Field-effect tetrode

Where the voltage on the first channel is ${\displaystyle V_{1}-V_{2}}$  and the voltage on the second channel is ${\displaystyle V_{3}-V_{4}}$ , the current in the first channel, ${\displaystyle I_{1}}$ , and the current in the second channel, ${\displaystyle I_{2}}$ , are given by:

${\displaystyle I_{1}=G_{1}(V_{1}-V_{2})\left[1-{\frac {2}{3V_{p}^{1/2}}}{\frac {(V_{1}-V_{3})^{(3/2)}-(V_{2}-V_{4})^{(3/2)}}{(V_{1}-V_{3})-(V_{2}-V_{4})}}\right]}$ 

and

${\displaystyle I_{2}=G_{2}(V_{3}-V_{4})\left[1-{\frac {2}{3V_{p}^{1/2}}}{\frac {(V_{3}-V_{1})^{(3/2)}-(V_{4}-V_{2})^{(3/2)}}{(V_{3}-V_{1})-(V_{4}-V_{2})}}\right]}$ ,

where the ${\displaystyle G_{i}}$  are the low-voltage conductance of the channels and ${\displaystyle V_{p}}$  is the pinch-off voltage (assumed to be the same for each channel).

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. The signal does not modulate the depletion layer, so the tetrode can perform at high frequencies. The tuning ratio can be very large – the high resistance limit is in the megohms range for symmetrical pinch-off conditions.^[2]

• Field-effect transistor
• Multigate device, other multi-gate devices
• Tetrode transistor, any transistor having four active terminals