A simple common-mode feedback circuit for a folded-cascode...

A simple common-mode feedback circuit for a folded-cascode opamp is presented in Fig. P6.57. If the bias currents of all transistors are $\mathrm{I}_{\mathrm{D}}=100 \mu \mathrm{A}$ and device lengths are $\mathrm{L}=0.25 \mu \mathrm{m}$, choose the device widths $\mathrm{W}_{7-10}$ identically so that the output common-mode level is $0.8 \mathrm{~V}$. Next, select $\mathrm{V}_{\mathrm{B} 3}$ to maximize the available output signal swing.

Key Concepts

Common-Mode Feedback

This concept involves the use of a feedback network that controls the common-mode level of an amplifier’s output. By monitoring the average of the output voltages and feeding back a correction signal, common-mode feedback ensures that the output common-mode voltage remains stable and at the desired level, which is particularly important in differential amplifier circuits and single-supply designs.

Folded-Cascode Amplifier

A folded-cascode amplifier is an opamp architecture that enhances performance by combining the high gain of a cascode stage with improved voltage swing characteristics. This design 'folds' the input signal path, allowing for a wider output voltage range and better biasing arrangements, which is crucial for maintaining proper operation in low-voltage environments.

MOSFET Biasing and Sizing

Biasing and sizing in MOSFET circuits involve choosing the appropriate device dimensions (width and length) and bias currents to set operating points and ensure that transistors operate in the desired region (typically saturation). Proper sizing directly influences the performance parameters such as gain, output voltage level, and overall stability of the circuit, as it dictates device transconductance and current density.

Output Signal Swing

Maximizing the available output signal swing refers to optimizing the biasing conditions and voltage headroom so that the amplifier can handle a maximum range of output voltages without distortion or clipping. This requires careful selection of bias voltages and transistor dimensions, ensuring that the active devices remain in saturation over the desired range and that the circuit can accommodate the full dynamic range of the input signal.

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