Using avalanche multiplication effect, semiconductor photodiode (APD) with internal gain can be obtained, and using general transistor amplification principle, another photovoltaic detector with internal gain of current, namely phototransistor, can be obtained. Its common bipolar transistors are very similar, both of which are composed of two very close pn junctions-emitter junction and collector junction, both of which have the function of current generation. In order to fully absorb photons, phototransistor needs a larger light receiving surface, so its response frequency is much lower than that of photodiode. [ 1]
2. 1 mechanism and working principle
Phototransistor is an ordinary triode, which is equivalent to a photodiode connected between the base and the collector. Therefore, its structure is similar to the general transistor, but it also has its own special features. As shown in figure 2. 1. 1. In the figure, e.b.c stands for emitter, base and collector of phototransistor respectively. In normal operation, the base-collector junction (B-C junction) is guaranteed to be in a reverse bias state and used as a light-receiving junction (that is, the base region is an illumination region). Phototransistors usually have two structures: npn and pnp. Commonly used materials are silicon and germanium. For example, the npn structure made of silicon has 3DU type and pnp type has 3GU type. The dark current of silicon-containing npn phototransistor is smaller than that of germanium phototransistor, and it is less affected by temperature change, so it has been widely used. [2]
There are two processes in the operation of phototransistor, one is photoelectric conversion; The second is photocurrent amplification. The photoelectric conversion process is carried out in the collector-base junction, just like the general photodiode. [3] The B-C junction is in a reverse bias state when the collector is relatively emitting a DC voltage and the base is open (see Figure 2. 1. 1(b)). When there is no illumination, the minority carriers generated by thermal excitation, electrons enter the collector from the base, holes move from the collector to the base, and current (dark current) flows in the external circuit. When light strikes the base region, an electron-hole pair is generated in this region, and the photogenerated electrons drift to the collector under the action of the internal electric field, forming a photocurrent, similar to a photodiode. At the same time, holes remain in the base region, which makes the base potential rise, and a large number of electrons flow from the emitter to the collector through the base. The total collector current is
IC = IP+βI P =( 1+β)IP 2. 1. 1
Figure 2. Structure and working principle of1.1phototransistor
Where β is the * * * emitter current amplification factor. Therefore, the phototransistor is equivalent to the parallel connection of a photodiode and the base-collector junction of an ordinary transistor. It is a photodetector, which amplifies the current (photocurrent IP) of the base-collector photodiode by β times, which can be shown in Figure 2. 1. 1(c). Unlike ordinary transistors, the collector current IC is controlled by the photocurrent IP=Ib generated by the base-collector junction. In other words, the collector junction plays a dual role. One is to turn the optical signal into an electrical signal and act as a photodiode. The second is to amplify the photocurrent and act as the collector of a general transistor. [4]
2.2 Equivalent circuit of photoelectric triode
According to the working principle of phototransistor, we can easily draw its equivalent circuit. Because the collector junction barrier capacitance Ccb is much smaller than the emitter junction barrier capacitance Cbe, we can get the AC equivalent circuit of the phototransistor as shown in Figure 2.2. 1, where ip is the current source of the collector junction photodiode and Cbe is the emitter junction capacitance; Rbe is forward differential AC resistance of emitter junction; ILw is an amplified current source; iL =βIP； β is the magnification of phototransistor; Rce is the collector-emitter resistance; Cce is the capacitance between collector and emitter; RL is the load resistance. Through that equivalent circuit in figure 5-40,
It can be obtained that the output voltage V0 across the load resistor is
2.2. 1
Where is the angular frequency of the incident optical signal, and an appropriate load is selected so that the output voltage is
2.2.2
It can be seen from the above formula that when an optical signal is input, the effective output signal will be reduced due to the signal shunt caused by the relatively large transmission junction capacitance. In addition, the bypass of the capacitor will also reduce the output current. It is very convenient to use the equivalent circuit of phototransistor in computer and analyze its time response and output characteristics. [5]
2.3 Characteristic parameters of phototransistor
2.3. 1 Voltammetric characteristics
Figure 2.3. 1 shows the relationship curve of phototransistor. It can be seen from the figure that when the bias voltage is zero, the collector current of the phototransistor is zero. When there is illumination, the output current of phototransistor is twice that of photodiode under the same illumination. The curve also shows that when the optical power increases at equal intervals, the output current does not increase at equal intervals, because the current amplification increases with the increase of signal photocurrent.
