Transitron oscillator Dynatron oscillator

transitron oscillator



screen current , voltage in transitron oscillator. screen voltage vc2 becomes high enough suppressor grid voltage turns positive, electrons begin passing through suppressor grid reach plate. plate current increases , screen current decreases, giving screen negative resistance (grey region).

the transitron oscillator, invented cledo brunetti in 1939, (although similar effect observed in tetrodes balthasar van der pol in 1926, , edward herold described similar oscillator in 1935) negative resistance oscillator circuit using pentode vacuum tube, in which, instead of plate, screen grid has negative resistance due being coupled suppressor grid. see circuit @ right. in transitron, screen grid biased @ positive voltage (battery b1) above plate voltage while suppressor grid biased negatively (battery b2), @ or below cathode voltage. therefore, electrons reflected negative suppressor grid , none through plate. reflected electrons instead attracted screen grid, screen current high while plate current zero. however, if suppressor grid voltage increased, approaches 0 (the cathode voltage) electrons begin pass through , reach plate, number diverted screen grid, , screen current, decrease. since other grids don t take significant current cathode current





i

c





{\displaystyle \scriptstyle i_{\text{c}}}

split between plate





i

p





{\displaystyle \scriptstyle i_{\text{p}}}

, screen grid





i

g2





{\displaystyle \scriptstyle i_{\text{g2}}}

:

i

g2


=

i

c


−

i

p





{\displaystyle i_{\text{g2}}=i_{\text{c}}-i_{\text{p}}\,}

the division of current between screen grid , plate controlled suppressor voltage. inverse relationship indicated saying transconductance between screen , suppressor grid (the change in screen current Δig2 divided change in suppressor voltage Δvg3) negative.

since suppressor grid voltage , not screen grid voltage controls screen current, if suppressor , screen grid coupled capacitor (c2) there constant potential difference between them, increasing screen grid voltage increase suppressor voltage, resulting in decrease in screen current. means screen grid has negative differential resistance respect cathode, , can used create oscillations.

in transitron circuit, screen , suppressor grids coupled bypass capacitor (c2) has low impedance @ oscillation frequency, have constant potential difference. parallel tuned circuit (c1-l) connected between screen grid , cathode (through battery b1). negative resistance of screen grid cancels positive resistance of tuned circuit, causing oscillations. in dynatron oscillator control grid can used adjust negative resistance.

since transitron oscillator didn t depend on secondary emission far more reliable dynatron. however, because screen grid not designed handle high power, oscillator s output power limited. other tubes multiple grids beside pentode, such hexode , pentagrid converter tube, have been used make similar negative transconductance oscillators. pentode tubes used in circuit have negative transconductance of around -250 microsiemens, giving negative resistance of -4000Ω. tubes more grids, such pentagrid converter, can used make transitron oscillators higher transconductance, resulting in smaller negative resistance.