Capacitance pyroelectric element

For practical purposes, the very small signals generated by pyroelectric elements must be amplified. The most widely used first stage consists of a field effect transistor (FET) which responds to electric potential rather than to charge. In this case, it is advantageous for the material to have a low permittivity to match the low input capacitance of the FET. Therefore the compositions with high... 

The electrical impedance of a pyroelectric detector is almost that of a pure capacitance. Hence an output signal only appears when the input radiation is changing. For maximum output the rate of change of the input radiation should be comparable with the electrical (RC) time constant of the element. Figure 3.10 is the equivalent electrical circuit of a pyroelectric detector (Putley [3.11, 51J). Assume that the element receives radiation over an area A normal to the polar axis of the material and that this produces a modulated temperature rise (3.4). 

A typical pyroelectric detector element with a capacitance of 10 pF produces a charge of 10 C. It is necessary to use a low-noise, high-impedance field-effect transistor (FET) as a source-follower amplifier to enable these small charges to be detected. 

A simple detector consists of an electroded element of pyroelectric material with area A, thickness t, capacitance Cg and emissivity rj. The element is exposed to a radiation of power W, which is modulated at a frequency/. The temperature of the element will also be modulated at this frequency by an amount depending on the material s heat capacity H and thermal conductance to its surroundings G, with a thermal time constant... 

Equations (5.30) and (5.31) clearly show that materials with a high pyroelectric constant and small specific heat capacity are required. However, the final choice of material depends on the capacitance of the amplifier to be used and the size of the detector element. 

Since Johnson noise is not present in the imaginary part of an electrical impedance, only noise due to the loss tangent of the capacitor, the load resistor, and the amplifier input resistor contributes to Johnson noise. However, the basically capacitive nature of the element shunts Johnson noise to some degree. As a consequence the NEP [W Hz 5] of p5ux)electrical detectors due to Johnson noise is only proportional to the square root of frequency (Putley, 1977). Pyroelectric detectors can, therefore, be used to relatively high modulation frequencies. Temperature noise, due to thermal conduction and radiative exchange, is also present, as in all... 

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