AD843 CBFET Series 250 V/µs Op-Amp: High-Speed Performance and Stability Analysis

Executive Summary: What is the CBFET Series?

The CBFET Series (specifically the AD843) is a high-speed, fast-settling operational amplifier designed for precision signal conditioning and high-frequency buffering. It combines the low input bias current of a FET-input stage with the speed and drive capability of a complementary bipolar (CB) output stage.

• Market Position: High-end legacy component; remains a "gold standard" for specific high-speed precision niches.

• Top Features: 34 MHz Unity Gain Bandwidth, 250 V/µs Slew Rate, and a fast 135 ns settling time to 0.01%.

• Primary Audience: Ideal for aerospace engineers, high-end audio designers, and instrumentation specialists requiring low bias current at high speeds.

• Supply Status: Active (Legacy). While widely available, it is often treated as a premium part compared to modern CMOS alternatives.

1. Technical Specifications & Performance Analysis

The CBFET Series represents a unique intersection of FET impedance and Bipolar speed. By utilizing Analog Devices’ proprietary Complementary Bipolar process, the AD843 achieves performance levels that were traditionally difficult for standard JFET op-amps.

1.1 Core Architecture (CBFET Logic)

The "CBFET" designation refers to the combination of a Junction Field Effect Transistor (JFET) input stage and a Complementary Bipolar (CB) output stage. This architecture is chosen to provide extremely low input bias currents (0.6 nA typ) without sacrificing the high-speed switching and power bandwidth (3.9 MHz) required for pulse-driven applications.

1.2 Key Electrical Characteristics

Engineers and procurement managers should note the following critical parameters: 

*   Unity Gain Bandwidth: 34 MHz, making it suitable for high-frequency active filters. 

*   Slew Rate: An impressive 250 V/µs, ensuring minimal distortion in high-speed signal processing. 

*   Input Offset Voltage: Laser-trimmed to 1 mV max, reducing the need for external nulling in DC-coupled circuits. 

*   Quiescent Current: 13 mA max, which is higher than modern low-power parts but necessary for its "Class-A-ish" output performance.

1.3 Interfaces and Connectivity

As an analog component, the CBFET Series interfaces directly with high-speed ADC/DAC buffers. Its low input bias current allows it to interface with high-impedance sensors or large-value resistors in integrator circuits without introducing significant error.

2. Pinout, Package, and Configuration

The AD843 CBFET Series is typically found in industry-standard 8-pin configurations, ensuring compatibility with many existing PCB footprints.

2.1 Pin Configuration Guide

• Pin 2 & 3 (Inputs): Inverting and Non-Inverting inputs. The FET input ensures pico-ampere level leakage.

• Pin 4 & 7 (Power): Negative Supply (V-) and Positive Supply (V+). Requires robust decoupling.

• Pin 6 (Output): High-speed output capable of driving significant loads.

• Pin 1 & 5 (Offset Null): Used for fine-tuning the input offset voltage if the 1mV factory trim is insufficient.

2.2 Naming Convention & Ordering Codes

Ordering the correct suffix is vital for procurement: 

- AD843JN/KN: Plastic DIP (Dual In-line Package), suitable for through-hole prototyping. 

- AD843JR/AR: SOIC (Small Outline IC), intended for automated surface-mount assembly. 

- AD843SQ/BQ: Cerdip packages, often used in higher temperature or military-grade environments.

2.3 Available Packages

3. Design & Integration Guide (For Engineers & Makers)

Pro Tip: The AD843 is a "fast" part. Layout parasitics that are negligible at 1 MHz become critical at 34 MHz.

3.1 Hardware Implementation

• Bypass Capacitors: Use a 0.1 µF ceramic capacitor in parallel with a 10 µF tantalum capacitor on each supply rail, placed within 0.5 inches of the pins.

• PCB Layout: Use a ground plane. Minimize trace length on the inverting input to reduce parasitic capacitance which can lead to "ringing."

• Thermal Management: Because this chip draws ~12-13mA, it runs warm. Ensure the SOIC version has sufficient copper pour for heat sinking.

3.2 Common Design Challenges

• Issue: Oscillation Instability. Users often report instability when driving capacitive loads (like long cables).

• Fix: Isolate the load with a 10Ω to 100Ω series resistor at the output and add a small (2-5pF) feedback capacitor to compensate for input capacitance.

4. Typical Applications & Use Cases

4.1 Real-World Example: High-End Audio

In the audiophile community, the AD843 is a favorite for cMoy headphone amplifiers and DAC I/V stages. Its high slew rate results in "fast" transient response and detailed high-frequency reproduction. When used in a headphone amp, its ability to drive loads cleanly while maintaining the high input impedance of the FET stage makes it superior to standard jellybean op-amps like the TL072.

5. Alternatives and Cross-Reference Guide

If the CBFET Series is out of stock or too expensive for your BOM, consider these alternatives:

• Direct Replacements: Texas Instruments OPA627 is the closest high-end equivalent, offering even lower noise but at a higher price point.

• Better Performance (Modern): Analog Devices AD8065 (FastFET™) offers higher bandwidth (145 MHz) and lower power consumption for modern SMT designs.

• Cost-Effective Options: Analog Devices AD744 or TI OPA134 for audio-specific applications where the extreme speed of the AD843 isn't fully required.

6. Frequently Asked Questions (FAQ)

• Q: What is the difference between CBFET Series and standard JFET op-amps?

• A: Standard JFETs often trade speed for low bias current. The CBFET (Complementary Bipolar FET) uses a specialized manufacturing process to maintain high speed (250 V/µs) while keeping the FET input benefits.

• Q: Why does my AD843 feel hot to the touch?

• A: This is normal. With a 13mA quiescent current at +/-15V, the device dissipates nearly 400mW at idle. Ensure proper ventilation.

• Q: Can the CBFET Series be used in single-supply designs?

• A: It is optimized for dual supplies (e.g., +/-15V). For single-supply 5V designs, modern Rail-to-Rail op-amps are generally preferred.

7. Resources

• Datasheet: Search "AD843 Datasheet Analog Devices" for full PDF.

• Spice Models: Available via LTspice library for circuit simulation.

• Evaluation Boards: Available for SOIC-8 high-speed op-amp testing.