Dynatron oscillator
In electronics, the dynatron oscillator, invented in 1918 by
Negative transconductance oscillators,[8] such as the transitron oscillator invented by Cleto Brunetti in 1939,[12][13] are similar negative resistance vacuum tube oscillator circuits which are based on negative transconductance (a fall in current through one grid electrode caused by an increase in voltage on a second grid) in a pentode or other multigrid vacuum tube.[5][14] These replaced the dynatron circuit[14] and were employed in vacuum tube electronic equipment through the 1970s.[8][10][11]
How they work
The dynatron and transitron oscillators differ from many oscillator circuits in that they do not use
In the dynatron and transitron circuits, a vacuum tube is
An advantage of these oscillators was that the negative resistance effect was largely independent of frequency, so by using suitable values of inductance and capacitance in the tuned circuit they could operate over a wide frequency range, from a few hertz to around 20 MHz.[6][8][9] Another advantage was that they used a simple single LC tuned circuit without the taps or "tickler" coils required by oscillators such as the Hartley or Armstrong circuits.[16]
Dynatron oscillator
In the dynatron a tetrode tube is used.[4] In some tetrodes the plate (anode) has negative differential resistance, due to electrons knocked out of the plate when electrons from the cathode hit it, called secondary emission.[4][5] This causes a downward "kink" in the plate current vs. plate voltage curve (graph below, grey region) when the screen grid is biased at a higher voltage than the plate, as described below. This negative resistance was mostly a feature of older tubes, of 1940s or earlier vintage.[4] In most modern tetrodes, to prevent parasitic oscillations the plate is given a coating which drastically reduces the unwanted secondary emission, so these tubes have virtually no negative resistance "kink" in their plate current characteristic, and cannot be used in dynatron oscillators.[4]
The tetrode wasn't the only tube which could generate dynatron oscillations. Early
An advantage of the dynatron circuit was that it could oscillate over a very wide frequency range; from a few hertz to 20 MHz.
However the dynatron had some drawbacks. It was found that the amount of secondary emission current from the plate varied unpredictably from tube to tube, and also within a single tube over its operating life;[18][19] eventually it would stop oscillating. When replacing the tube, several might have to be tried to find one that would oscillate in a circuit. In addition, since dynatron oscillations were a source of instability in amplifiers, the tetrode's main application, tube manufacturers began applying a graphite coating to the plate which virtually eliminated secondary emission.[4] By 1945 the use of the dynatron circuit was declining.[10][11][19]
Secondary emission
In an electron tube, when
However, if the screen grid is operated at a higher potential than the plate, the secondary electrons will be attracted to it, and return to ground through the screen grid supply.[4] This represents a current of electrons IG2 away from the plate, which reduces the net plate current IP below the cathode current IC
Higher plate voltage causes the primary electrons to hit the plate with more energy, releasing more secondary electrons. Therefore, starting at the voltage at which the primary electrons have enough energy to cause secondary emission, around VP = 10V, there is an operating region (grey) in which an increase in plate voltage causes more electrons to leave the plate than the additional electrons arriving at the plate, and therefore a net reduction in plate current.
Negative resistance
Since in this region an increase in plate voltage causes a decrease in plate current, the AC plate resistance, that is the differential output resistance of the tube, is negative:
As with other
The frequency of oscillation is close to the
Design
As can be seen from the graphs, for dynatron operation the screen grid had to be biased at a considerably higher voltage than the plate; at least twice the plate voltage. The plate voltage swing is limited to the negative resistance region of the curve, the downward "kink", so to achieve the largest output voltage swing, the tube should be biased in the center of the negative resistance region.
The negative resistance of older tetrode tubes was around 10kΩ - 20kΩ, and can be controlled by varying the control grid bias. If the magnitude of the negative resistance |rP| is just small enough to start oscillation, just a little smaller than the positive resistance R of the tuned circuit, the oscillation frequency will be very stable, and the output waveform will be almost sinusoidal. If the negative resistance is made significantly smaller than the positive resistance, the voltage swing will extend into the nonlinear part of the curve, and the peaks of the sine wave output will be flattened ("clipped").
Transitron oscillator
The transitron oscillator, invented by Cledo Brunetti in 1939,
The division of current between the screen grid and plate is controlled by the suppressor voltage. This inverse relationship is indicated by saying the transconductance between the screen and suppressor grid (the change in screen current ΔIG2 divided by the change in suppressor voltage ΔVG3) is negative.
Since the suppressor grid voltage and not the screen grid voltage controls the screen current, if the suppressor and screen grid are coupled together with a capacitor (C2) so there is a constant potential difference between them, increasing the screen grid voltage will increase the suppressor voltage, resulting in a decrease in screen current. This means the screen grid has
In the transitron circuit, the screen and suppressor grids are coupled with a bypass capacitor (C2) which has a low impedance at the oscillation frequency, so they have a constant potential difference. The parallel tuned circuit (C1-L) is connected between the screen grid and the cathode (through battery B1). The negative resistance of the screen grid cancels the positive resistance of the tuned circuit, causing oscillations. As in the dynatron oscillator the control grid can be used to adjust the negative resistance.
Since the transitron oscillator didn't depend on secondary emission it was far more reliable than the dynatron. However, because the screen grid is not designed to handle high power, the oscillator's output power is limited. Other tubes with multiple grids beside the pentode, such as the
References
- ^ a b Kröncke, H. (March 24, 1926). "Oscillation without reaction" (PDF). Wireless World. 18 (12). London: 467–468. Retrieved March 20, 2015.
- ^ S2CID 51656451. Retrieved 2012-05-06.
- ISBN 978-0080524054.
- ^ ISBN 978-0080539386.
- ^ a b c d e Edson, William A. (1953). Vacuum Tube Oscillators (PDF). US: John Wiley and Sons. pp. 31–34. on Peter Millet's Tubebooks website
- ^ a b c d e f Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers. Dept. of the Army, US Government Printing Office. September 1952. pp. 68–69.
- . Retrieved May 3, 2013.
- ^ a b c d e Dietmar, Rudolph (17 December 2010). "Negative resistance oscillators". Principles of Schematics forum. Ernest Erb personal website. Retrieved 29 November 2013.
- ^ a b c d Worthen, Charles E. (May 1930). "The Dynatron" (PDF). The General Radio Experimenter. 4 (12). General Radio Co.: 1–4. Retrieved September 5, 2014.
- ^ a b c Shunaman, Fred (April 1945). "Transitron Oscillators" (PDF). Radio-Craft. 16 (7). New York: Radcraft Publication Inc.: 419. Retrieved September 6, 2014.
- ^ a b c Palmer, C. W. (March 1940). "Recent advances in oscillator circuits" (PDF). Radio-Craft. 11 (9). New York: Radcraft Publications, Inc.: 534–535. Retrieved September 6, 2014.
- ^ S2CID 51644322.
- .
- ^ a b c Gottlieb, 1997, Practical Oscillator Handbook, p. 78-81
- ^ ISBN 978-0199565917.
- ^ a b c Brunn, Brunsten (August 15, 1931). "Dynatron Oscillator Uses" (PDF). Radio World. 19 (22): 15. Retrieved September 5, 2014.
- ISBN 9781107636187.
- ^ a b c Spangenberg, Karl R. (1948). Vacuum Tubes (PDF). New York: McGraw-Hill Book Co. pp. 718–719.
- ^ a b Ghirardi, Alfred A. (May 1945). "Practical Radio Course, Part 34" (PDF). Radio News. 43 (5): 148–150. Retrieved September 5, 2014.
- ISBN 978-0750304931.
- S2CID 51656745.