Editing Tetrode (section)

==Anode characteristic of screen-grid valves==
The reason for the limited applicability of the screen-grid valve, and its rapid replacement by the RF [[pentode]] (introduced around 1930) was the peculiar anode characteristic (i.e. variation of anode current with respect to anode voltage) of the former type of tube.

In normal applications, the anode voltage was about 150&nbsp;V, while that of the screen-grid was about 60&nbsp;V (Thrower p 183).<ref name="Thrower" />  As the screen grid is positive with respect to the cathode, it collects a certain fraction (perhaps a quarter) of the electrons which would otherwise pass from the grid region to the anode.  This causes current to flow in the screen grid circuit.  Usually, the screen current due to this cause is small, and of little interest.  However, if the anode voltage should be below that of the screen, the screen grid can also collect [[secondary emission|secondary electrons]] ejected from the anode by the impact of the energetic primary electrons.  Both effects tend to reduce the anode current. If the anode voltage is increased from a low value, with the screen grid at its normal operating voltage (60V, say) the anode current initially increases rapidly because more of those electrons which pass through the screen-grid are collected by the anode rather than passing back to the screen grid.  This part of the tetrode anode characteristic resembles the corresponding part of that of a [[triode]] or [[pentode]].  However, when the anode voltage is increased further, the electrons arriving at the anode have sufficient energy to cause copious secondary emission, and many of these secondary electrons will be captured by the screen, which is at a higher positive voltage than the anode. This causes the anode current to fall rather than increase when the anode voltage is increased.  In some cases the anode current can actually become negative (current flows out of the anode); this is possible since each primary electron may produce more than one secondary. Falling positive anode current accompanied by rising anode voltage gives the anode characteristic a region of negative slope, and this corresponds to a [[negative resistance]] which can cause instability in certain circuits.  In a higher range of anode voltage, the anode voltage sufficiently exceeds that of the screen for an increasing proportion of the secondary electrons to be attracted back to the anode, so the anode current increases once more, and the slope of the anode characteristic becomes positive again.  In a yet higher range of anode voltages, the anode current becomes substantially constant, since all of the secondary electrons now return to the anode, and the main control of current through the tube is the voltage of the control grid. This is the normal operating mode of the tube.<ref name="Terman">{{cite book   
  | last = Terman
  | first = F.E.
  | title = Electronic and Radio Engineering
  | publisher = [[McGraw-Hill Book Company]] Ltd
  | year = 1955
  | location = New York, Toronto, London
  | pages = [https://archive.org/details/electronicradioe00term/page/196 196–8]
  | url = https://archive.org/details/electronicradioe00term
  | url-access = registration
  }}</ref>

[[File:6L6 triode anode characteristics.jpg|thumb|right|Typical triode anode characteristics]]
The anode characteristic of a screen-grid valve is thus quite unlike that of a [[triode]]. Where the anode voltage is less than that of the screen grid, there is a distinctive [[negative resistance]] characteristic, called the ''dynatron region''<ref>Happell, Hesselberth, (1953). [https://archive.org/details/Engineering_Electronics_George_Happell_Wilfred_Hesselberth_1953/page/n97/mode/2up ''Engineering Electronics'']. New York: McGraw-Hill. p. 88</ref> or ''tetrode kink''. The approximately constant-current region of low slope at anode voltages greater than the screen grid voltage is also markedly different from that of the triode, and provides the useful region of operation of the screen grid tube as an amplifier.<ref>John F. Rider, (1945). pp. 293 - 294</ref> The low slope is highly desirable, since it greatly enhances the voltage gain which the device can produce. Early screen-grid valves had amplification factors (i.e. the product of [[transconductance]] and anode slope resistance, R{{sub|a}}) fifty times or more greater than that of comparable triode.<ref name="rider03"/>  The high anode resistance in the normal operating range is a consequence of the electrostatic shielding action of the screen grid, since it prevents the electric field due to the anode from penetrating to the control grid region, where it might otherwise influence the passage of electrons, increasing the electron current when the anode voltage is high, reducing it when low. 
[[File:6J7 pentode anode characteristics.jpg|thumb|right|Typical pentode anode characteristic. There are a wide range of anode voltages over which the characteristic has a small positive slope. In a screen-grid tube this region is restricted to anode voltages greater than that of the screen grid.]]

The negative resistance operating region of the tetrode is exploited in the [[dynatron oscillator]], which is an example of a negative resistance oscillator.(Eastman, p431)<ref name="Eastman" />