Analog circuit design using amorphous silicon thin film transistors

Amorphous silicon thin film transistors (TFTs) can be important circuit elements in large area imaging sensors because of their advantageous material and optical properties. These TFTs bring with them a gate-bias-induced threshold voltage (V_T) shift, which is a highly undesirable effect for analog circuits. This paper addresses two techniques for reducing the effect of V_T shift in TFT circuit applications. Internal compensation for V_T shift was achieved to a first order by optimal bias conditions designed in the circuit. In a second circuit negative feedback was used to make amplifier circuits less sensitive to changes in V_T. Results are presented for both measured circuits and for simulations using the ASIA-1 amorphous silicon TFT model on the AIM Spice simulator. Experiments demonstrate that a differential amplifier with ~20 dB of gain, varies less than 2% change when the V_T of the TFTs change by up to 300%. Alternatively, a three-stage amplifier, with an overall gain of 13 dB, displays only a 5% change of overall gain when employing negative feedback