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Circuit design of weak optical signal preamplifier in photoelectric detection technology

Author: WeiZhi From: www.yourpaper.net Posted: 2009-03-31 13:17:03 Read:
0 Introduction
photoelectric detection technology is optical and electronics are combined to produce a new detection technology (1).It is mainly the use of electronic technology to detect optical signals, and further transfer, storage, control, calculation and display (2).Photoelectric detection technology from the principle of speaking can detect all possible effects of light and optical properties of non-electric quantity.It can be through the optical system to become the optical information easy to accept the non-electrical information transform to be detected, and then use the photoelectric detector optical information quantity is transformed into electricity, and further through the amplification circuit, processing, in order to achieve the purpose of output signals (3).Then the electronics, information theory, Computer and physics method to analyze the causes and laws of noise, in order to facilitate the corresponding circuit is improved, better study characteristics and correlation of weak useful signal noise, so as to understand the state of non-electric quantity.The purpose of weak signal detection is from strong noise in extraction of useful signal, at the same time to improve the signal-to-noise ratio detection system output signal.
The basic structure of 1 photoelectric detection circuit
Signals of the photoelectric detector receives are generally very weak, and the light detector output signals are often buried in noise, therefore, needs to deal with weak signal that, generally must first pretreatment, the most noise filtering, voltage amplitude and the weak signal amplification to the subsequent processor requirements.In this way, they need through the preamplifier circuit, filter circuit and main amplifier circuit to output a signal to be detected is appropriate, and filtered out most of the noise.The photoelectric detection module block diagram as shown in figure 1.
The block diagram of src='http://paperimg.yourpaper.net/getimg/?s=st&a=%2fNewspic%2f2008810%2f1643134078.jpg'width=300 border=0> photoelectricThe working mode and equivalent 2 photodiode model
The working mode of
2.1 photodiode
The photodiode has two modes: zero bias and reverse bias, shown in Figure 2 is a bias circuit two modes for photodiode.In the picture, in the photovoltaic mode, linear photodiode can be very precise; while in the optical mode, photodiode can realize switching speed is higher, but at the expense of certain linear.In fact, in the reverse bias conditions, even if no light, there is still a very small current (called the dark current or not according to current 1.And there is no dark current at zero bias, then diode noise basically is thermal noise shunt resistance; in reverse bias, because the shot noise conduction have become additional noise source.Therefore, in the design of photoelectric diode circuit, is usually carried out optimization design for photovoltaic or photoconductive one of two modes, instead of the two models are optimization design (4).
Two models of photodiode
Generally speaking, in the photoelectric precision measurement, the measured signal is relatively weak, therefore, dark current effect is very obvious.Signal, because the design of a signal to be detected is discussed is very weak so, try to avoid the noise is the primary task, therefore, the photovoltaic mode design.
Equivalent circuit model of
2.2 photodiode
Photodiode model that works in the photovoltaic mode as shown in Figure 3, it consists of a radiation source current, light excitation of an ideal diode, junction capacitance and parasitic series and shunt resistance.In the picture, IL diode leakage current; ISC diode current for the parasitic resistance; RPD; CPD photodiode parasitic capacitance; ePD noise source; Rs series resistance.
Model of src='http://paperimg.yourpaper.net/getimg/?s=st&a=%2fNewspic%2f2008810%2f1643142254.jpg'width=280 border=0> photodiodeBecause there is no pressure drop photodiode operating in the photovoltaic mode, so the zero bias.In this way, key parasitic element circuit performance for CPD and RPD, they will affect the stability of frequency and noise performance of optical detection circuit.CPD is composed of P type and N type material photodiode depletion layer width between the.Depletion layer is narrow, junction capacitance value is greater.Conversely, depletion of wide (such as PIN photodiode) would exhibit a broad spectrum response.The numerical range of silicon diode junction capacitance of about 20 or 25pF to thousands of more than pF.The photodiode parasitic resistance RPD (also known as the 'triage' resistance or the 'dark' resistance), and the photodiode bias associated.
And photovoltaic voltage mode instead, photodiode optical methods are both ends of a reverse bias voltage to the light sensing element.When the voltage is applied to the optical detection devices, the depletion layer width will increase, thus greatly reducing parasitic capacitance of CPD.To reduce parasitic capacitance value is conducive to high-speed work, however, linearity and offset errors not optimization.The compromise design this question will increase the leakage current of IL and linear error diode.
3 circuit design
3.1 main amplifier design
Weak optical signal many need checking is usually through a variety of sensors to convert non-electronic, thus enabling the detection of objects into electricity (voltage or current).Because the target itself is weak, at the same time by the sensitivity of various sensor is limited, so the quantity is small signal, can not be directly used for sampling and processing.The design of the photodiode preamplifier circuit mainly plays the role of current to voltage, but the subsequent circuit for A / D conversion circuit, the voltage is 2 V.However, even so, while the output voltage signal of the general also need to continue to enlarge the hundreds of times, it is application of main amplifier circuit.The typical amplification circuit as shown in figure 4.
src='http://paperimg.yourpaper.net/getimg/?s=st&a=%2fNewspic%2f2008810%2f1643153015.jpg'width=230 typical amplification circuit
Magnification of the main amplifier for A=lR2/R3, where R2 is the feedback resistor.In order to follow the normal operation of the circuit, the design needs to set reasonable R2 and R1 values, in order to get the required output voltage amplitude.That is
src='http://paperimg.yourpaper.net/getimg/?s=st&a=%2fNewspic%2f2008810%2f1643155432.jpg'width=250 formula
3.2 filter design G>
In order to make the circuit design is simple and has a good signal to noise ratio, the design also need to use band pass filter to process signals.In order to ensure the measurement accuracy, this design after the preamplifier circuit with two order band pass filter circuit, the noise except the useful signal band, including noise, environmental noise and introduce the preamplifier.Here is the active bandpass filter optional signal in a frequency band, and inhibitory signals in the spectrum outside.The amplitude-frequency characteristics of the filter as shown in figure 5.In Figure 5, F1, F2 respectively on the lower cutoff frequency, F0 as the center frequency, the frequency band width:

In the formula, Q is the quality factor, Q value is large, along with the change of frequency, the gain attenuation faster.This is because the center frequency, Q value is greater, the pass band is narrower, the selectivity of the filter.
Active power filter is a filter circuit containing a semiconductor triode, integrated operational amplifier and other active devices.This filter with respect to the characteristics of the passive filter is of small volume, light weight, low cost, firm structure, can be integrated.The operational amplifier has lost the open loop gain and good stability, high impedance, low output impedance, high, and simple structure and excellent performance.This design uses a place for the amplifier design.
This design uses a place for the amplifier design.
Figure 6 shows the two order bandpass filter is a kind of two order voltage-controlled voltage source (VCVS) bandpass filter, the filter circuit with active power filter, and the two voltage-controlled voltage source (VCVS) low-pass filter and two order voltage-controlled voltage source high-pass filter connected bandpass filter.
For the first part, namely the low pass filter, low pass requirements system cut-off frequency is FC, the transfer function:
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