A regular op-amp takes a differential voltage and produces an output voltage. An Operational Transconductance Amplifier (OTA) takes a differential voltage and produces an output current. That single change unlocks a wildly different circuit design style — one where you tune parameters electronically by adjusting a bias current.

The transfer function is simply:

I_out = g_m × (V₊ − V₋)

The critical part: g_m is proportional to an external bias current called I_ABC (Amplifier Bias Current). For the classic LM13700, g_m ≈ 19.2 × I_ABC (in siemens when I_ABC is in amps). Sweep I_ABC from 1 µA to 1 mA and g_m swings across three decades.

Why this matters: if you load the output with a capacitor, you've built an integrator with an electronically-tunable time constant. Load it with a resistor R, and you get a voltage gain of g_m·R that you can dial by adjusting a current. Every parameter that used to be nailed down by resistor ratios becomes a knob.

Real-world example — the analog synthesizer VCF: The Roland TB-303, ARP 2600, and countless modular synth filters are built from OTA cascades. A four-pole low-pass filter uses four OTA-integrator stages; the cutoff frequency is directly controlled by a bias current derived from a 1V/octave exponential converter. Play a keyboard, get an exponentially-scaled control current, sweep the cutoff — that's the entire filter architecture in one sentence.

Rule of thumb — input signal amplitude: OTAs are only linear over roughly ±20 mV of differential input (limited by the input diff-pair's V_T ≈ 26 mV). Exceed that and distortion rises fast. So always attenuate your input. A voltage divider ahead of the OTA input (often 100:1) keeps you in the linear region. The LM13700 includes linearizing diodes on-chip that stretch the linear range to about ±100 mV when biased.

Design walkthrough — variable-gain amplifier:

• Pick load resistor R_L = 10 kΩ at the output
• Choose I_ABC = 100 µA → g_m = 19.2 × 100 µA = 1.92 mS
• Voltage gain = g_m · R_L = 1.92 mS × 10 kΩ = 19.2 V/V
• Drop I_ABC to 10 µA → gain drops to 1.92 V/V

Watch out for: OTA outputs are high-impedance current sources — always buffer with an emitter follower or op-amp before driving anything resistive that isn't your intended load. And keep the output voltage swing inside the compliance range, or the internal current mirrors saturate and g_m collapses.