(1) Explain the bias situation. Photodetectors basically need to work under some form of bias conditions. For example, the bias of the photoelectromagnetic detector is a magnetic field, while the resistance measuring radiant heat gas and photoconductive detector require a DC bias power supply. The noise and signal of the detector are usually related to the bias, so the bias must be noted. (2) Area of photosensitive surface. The size and area of the photosensitive surface also affects the noise and signal of the detector. Normally, the area of the photosensitive surface is proportional to the signal noise of the detector. (3) Working temperature. For most detectors, the operating temperature will affect their noise and signal, among which detectors made of semiconductor materials are the most obvious. Therefore, the operating temperature of the detector must be clarified. Commonly used working temperatures are: liquid hydrogen temperature (22.4K), liquid chlorine temperature (4.2K), liquid nitrogen temperature (77K) and so on. (4) The broadband and passband of the circuit. The detector loses its extreme performance due to the influence of noise. The current and voltage of the noise are proportional to the square root of the broadband. Therefore, when describing the performance of the detector, the bandwidth and passband of the circuit must be determined. (5) The spectral distribution of the radiation source. The response signal of most photodetectors is related to the wavelength of the radiation source, and they all respond to the radiation wavelength signal within a specific range, and the photon detector performs the most prominently. The spectral response characteristics that depend on the wavelength of the signal radiation determine the effectiveness of the detector for detecting the target. Therefore, when establishing the performance of the detector, it is necessary to clarify the spectral distribution of the radiation source to be measured. If the radiation source is a black body, then the temperature of the black body must be clarified; if the radiation source is monochromatic radiation, then the wavelength of the monochromatic wave source must be clarified; if the radiation is modulated, then the frequency of the modulation must be given.
2.1 The design of the photomultiplier tube preamplifier circuit
PMT is the photomultiplier tube. Its working process is: When the photocathode is illuminated by the light source, it will emit photoelectrons into the vacuum, and the emitted photoelectrons are under the action of the focusing polar electric field When moving into the multiplication system, the multiplication increases significantly after the second emission, and the enlarged electrons are transported to the anode by the electric field force, and the anode collects electrons as a signal output. Since the photomultiplier tube is a high-voltage electronic tube, if it has a high-voltage output, it is prone to problems. Therefore, in order to ensure the safety of the signature amplifier, it is necessary to design a protective transistor and resistor to form a protection circuit [1].
2.2 Silicon photocell preamplifier circuit design
(1) The photocell outputs the circuit in the form of an open circuit. The maximum output voltage of the two photovoltaic cells, silicon and selenium, will not be higher than 0.6 V, and the linear relationship between their output voltage and light is not obvious. When you want to obtain a high output voltage but do not have high requirements on the linear relationship, you can use a high input impedance preamplifier, at this time the photovoltaic cell is close to an open circuit. The first stage is composed of VT 1 and VT 2. Both VT1 and VT2 are FETs with low input impedance. Connect VT2 to the source follower and work in a constant current environment, which not only improves the linearity of the circuit but also The input impedance is enhanced, up to 109 ohms. (2) A circuit in which the photovoltaic cell outputs in the form of current. If the linearity requirement is relatively high when performing photoelectric detection, then a circuit with low input impedance should be used to achieve amplification. A circuit form similar to the current-voltage converter often used in photomultiplier tubes, the input resistance of the amplifier is compared with the internal resistance of the photocell, which is equivalent to a short circuit. After amplification, the linear relationship will be well improved. .