In short: A CMOS OTA is a transconductance amplifier whose output current is set by its differential input voltage through the transconductance gm. The telescopic cascode gives the highest gain and speed but a tight output swing, while the folded cascode trades a little gain and power for much wider swing and an input range that can include the supply rails.
A cmos ota (operational transconductance amplifier) is an amplifier that converts a differential input voltage into an output current. Unlike a classic op-amp that is designed to drive resistive loads through a low-impedance output buffer, an OTA presents a high-impedance current output and is almost always used to drive capacitive loads such as the sampling capacitor of an ADC or the compensation network of a switched-capacitor filter. This single-stage philosophy is what makes the OTA the workhorse of analog CMOS design.
The defining parameter is the transconductance, gm, which links input and output:
Because the gain depends on Rout, designers chase large output resistance — and that is exactly why cascode structures dominate OTA topologies.
The transconductance does double duty. It sets the gain through gm · Rout, and it sets the speed through the unity-gain frequency of a capacitively loaded stage, ωu = gm / CL. That coupling is the central tension in OTA design: you want gm large for both gain and bandwidth, but larger gm usually means more bias current or wider devices, both of which cost power and area.
The telescopic cascode stacks cascode transistors directly on top of the input differential pair and its current-source load, all in one vertical "telescope" of devices between the rails. This is the most power- and speed-efficient OTA because every branch carries the same current, and there are no extra poles from mirror nodes.
The folded cascode "folds" the signal path: the drain current of the input pair is steered into a cascode branch biased from the opposite rail, rather than stacked on top of the pair. By separating the input-pair bias from the cascode-output bias, the topology relaxes nearly every limitation of the telescopic version at a modest cost.
Output swing is the headroom the output node can travel before a cascode device leaves saturation. Every stacked transistor steals roughly one VDSsat (~100–200 mV each) from the available range, so swing is the currency you spend to buy gain. A practical way to read the two topologies:
Designers also watch phase margin: the dominant pole sits at the output node (gm/CL), and the folding/mirror node sets the first non-dominant pole, which must stay well above the unity-gain frequency for stability.
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Explore CourseTron Courses →An op-amp is designed to output a voltage into resistive loads through a low-impedance buffer stage, while an OTA outputs a current from a high-impedance node and is meant to drive capacitive loads. In integrated CMOS, the OTA is usually the native building block because it stays single-stage and is simple to compensate.
Its folding node must sink the input-pair current plus the cascode branch bias current simultaneously, so for an equal transconductance it typically draws about twice the current of a telescopic design. You pay that power for a wider output swing and a rail-inclusive input range.
By raising the output resistance. Cascoding multiplies Rout by the cascode device's intrinsic gain, and gain-boosting (a regulated/active cascode) adds a local feedback amplifier that multiplies it again, reaching 80–100 dB while keeping the amplifier single-pole.
The number of stacked transistors between the supplies. Each device needs at least its VDSsat to stay in saturation, so with a tail plus two cascode levels the usable swing shrinks to roughly VDD minus four saturation drops minus the tail headroom.
Related: op-amp design basics · Glossary · all courses.