A PVT-Independent Constant- Bias Technique Based on Analog Computation

This brief presents a new process, voltage, and temperature (PVT)-independent constant-G_m bias technique for any Iout(V_in)-monotonic convex (or concave) transconductors. In this brief, the conventional constant-g_m biasing is formulated into an analog computation process that calculates the constant-Gm bias voltage V₀ from an input current. An analog computer measures the effective G_m from two matched transconductors and converges it to the inverse of a precision resistance by adjusting V₀. Through this, a well-defined large-signal constant-G_m bias voltage can be found. The proposed technique eliminates the power-law or exponential-law assumptions on the Iout(V_in) characteristics in a conventional design. An interpolation calculation follows to find the optimal small-signal constant-g_m bias voltage V_out. The fundamental limitations of the proposed technique are the required monotonicity and convexity (or concavity) of the transconductor´s I_out(V_in). Error sources include mismatches of paired devices and the input offset voltage and open-loop gain of the operation amplifier (OpAmp). These errors are analyzed in detail in this brief. Biasing circuits based on different transistor types are designed using the Taiwan Semiconductor Manufacturing Company (TSMC) 65-nm CMOS process. Computer simulations show that the proposed biasing circuitry can achieve ±0.22% g_m variations from -60 °C to 130 °C.