Enhancing Voltage-Controlled Resistor (VCR) Performance with Buffered Global Feedback Using N-Channel JFETs

In precision analog applications, minimizing distortion while maintaining frequency response is essential when implementing a voltage-controlled resistor (VCR) using an N-Channel JFET.

One effective method to achieve this is through buffered global feedback. In this design, the feedback path to the JFET gate is derived from the output of an op-amp (U1A) rather than using a traditional drain-to-gate feedback resistor. The feedback network, composed of R3 and R4, connects the op-amp output directly to the gate, allowing for:

• Significantly Reduced Signal Distortion: Buffered feedback isolates the gate drive from the drain voltage, improving linearity across the operating range.

• Improved High-Frequency Response: Because the feedback is sourced from the op-amp, R3 and R4 values can be lower, resulting in minimized parasitic capacitance effects and broader bandwidth operation.

This architecture removes the need for a direct feedback resistor across the JFET and enables the VCR to perform effectively at higher frequencies compared to conventional approaches.

For design engineers focused on optimizing low-noise and high-linearity circuits, this method offers a robust solution using N-Channel JFETs.

Reference:EDN - A Guide to Using FETs for Voltage-Controlled Circuits, Part 2

Learn more about Linear Systems' ultra-low-noise, precision JFET solutions at www.linearsystems.com.

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