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tecting and measuring extremely small currents for example the general electric fp54 was described as a space charge grid tube designed to have a very high input impedance and a very low grid current it is designed particularly for amplification of direct currents smaller than about 10 9 amperes and has been found capable of measuring currents as small as 5 x 10 18 amperes it has a current amplification factor of 250 000 and operates with an anode voltage of 12v and space charge grid voltage of 4v 11 the mechanism by which the space charge grid lowers control grid current in an electrometer tetrode is that it prevents positive ions originating in the cathode from reaching the control grid 12 note that when a space charge grid is added to a triode the first grid in the resulting tetrode is the space charge grid and the second grid is the control grid bi grid valve edit 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 9 the tube was intended to be used in a reflex circuit for example the single valve ship receiver type 91 13 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 14 15 tetrode of the bi grid valve type circuit using bi grid tetrode 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 16 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 17 another very similar application of the bi grid valve was as a self oscillating frequency mixer in early superhet receivers 18 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 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 the superheterodyne receiver edit main article superheterodyne receiver the principle of the modern superheterodyne or superhet receiver originally named the super sonic heterodyne receiver because the intermediate frequency was at an ultrasonic frequency was invented in france by lucien levy in 1917 19 p 66 though credit is usually also given to edwin armstrong the original reason for the invention of the superhet was that before the appearance of the screen grid valve amplifying valves then triodes had difficulty amplifying radio frequencies i e frequencies much above 100 khz due to the miller effect in the superheterodyne design rather than amplifying the incoming radio signal it was first mixed with a constant rf oscillator the so called local oscillator to produce a heterodyne of typically 30 khz this intermediate frequency if signal had an identical envelope as the incoming signal but a much lower carrier frequency so it could be efficiently amplified using triodes when detected the original modulation of the higher frequency radio signal is obtained 20 a somewhat complicated technique it went out of favor when screen grid tetrodes made tuned radio frequency trf receivers practical citation needed however the superheterodyne principle resurfaced in the early 1930s when their other advantages such as greater selectivity became appreciated and almost all modern receivers operate on this principle but with a higher if frequency sometimes higher than the original rf with amplifiers such as the tetrode having surpassed the triode s limitation in amplifying high radio frequency signals the superheterodyne concept could be implemented using a valve as the local oscillator and a separate valve as the mixer which takes the antenna signal and the local oscillator as input signals but for economy those two functions could also be combined in a single bi grid tetrode which would both oscillate and frequency mix the rf signal from the antenna 18 in later years this was similarly accomplished by the pentagrid converter tube a similar two input amplifying oscillating valve but which like pentode tubes incorporated a suppressor grid and in this case two screen grids in order to electrostatically isolate the plate and both signal grids from each other in today s receivers based on inexpensive semiconductor technology transistors there is no cost benefit in combining the two functions in one active device screen grid valve edit view of the interior of an osram s23 screen grid valve in this valve the anode is in the form of two flat plates the wires of the screen grid can also be seen the anode connection is at the top of the envelope to minimise anode grid capacitance at anode voltages less than that of the screen grid the tetrode characteristic curves are kinked due to secondary emission from the anode in the normal range of anode voltages the anode current is substantially constant with respect to anode voltage both features are quite unlike the corresponding curves for a triode for which anode current increases continuously with increasing slope throughout the marconi osram s625 the first commercially produced screen grid tube the screen is a cylinder with a metal gauze face that completely surrounds the anode and the tube is double ended with the anode terminal at one end and the grid at the other to improve isolation between the electrodes the screen grid tube provides much smaller control grid to anode capacitance and much greater amplification factor than a triode radio frequency amplifier circuits using triodes were prone to oscillation due to the grid to anode capacitance of the triode 21 in the screen grid tube a grid referred to as the screen grid shield grid or sometimes accelerating grid is inserted between the control grid and the anode the screen grid provides an electrostatic shield between the control grid and the anode reducing the capacitance between them to a very small amount 21 22 to reduce the influence of the anode s electric field on the cathode space charge and on the control grid during 1915 1916 physicist walter h schottky developed the first tubes having a grid positioned between the anode and the control grid to provide an electrostatic shield 23 24 schottky patented these screen grid tubes in germany in 1916 and in the u s in 1919 25 26 these tubes were produced in germany and known as siemens schottky tubes 24 in japan hiroshi ando patented improvements to the construction of the screen grid in 1919 27 during the latter half of the 1920s neal h williams and albert hull at general electric h j round at mov and bernard tellegen at phillips developed improved screen grid tubes these improved screen grid tubes were first marketed in 1927 28 feedback through the anode to grid capacitance miller effect of the triode could cause oscillation especially when both anode and grid were connected to tuned resonant circuits as is usual in a radio frequency rf amplifier 29 for frequencies above about 100 khz neutralizing circuitry was necessary a typical triode used for small signal amplification had a grid to anode capacitance of 8 pf while the corresponding figure for a typical screen grid valve was 0 025 pf 30 neutralizing circuits were not required for a well designed screen grid tube rf amplifier stage 31 32 the screen grid is connected to a positive dc voltage and at ac ground as insured by a bypass capacitor to ground 21 the useful region of operation of the screen grid tube as an amplifier is limited to anode voltages greater than the screen grid voltage at anode voltages greater than the screen grid voltage some electrons from the cathode will hit the screen grid producing screen current but most will pass through the open spaces of the screen and continue to the anode 21 as the anode voltage approaches and falls below that of the screen grid screen current will increase as shown in the plate characteristics image an additional advantage of the screen grid became apparent when it was added the anode current becomes almost completely independent of the anode voltage as long as the anode voltage is greater than the screen voltage this corresponds to a very high anode dynamic resistance thus allowing for a much larger voltage gain when the anode load impedance is large 33 the anode current is controlled by the control grid and screen grid voltages consequently tetrodes are mainly characterized by their transconductance change in anode current relative to control grid voltage whereas triodes are characterized by their amplification factor mu their maximum possible voltage gain at the time of the introduction of screen grid valves a typical triode used in radio receivers had an anode dynamic resistance of 20 kω or less while the corresponding figure for a typical screen grid valve was 500 kω a typical triode medium wave rf amplifier stage produced voltage gain of around 14 but screen grid tube rf amplifier stages produced voltage gains of 30 to 60 34 two s23 screen grid valves in a 1929 osram music magnet receiver to take full advantage of the very low grid anode capacitance the shielding between anode and grid circuits was observed in the construction of the radio the s625 valve was mounted in a grounded plane metal shield aligned to correspond with the position of the internal screen grid the input or control grid circuit was on one side of the shield while the anode or output circuit was on the other in the receiver shown using s23 tubes each entire stage of the 2 stage rf amplifier as well as the tuned detector stage was enclosed in an individual large metallic box for electrostatic shielding these boxes have been removed in the illustration but the up turned edges of the bases of the boxes can be seen thus screen grid valves permitted better radio frequency amplification in the medium and high frequency ranges in radio equipment they were commonly used in the design of radio frequency amplification stage s of radio receivers from late 1927 through 1931 then were superseded by the pentode tube anode characteristic of screen grid valves edit main article vacuum tube characteristics 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 v while that of the screen grid was about 60 v thrower p 183 9 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 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 35 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 36 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 37 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 a fifty times or more greater than that of comparable triode 33 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 t...
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