getting higher so what you gain by keeping Grid capacitance down you loose by higher HT voltage. They used the Cascade in Valve TV tuners to get VHF UHF performance at low cost.
I ran a simulation for a Cascade and copy the LTspice circuit below:
I'm just not getting excited about Cascade and any idea that (for solid state) a Cascade configuration is anywhere near correctly biased Common base.
As for Miller effect well hardly much justification for having to call it Miller effect because its glaringly obvious that the Capacitance between Grid and Anode will be static Grid Capacitance times the increased Anode voltages that increases as function of gain.
Last edited:
It’s not bad jargon, and I don’t see how a name affects the use of a circuit. Wikipedia gives the origin of the name https://en.wikipedia.org/wiki/Cascode where it was coined for good reason.
In applications like the Eddystone receiver, the base voltage is varied to change the emitter resistance, and hence the signal current into the emitter. That is a cheap and nasty thing to do, but 60 years ago it was the best solution possible in RF design. That isn’t a typical use of a common base design as linearity changes significantly with bias (and in the case of the circuit originally posted, Q).