Engineering:Field effect tetrode

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.

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:

$I_1=G_1(V_1-V_2)[1-\frac{2}{3V_p^{1/2}}\frac{(V_1-V_3{)}^{(3/2)}-(V_2-V_4{)}^{(3/2)}}{(V_1-V_3)-(V_2-V_4)}]$

and

$I_2=G_2(V_3-V_4)[1-\frac{2}{3V_p^{1/2}}\frac{(V_3-V_1{)}^{(3/2)}-(V_4-V_2{)}^{(3/2)}}{(V_3-V_1)-(V_4-V_2)}]$

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.

See also

• Field effect transistor

External links

• Jedec definition.