Dual-gate depletion-mode DMOS transistor for linear gain-control application

A conveniently implemented means is described for obtaining linear voltage control over the gain of dual-gate DMOS transistor amplifiers. This approach is applicable to surface acoustic wave (SAW) transversal filters and signal-processing modules. The system makes use of dual-gated, depletion-mode, n-channel, DMOS transistors. When two dual-gate DMOST's with their associated transducers are connected in a suitable differencing circuit, the gain (tap weight) depends linearly upon an applied control voltage to a deviation of only ±1 dB over a dynamic range of more than 50 dB. This linear property greatly simplifies the setting of tap weights in a transversal filter and provides versatility for acoustic-wave signal processing. A simple four-transistor model is proposed for the dual-gate DMOST. Theoretical analysis agrees well with experimental results and justifies the validity of the differencing scheme. An analytical solution for the device transconductance as a function of the control voltage is derived. Analysis based on this model gives insight into the operation of the dual-gate DMOST, and the design and optimization of the sensor and amplifying devices.