When an operational transconductance amplifier (OTA) is configured as a Schmitt trigger, which control quantity sets and therefore determines the upper and lower trigger thresholds (by adjusting the device transconductance gm)?

Introduction / Context:
An operational transconductance amplifier (OTA) converts an input voltage into an output current with transconductance gm. In many OTA implementations, gm is programmable via a bias current IBIAS. Using an OTA in a Schmitt-trigger (hysteretic comparator) arrangement exploits this controllability to set switching thresholds.

Given Data / Assumptions:

• OTA exhibits gm proportional to a bias current IBIAS over its operating range.
• Schmitt-trigger topology adds positive feedback to create hysteresis (distinct upper and lower thresholds).
• Device is operated within its linear/controlled gm region for predictable thresholds.

Concept / Approach:
In an OTA-based Schmitt trigger, the effective loop gain and the translation between input voltage and output current depend on gm. Since gm ∝ IBIAS, changing IBIAS scales the currents in the feedback network, thereby moving the input voltages at which the circuit toggles. Iout is a result of gm * Vin; it does not independently set thresholds. Vout follows the comparator/regeneration behavior and likewise does not directly control thresholds; it is the outcome of the gm-controlled loop.

Step-by-Step Solution:
Express OTA current: Iout = gm * Vin.Relate gm to bias: gm = k * IBIAS (k depends on process and topology).Positive feedback creates two switching points proportional to gm.Adjust IBIAS to scale gm → thresholds shift accordingly.

Verification / Alternative check:
Device datasheets and application notes show threshold equations where IBIAS appears explicitly, confirming that altering IBIAS repositions the Schmitt trigger’s upper and lower trip points.

Why Other Options Are Wrong:
Iout is a dependent variable set by gm and input; it does not independently program thresholds.

Vout is the end result of crossing the threshold; it does not set it.

“Both Iout and IBIAS” overstates the role of Iout; the controllable parameter is IBIAS.

Supply voltage alone does not determine hysteresis thresholds in an OTA-based design.

Common Pitfalls:
Assuming thresholds are fixed by passive resistors only; in OTA designs, gm (and therefore IBIAS) is a powerful knob for tuning hysteresis without changing resistors.

Final Answer:
the IBIAS