JFET Design Spotlight 🔍 | From Early OCR Systems to 45 Years of FET Innovation

"In 1979, we teamed up at DEST Corp. to design the first desktop OCR machine for scanning standard letter and legal pages with a linear Reticon stored-charge diode array.

Internal to that sensor were two identical 2048-pixel diode strings. Light would deplete the diode pixel reset charge and be read as exposure versus time through a charge integrator.

Because the CMOS diode-select shift registers created substantial charge injection noise, the dark string was differentiated with the active string for noise cancellation and then sampled. This sample-and-hold circuit had to be fast and free of artifacts. Unlike many feedback S&H circuits, this is an errorless open-loop stored-voltage sampler with no loop recovery or compensation time.

A capacitor, shunt-switched by DMOS, stores the dark voltage value before a scan so differenced pixel voltages are DC referenced. Pixel voltages are then capacitively coupled into a high-speed, high-impedance JFET buffer to the sampling switch.

DMOS parts are often substrate-biased to lower inter-terminal capacitance, resulting in reduced charge injection when used as switches. Buffered pixel voltages are switch-sampled through a DMOS part compensated for blow-by capacitance in the off-state, charge injection cancellation, and constant video tracking of source-to-substrate bias.

The gate switching voltage of -0.7 V to +5 V also tracks the source voltage for constant charge management. The sampled pixel voltage is then buffered through a JFET-controlled variable gain amplifier for setting an automatic gain control to keep text video at a constant amplitude.

Circuits like this use bipolar, JFET, and DMOS parts for their unique and diverse properties. The Linear Systems DMOS used here was originally made by Signetics at the time, the JFETs were from Siliconix, and the bipolar dual, an MP352, was from an earlier John Hall company, Micro Power Systems, before LIS.

Overall, the signal-to-noise performance was over 70 dB and, moreover, was very repeatable with medium-tolerance components in manufactured quantities in the thousands.

This circuit is just one example of the many uses of FETs as switches, amplifiers, and variable resistors. In other portions of this system, an SD5000 quad DMOS was used for isolation and integrator reset on 5-picocoulomb charge integrators following the Reticon RL2048 outputs. Matching of the quad DMOS on die made charge injection cancellation relatively simple and repeatable.

I've now been using these parts for the last 45 years and will continue to do so." - Kirkwood Rough

Why This Design Still Matters Today

While OCR technology has evolved dramatically since 1979, the fundamental analog design challenges remain the same: maximizing signal integrity, minimizing noise, managing charge injection, and preserving accuracy when processing extremely small signals.

Kirkwood's experience highlights an important principle that continues to drive modern circuit design: selecting the right device technology for each function.

JFETs remain valuable for their high input impedance and low noise performance. DMOS devices continue to excel in high-speed switching applications where low capacitance and controlled charge injection are critical. Bipolar devices provide precision gain and matching characteristics that complement both technologies.

Many of today's advanced systems—including image sensors, medical instrumentation, industrial sensing, scientific equipment, and precision measurement systems—still rely on these same analog fundamentals.

At Linear Integrated Systems, we continue to develop and manufacture JFET, DMOS, and bipolar technologies that enable engineers to solve challenging signal chain problems across a wide range of applications.

Interested in learning more about our analog portfolio or discussing your application? Contact our engineering team or request samples using the links below.

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