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Field effect tetrode

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The field effect tetrode is a solid-state device, constructed by creating two field effect channels back-to-back, with a junction between. It is a four terminal device with interesting properties. It does not have specific gate terminals since each channel is a gate for the other, the voltage conditions modulating the current carried by the other channel.

Contents

Current voltage relationship

Where the current in the first channel is I 1 , the current in the second channel is I 2 , the voltage of the first channel is V 1 - V 2 and in the second channel V 3 - V 4 we have:

I 1 = G 1 ( V 1 − V 2 ) [ 1 − 2 3 V p 1 / 2 ( V 1 − V 3 ) ( 3 / 2 ) − ( V 2 − V 4 ) ( 3 / 2 ) ( V 1 − V 3 ) − ( V 2 − V 4 ) ]

and

I 2 = G 2 ( V 3 − V 4 ) [ 1 − 2 3 V p 1 / 2 ( V 3 − V 1 ) ( 3 / 2 ) − ( V 4 − V 2 ) ( 3 / 2 ) ( V 3 − V 1 ) − ( V 4 − V 2 ) ]

Where the G i are the low-voltage conductance of the channels and V p is the pinch-off voltage (assumed to be the same for each channel).

Applications

The field effect tetrode can be used as a highly linear electronically variable resistor - resistance is not modulated by signal voltage. Signal voltage can exceed bias voltage, pinch-off voltage and junction breakdown voltage. The limit is dependent on dissipation. Signal current flows in inverse proportion to the channel resistances - signal does not modulate the depletion layer, meaning the tetrode can perform at high frequencies. The tuning ratio can be very large - the high resistance limit in the megohms range for symmetrical pinch off conditions.

References

Field effect tetrode Wikipedia
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