|OddMix VACUUM TUBE - AMPLIFIERS|
Inverted Electron Tube High Input Impedance Circuit Amplifier Electrometer
|Fig. 1. Inverted Tube Triode Circuit|
By an unusual connection a conventional vacuum tube can have special characteristics. If electron tube electrodes are employed in an unusual circuit connection or by applying different than intended combinations of electrode voltages to some of the tube's electrodes, special, useful results can be achieved.
It is possible to interchange the function of the control grid and the anode electrode in the conventional triode. By making the grid positive with respect to the cathode, and using it as the new circuit's anode electrode by using the anode as a negative control electrode an inverted circuit is made.
Figure 1 shows a simple three-electrode inverted tube amplifier circuit. For proper operation the vacuum tube amplifier circuit requires three different voltage sources marked on the schematic as "VA" filament or heater, "VB" plate or anode battery and "VC" as grid or bias battery.
|Picture 1. Beckman 932 Electrometer Tube|
Because the control grid electrode is much closer to the cathode, with a small voltage it exerts a strong pull to the space charge that surrounds the cathode. The flow of the electron current is dependent mostly of the electrostatic field conditions within the tube. Thus it is possible to use the negative anode terminal to control some of this cathode to grid current and to serve as the new control electrode.
When the tube is connected in that initial arrangement, its amplification factor is low - approximately 1/u where u [mu] is the amplification factor of the normally connected vacuum tube. Because the usual anode terminal is farthest from the cathode and out of the way of the electron flow, it has exceedingly small current and correspondingly high input resistance.
By careful selection of the vacuum tube, it is possible to achieve an input resistance of 10,000 MegOhms or more. The limiting factor for the resistance is the glass and the external and internal conductive paths on the tube. And because the electron tubes are made of glass and built to operate with normally high voltage, they can operate with high voltage control signal.
The dynamic anode resistance of the inverted tube circuit is much lower then a normally connected tube. By using the lower output and the much higher input resistance, high input voltage signals can be used to control high current without using any input power.
The inverted amplifier configuration is exploited often for electrometer circuits. Electrometers are useful to measure the electric field around us in a normal surround environment. Vacuum tube circuits having the advantage to survive very high voltage, temporary static discharges, that would fry all other semiconductor devices, even protected gate MOS's.
Picture 1 depicts a new, unused, factory fresh professional electrometer tube the Beckman 931. Another, a Beckman 932 type tube is also available. For not cutting edge circuit design, when professional tubes are out of the budget, good results can be obtained by careful selection or standard electron tubes. Direct filament type tubes are most popular for that type of experimentation, because they accomodate battery operation better.