Editing Tetrode (section)

==Bi-grid valve==
In the bi-grid type of tetrode, both grids are intended to carry electrical signals, so both are control grids.  The first example to appear in Britain was the Marconi-Osram FE1, which was designed by [[H. J. Round]], and became available in 1920.<ref name="Thrower"/> The tube was intended to be used in a [[reflex receiver|reflex]] circuit (for example the single-valve ship receiver Type 91<ref name="Scott-Taggart">{{cite book   
  | last = Scott-Taggart
  | first = J.
  | title = Elementary Text-Book on Wireless Vacuum Tubes, 4th Edition
  | publisher = [[Radio Press]] Ltd
  | year = 1922
  | pages = [https://archive.org/details/thermionictubesi00scotrich/page/207 207]–8
  | url = https://archive.org/details/thermionictubesi00scotrich
  }}</ref>) where the same valve performed the combined functions of RF amplifier, AF amplifier, and diode detector.  The RF signal was applied to one control grid, and the AF signal to the other.  This type of tetrode was used in many imaginative ways in the period before the appearance of the screen-grid valve revolutionised receiver design.<ref name="Goddard">{{cite book   
  | last = Goddard
  | first = F.
  | title = The Four-Electrode Valve
  | publisher = Mills & Boon, Ltd
  | year = 1927
  | location = London
  }}</ref><ref name="Morrow">{{cite book   
  | last = Morrow
  | first = G.L.
  | title = A Four Electrode Valve Receiver
  | publisher = E.W.
  | date = June 1924
  | pages = 520–24
  }}</ref>
[[File:Thorpe K4 Bi-Grid Valve.jpg|thumb|alt=Tetrode of the Bi-Grid Valve type|Tetrode of the Bi-Grid Valve type]]
[[File:Bigrid transmitter.jpg|thumb|right|Circuit using bi-grid tetrode [[electronic oscillator|oscillator]] as AM transmitter. H is a source of high voltage.]]
One application is shown in the illustration.  This is recognisable as an AM telephony transmitter in which the second grid and the anode form a power [[oscillator]], and the first grid acts as a modulating electrode. The anode current in the valve, and hence the RF output amplitude, is modulated by the voltage on G1, which is derived from a carbon microphone.
<ref name="Scott-Taggart 1921">{{cite book   
  | last = Scott-Taggart
  | first = John
  | title = Thermionic Tubes in Radio Telegraphy and Telephony
  | publisher = [[Wireless Press]]
  | date = 1921
  | location = London
  | pages = 377
  | url = https://archive.org/stream/thermionictubesi00scotrich#page/n3/mode/2up
  }}</ref>
A tube of this type could also be used as a direct conversion CW (radiotelegraphy) receiver. Here the valve oscillates as a consequence of coupling between the first grid and the anode, while the second grid is coupled to the antenna.  The AF beat frequency is audible in the headphones.  The valve acts as a self-oscillating [[product detector]].<ref name="Scott-Taggart 1919">{{cite book   
  | last = Scott-Taggart
  | first = John
  | title = British Patent 153,681
  | date = 14 August 1919
  | location = London
  }}</ref>
Another, very similar application of the bi-grid valve was as a self oscillating [[frequency mixer]] in early superhet receivers<ref name="Williams">{{cite book   
  | last = Williams
  | first = A.L.
  | title = Supersonic Heterodyne Receiver Employing a Four-Electrode Valve
  | publisher = E.W.
  | date = 1 June 1924
  | pages = 525–26
  }}</ref> 
One control grid carried the incoming RF signal, while the other was connected into an [[oscillator]] circuit which generated the local oscillation within the same valve.  Since the anode current of the bi-grid valve was proportional both to the signal on the first grid, and also to the oscillator voltage on the second grid, the required multiplication of the two signals was achieved, and the intermediate frequency signal was selected by a [[tuned circuit]] connected to the anode.  In each of these applications, the bi-grid tetrode acted as an unbalanced [[analog multiplier|analogue multiplier]] in which the plate current, in addition to passing both input signals includes the product of the two signals applied to the grids.