Introduction
Most pyroelectric sensors include an integrated JFET (Junction Field Effect Transistor) configured as a source follower. This buffer converts the extremely high impedance of the pyroelectric element to a more manageable level and provides initial amplification.
Why a JFET Buffer is Needed
The pyroelectric element has an impedance of 10^10-10^14 Ω. Directly connecting this to external circuitry would result in:
- Signal loss due to loading.
- Noise pickup.
- Unstable operation.
The integrated JFET provides a low-impedance output (a few kΩ) that can drive external loads.
JFET Source Follower Configuration
The JFET is typically connected as a source follower (common drain):
- Drain connected to VCC (often through an external resistor).
- Source connected to GND through an external resistor.
- Gate connected to the pyroelectric element (internal).
Voltage gain is slightly less than 1 (typically 0.7-0.9), but the circuit provides current gain and impedance transformation.
External Components
The JFET requires external resistors to operate:
- R_d (drain resistor): Connected from drain to VCC. Sets the operating point. Typical value: 10-100 kΩ.
- R_s (source resistor): Connected from source to GND. Completes the source follower. Typical value: 10-100 kΩ.
Some sensors include these resistors internally; others require external addition. Check the datasheet.
Output Signal
The output is taken from the source (or drain, depending on configuration). With a source follower, the output voltage is approximately:
V_out = V_gate – V_GS
Where V_GS is the gate-source voltage (typically 0.5-1.5V). The output is DC-biased at this level, with the AC signal from the pyroelectric element superimposed.
Biasing and Operating Point
The JFET’s operating point is set by the resistor values. The source voltage should be in the middle of the supply range to allow maximum signal swing. If the sensor is used in a circuit with AC coupling, the exact DC level is less critical.
Temperature Effects
JFET parameters change with temperature, affecting the DC bias point. In precision applications, consider this drift or use AC coupling.
Noise Considerations
The JFET contributes noise, but modern sensors use low-noise JFETs. The external resistors also contribute thermal noise. Choose resistor values carefully – too low increases current and noise; too high may not provide enough bias current.
Alternative: Op-Amp Transimpedance Amplifier
Some high-end designs omit the JFET and use an external op-amp in transimpedance configuration directly with the pyroelectric element. This can achieve lower noise but is more complex and requires careful PCB layout to avoid leakage.
Testing the JFET
To test if the JFET is working:
- Power the sensor with appropriate drain and source resistors.
- Measure the source voltage. It should be between 0.5V and VCC-0.5V.
- Apply heat (finger) near the sensor. The source voltage should change.
- If source voltage is 0 or VCC, the JFET may be damaged.
Conclusion
The JFET buffer is a critical part of the PIR sensor. Understanding its operation helps in designing the interface circuit and troubleshooting sensor issues.