There are two parts of Thermionic emitters:-
A) Directly heated emitter
(i) Cathode is directly heated by passing current.
(ii) Thermionic current is less.
(iii) Energy consumption and life is short.
B) Indirectly heated emitter
(i) Cathode is indirectly heated.
(ii) Thermionic current is more.
(iii) Energy consumption and life is long.