The FET tetrode: A high-performance, small-signal amplifier

A new device has been developed that is similar to the FET triode, but which offers several significant advantages over the latter. By the addition of a shield electrode (gate) between the gate and drain, it is possible to obtain a high degree of isolation between the output and the input. Most significantly, the de-coupling provides: 1. A substantial reduction of CGD, by nearly two orders of magnitude, compared to a conventional triode of similar dimensions. 2. An increase of rdby about the same amount, and hence the amplification factor, µ = gmrd. 3. The quiescent gate leakage current is low and essentially independent of drain voltage. In addition to its primary function as a shield, the second gate may also be considered as an auxiliary input point. Since the pinch-off voltage is nearly the same as the control gate, it may be used for AGC or similar functions. The device is made by the planar process and hence may be made quite small. It is, therefore, suitable for high-frequency operation. Design trade-offs will be considered, and a model presented for the characterization of the tetrode.