When using an operational transconductance amplifier (OTA) as an amplitude modulator, the stage’s voltage gain is intentionally varied by applying a _________ voltage to the ______ input that controls gm.

Introduction / Context:
OTAs provide voltage-to-current conversion with a transconductance gm that is set by a control (bias) input. This makes them excellent for voltage-controlled gain blocks, amplitude modulators, and voltage-controlled filters in analog signal processing and synthesizers.

Given Data / Assumptions:

• OTA has a bias control input (or bias current) that sets gm.
• Voltage gain for an OTA-based stage is proportional to gm.
• Modulation means varying the amplitude according to a control signal.

Concept / Approach:
Amplitude modulation is achieved by varying the gain of a carrier path with a control waveform. In an OTA, the gain scales with gm, and gm is governed by the bias input. Therefore, applying the modulation signal to the bias input varies gm over time, which varies the effective gain.

Step-by-Step Solution:

Verification / Alternative check:
Application notes show OTA VCA (voltage-controlled amplifier) topologies with bias-controlled gm. Replacing the steady bias with a modulating voltage yields amplitude modulation.

Why Other Options Are Wrong:
Demodulation, bias: demodulation extracts envelopes; it does not control gain.

Bias, modulation: reverses roles; the modulation must act on the bias control.

Low-frequency, bias: frequency isn’t the defining factor; the role is modulation control via the bias input.

Common Pitfalls:
Driving the OTA signal input with the modulation rather than the bias control. Forgetting bias linearity range and headroom constraints that limit distortion.

Final Answer:
modulation, bias