frequency response
The frequency response of phototransistor is related to the junction structure and external circuit. Usually, we need to consider: the charging and discharging time of minority carriers to the barrier capacitance (sum) of emitter junction and collector junction; Time required for minority carriers to cross the base region; The transit time of minority carriers sweeping through the collection barrier region; The current reaching the collection region through the collection junction flows through the collection region, and the junction voltage generated by the external load resistance will change the time constant of collecting junction charges. Therefore, the total response time of phototransistor should be the sum of the above time. Therefore, the response time of phototransistor is much longer than that of photodiode. Because phototransistors are widely used in various photoelectric control systems, their input optical signals are mostly pulse signals, that is, they work in large signal or switch state, so the response time or frequency of phototransistors will be an important parameter of phototransistors. [6]
In order to improve the response frequency of phototransistor, we can know from the equivalent circuit of phototransistor that the sum time constant should be reduced as much as possible. On the one hand, the junction capacitance is reduced as much as possible in technology. On the other hand, the load resistance should be selected reasonably to reduce the circuit time constant. Figure 2.3.2 shows the relationship between the relative value of the output voltage of the phototransistor and the modulation frequency of the incident light under different load resistances. As can be seen from the figure, the higher the frequency, the worse the high-frequency response. Reduction can improve frequency characteristics. However, this reduction will lead to a reduction in the output voltage. Therefore, in practical use, reasonable selection and use of high-gain operational amplifier for post-stage voltage amplification can obtain higher output voltage and improve frequency response. In addition, in order to improve frequency response, reduce volume and increase gain, operational amplifiers with high gain and low input impedance are often used in circuits. Figs. 2.3.3(a)(b a), (b) and (b) respectively show the integrated circuit schematic diagram of Darlington phototransistor. In fact, phototransistors with base leads are usually used, which provide a certain base current. For the phototransistor without base lead, the background light with a certain illumination is given to make it work in the linear amplification region to obtain a larger collector current, which will be beneficial to improve the frequency response of the phototransistor. Figure 2.3.4 shows the relationship between the response time of phototransistor and collector current. It can be seen from the figure that increasing the collector current can reduce the response time of the phototransistor, that is, increase the working frequency of the phototransistor. [7]
Compared with photodiode, phototransistor has lower frequency response and is not suitable for high-speed and broadband photoelectric detection system, but it is still widely used in general photoelectric detection system because of its higher response and internal current gain.
Design an alarm. The circuits shown in Figure 3. 1(a) and (b) are the circuit diagrams of infrared transmitter, infrared receiver and wireless transmitter respectively.
The circuit shown in figure 3. 1(a) is an infrared transmitter circuit. A 300Hz self-excited oscillator consists of VT 1, VT2, C 1, R 1, and its oscillator frequency is mainly determined by the time constant R 1 C 1. The infrared light-emitting diode is connected in series in the collector loop of VT2. During the oscillation of the oscillator, whenever VT2 is turned on, the light-emitting diode emits light. R3 is used for current limiting, so that the current of VT2 does not exceed 500mA.
Infrared emitter
(b) infrared receiving wireless transmitter
Figure 3. 1 Shading infrared monitoring wireless alarm circuit
In the circuit shown in fig. 3. 1(b), the infrared converging tube VD3 is a tube type (with the same light wavelength) matching with the emitting tube. VD3 converts the irradiated infrared light into an electrical signal, which is applied to the inverting input terminal of IC 1-a through C2 and R5. IC 1 adopts dual operational amplifiers TL072 (or LM358, R4558, NE5532), and its noninverting terminal is externally connected with 6V cheating voltage. The magnification of this stage is K=20lg(R8/R5), and the parameters shown give a magnification close to 53dB. The output of IC 1-a is rectified by VD4, C3, etc. And then applied to the inverting input of IC 1-b in the form of DC voltage. IC 1-b, R 10, R 12, RP 1 etc. Forming a voltage comparator. When VD3 is always irradiated by infrared light, the potential at point B is VB.
When someone sets foot in the infrared monitoring area, the infrared beam is blocked, IC 1-a has no signal input, and the output is low level, so the power supply voltage charges C3 through R9, resulting in that pin ⑦ of VB >: VA, IC 1-B is at low level, VT3 is turned off, and then pin ⑦ of IC2 is connected to the power supply through R 14. The oscillation frequency is f =1.44/[(r15+2r16) C4], and the oscillation frequency of the illustrated parameters is about 1000Hz.
The audio pulse signal output by IC2 is applied to the base of VT4 through R 17 and C6. VT4, L, C9, C 10, etc. A high frequency oscillator is formed, and its oscillation frequency mainly depends on the frequency selection loop formed by L and C9. Adjust C9 to make the oscillation frequency within the FM band of 88- 108MHz. At the same time, the oscillation stage is in the state of frequency modulation oscillation under the excitation of the input pulse signal, because the collector capacitance of VT4 changes with the high and low levels of the modulation pulse, thus realizing frequency modulation. The FM carrier signal is transmitted through the antenna.