Precision Light Meter Circuit Featuring BPX 63 Photodiode and JFET Feedback

JC - Linear Systems
3 days ago
2 min read

At Linear Systems, we're continually inspired by the creative ways engineers use discrete components like JFETs to build elegant analog solutions. This Friday FET Design features a voltage-integrating light meter that leverages a Siemens BPX 63 photodiode for exceptional low-light sensitivity, combined with a clever JFET-based feedback network to control integration timing and minimize distortion.

📐 Circuit Overview

This design ensures that only useful light is used to affect aperture control—excluding noise and ambient artifacts. It also offers fast recovery from light bursts, critical when operating in unpredictable lighting environments such as astrophotography or low-light film capture.

⚙️ Key Features:

Photodiode BPX 63: Delivers 10 nA/lux sensitivity.
JFET T1 & T2: Manage fast recovery and low-noise gate control.
High-Gain Amplification: Amplification exceeds 3000 when S1 and S2 open.
C1 Capacitor Integration: Produces a linear voltage output based on incident light.
T3 Feedback Switch: Ends exposure when output reaches 1V, improving timing accuracy.
+/-3V Supply: Ideal for compact, portable designs.

🧠 How It Works:

When the camera shutter is closed, S1 and S2 are closed, shorting the op-amp’s feedback loop through T1, preventing unnecessary charge accumulation.
As the shutter opens, S1 and S2 open, and the amplifier gain increases. The integrating capacitor C1 starts charging with the photocurrent.
When voltage reaches 1V, T3 (BCW 60) conducts, signaling the end of exposure by discharging the load path and resetting the system.

This buffered design improves upon classic drain-to-gate feedback by taking global feedback from the op-amp’s output (U1A), reducing distortion and enabling operation at higher frequencies. The use of discrete JFETs in key signal paths ensures minimal noise and excellent control characteristics.