Abstract: An automatic gain control (AGC) transimpedance amplifier (TIA) uses a differential structure with feedback PIN diodes to adjust the loop gain of the amplifier automatically to maintain stability over a wide dynamic range when converting optical power using a photodiode to an electrical signal. A stable DC current derived from the photodiode current sets the voltage gain of the amplifier. The use of ultra-linear long carrier lifetime PIN diodes assures the transimpedance feedback resistance is linear. The AGC function adjusts the gain of the TIA to provide a linear stable differential transresistance controlled by the photodiode current; a linear stable AGC function using current supplied by the photodiode; an improvement of about 10 db of the transresistance dynamic range; and reduces the need for internal and external circuitry needed to provide the same function. The TIA is applicable to CATV optical systems which have very strict linearity requirements.

Abstract: The present invention is a FET having a p-doped or acceptor-doped layer underneath a FET channel to enable E/D Mode operation. A FET threshold voltage is tunable through a voltage applied to the p-doped layer via a metal contact such as a threshold-control terminal (TCT). The present invention has a dual E/D mode operation of a single FET device, and also a dual E/D mode operation with a single-polarity positive power supply voltage. The FET of the present invention is fabricated to enable dual enhancement-mode/depletion-mode (E-Mode/D-Mode) high electron mobility transistors (HEMTs), to enable dual E/D Mode operation by incorporating a p-doped or acceptor doped region underneath the channel, to achieve a tunable threshold voltage by varying the bias voltage on a fourth terminal called the threshold-control terminal (TCT) that contacts the p-doped layer, and to enable Dual E/D-Mode operation of a HEMT with a single-polarity positive power supply voltage.