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Design and research of photodetector preamplifier circuit
December 01,2020 | News    Browsing volume:852
     The photodetector converts the weak light signal into a current signal, and then converts the current signal into a voltage signal through an amplifying circuit. Because the current signal is relatively weak, it can only be recorded after being effectively amplified. This function can be realized, but if the pre-circuit design is unreasonable, it will amplify the doped noise signal, etc., thereby affecting the quality of the current. Therefore, the design and research of the photodetector pre-amplification circuit has important meaning.

1 Working conditions of photodetector

       (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 The design of three kinds of detector preamplifier circuit


      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. .


  2.3 PIN photodiode preamplifier circuit design

  The working principle of this type of diode is similar to that of ordinary diodes, and they all work on the basis of PN junction photoelectric effect. The difference lies in the structure, in which a thicker layer of this card semiconductor is doped in the N-type semiconductor and the P-type semiconductor. This greatly improves the sensitivity and quantum efficiency of the long wave. In addition, due to the relatively thick I layer, it can withstand a relatively high reverse bias when working under reverse bias, thus widening Linear output range. Therefore, according to the long-wave response rate, high sensitivity, and fast response speed of the P I N photodiode, two types of amplifying circuits can be selected, namely: high-speed response amplifying circuit and current amplifying I-V conversion circuit [2].

3 Several factors affecting the precision of the preamplifier circuit


  ( 1) Working environment. In the actual working process of the photodetector preamplifier circuit, it should be placed in a grounded shielded box to avoid external electromagnetic interference. (2) Layout and routing. When designing the circuit, in order to ensure that the subsequent stage can provide a stable and reliable signal, the layout must be strictly in accordance with the signal flow, and there should be no cross-winding of the front and rear stages to avoid the phenomenon of large and small signal coupling. (3) Temperature compensation. Generally, the photodetector must be equipped with a temperature compensation circuit, especially the avalanche photodiode is the most prominent. When necessary, a temperature compensation circuit should also be designed in the preamplifier circuit.

4 Conclusion


      Through the design and research of the photodetector preamplifier circuit, it is known that the designed preamplifier circuit should have low input impedance, high gain and other performance characteristics, and the bandwidth of the operational amplifier and environmental factors should also be considered in the design. The influence of the pre-amplifier circuit can make the functional properties of the designed pre-amplifier circuit reach the best state.