Choosing an amplifier for amplitude modulation (AM): Which amplifier class is ideal when the application requires gain that can be directly controlled by a current (i.e., a transconductance-based gain element)?

Introduction / Context:
Amplitude modulation (AM) requires varying a carrier’s amplitude proportional to a modulating input. A convenient way to realize this electronically is to control an amplifier’s gain with a control signal. Operational transconductance amplifiers (OTAs) offer current-controlled transconductance, making them well-suited for implementing voltage-controlled amplifiers and modulators.

Given Data / Assumptions:

• The design needs carrier amplitude to vary with a modulation signal.
• A gain element whose transconductance (gm) is controllable by a bias current is available.
• Linearity and bandwidth must be sufficient for the carrier and modulating signals.
• We ignore detailed RF matching and focus on the base modulation mechanism.

Concept / Approach:
An OTA outputs current proportional to differential input voltage: Iout = gm * Vdiff. Because gm depends on a control current (Ibias), gm can be swept to implement gain control. Cascading or loading the OTA with an appropriate resistor or active load converts the controlled current back to a voltage, yielding an amplitude-modulated output when the control input is the modulating signal.

Step-by-Step Solution:

Verification / Alternative check:
Compare with multiplier ICs or Gilbert cells: these also enable AM by multiplying signals. However, an OTA provides a straightforward current-controlled gain approach that is common in analog synthesizers and signal-conditioning circuits.

Why Other Options Are Wrong:

Common Pitfalls:
Overdriving the OTA into nonlinearity; ignoring gm vs. temperature drift; forgetting to filter bias control to avoid injecting unwanted ripple into the carrier.

Final Answer:
operational transconductance