MOSFET Vs.  VACUUM TUBES

TUBES (VALVES in England), or gas state devices, are voltage controlled devices. In the TUBE's simplest configuration, triode amplifier, the plate current is function of grid voltage, as drain current is function of gate voltage in a MOSFET

TUBES and MOSFET's has very similar curves characteristics, in particular if we compares TUBE's in pentode configuration with the called  "depletion mode MOSFET".

There are more similitude between MOSFET's and TUBE's than MOSFET's and BJT's. 

The characteristics curves define the Total Harmonic Distortion composition. A deeply analysis can see in PAPER

 

Output characteristic of vacuum tube 6L6 in pentode configuration.

      

There are three basic tube amplifier configurations: triode , pentode and a feedback pentode variant called ultralinear. Pentode configuration is more efficient than triode in terms of load power. Triode amplifier has more natural sound because the THD composition has low order harmonics, but has narrow bandwidth and less efficience.

Tubes has very low transconductance (around 0.01 mhos) and low maximum plate current. There are a very few tubes amplifiers with direct load connection. The vast majority has output transformer couple. This transformer allow operate with reasonable efficiency with 8 ohms load.  Output transformer limit the bandwidth, in amplitude a phase, and introduce distortion. Is very expensive to build a hi-end output transformer.

In order to have more plate current, two or more tubes may put in parallel. This is a difficult task because tubes parameters varies with time, and the matching parameter, the transconductance, is difficult to measure.

 

MOSFET advantage TUBES in all aspects:

-Very higher transconductance (two magnitude order, typically 3,5 mhos)

-Direct load coupling (transformless) this mean higher bandwidth, less distortion, more damping factor.

-Better gain bandwidth product.

-Power bandwith one magnitude order higher than TUBES.

-Easy paralleling.

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