An Autozeroing Floating-gate Bandpass Filter

We have developed a bandpass oating-gate ampli er that uses tunneling and pFET hot-electron injection to set its DC operating point adaptively. Because the gate currents are small, the circuit exhibits a high-pass characteristic with a cuto frequency less than 1 Hz. The high-frequency cuto is controlled electronically, as is done in continuous-time lters. We have derived analytical models that completely characterize the ampli er and that are in good agreement with experimental data. This autozeroing oating-gate ampli er demonstrates how to use continuous-time, oatinggate adaptation in ampli er design. We previously introduced the autozeroing oating-gate ampli er (AFGA)[1]; here, we use the AFGA as an integrated continuous-time bandpass lter. Figure 1 shows the autozeroing oating-gate ampli er. The open-loop ampli er consists of a pFET input transistor and an nFET current source. With capacitive feedback, the input signal is ampli ed by a closed-loop gain approximately equal to C1 C2 . We present data from an AFGA fabricated in the 2 m nwell Orbit CMOS process available through MOSIS. Typical operating values for Vtun were between 33V and 42V ; those for Vdd were between 6V and 12V . We obtained similar data in the 1:2 m nwell Orbit CMOS process, but with typical operating values for Vtun between 26V and 31V . For more modern processes, the typical operating voltages will decrease, because of thinner gate oxides and higher dopant impurity concentrations. 1. TIME-DOMAIN AFGA RESPONSE The complementary nFET and pFET currents adjust the output voltage such that the ampli er's oating-gate voltage returns to its steady-state value on an intermediate time scale. This circuit behavior translates to a low-pass lter with a short time constant at the output voltage. The quiescent source current (Is0) is set by the nFET current source. We model the nFET transistor as an ideal current source setting the bias current, Iso, for the AFGA. We describe the channel current of the pFET by Ip = Iso exp Vg UT ; (1) where is the fractional change in the pFET surface potential due to a change in Vg, and UT is the thermal Figure 1: An autozeroing oating-gate ampli er (AFGA) that uses pFET hot-electron injection. The ratio of C2 to C1 sets the gain of this inverting ampli er. The nFET is a current source, and it sets the current through the pFET. Steady state occurs when the injection current is equal to the tunneling current. The capacitances, Cw and CL, represent both the parasitic and the explicitly drawn capacitances. Between Vtun and Vfg is our symbol for a tunneling junction, which is a capacitor between the oating-gate and an nwell. voltage, kT q . We showed previously that for our operating conditions the AFGA's open-loop gain also is large ( 1000) and nearly constant [2]. Since the output capacitances are charged or discharged by currents on the scale of Is0, the cuto frequency will be directly dependent on the bias current. Continuous-time integrators operate on a similar principle [3, 4]. The complementary tunneling and hot-electron injection processes adjust the oating-gate charge such that the ampli er's output voltage returns to its steady-state value on a slow time scale. The combination of electron tunneling and pFET hot-electron injection applies the appropriate negative feedback to stabilize the output voltage. If the output voltage is below its equilibrium value, then the injection current exceeds the tunneling current, decreasing the charge on the oating gate; that, in turn, increases the 0-7803-4455-3/98/$10.00 (c) 1998 IEEE 0 5 10 15 20 25 30 35 40 45 50 1 1.5 2 2.5 3 3.5 4 4.5 5 Time (seconds) V ol ta ge ( V )