1. Multi-CCD Image Synchronization Acquisition System CCD (Charge Coupled Device) image sensors possess high spatial resolution, high photoelectric sensitivity, and a large dynamic range, making them a hot research and development area in radiographic imaging, particularly in medical imaging. In specific applications, such as using multiple CCDs to sense the same flash image, it is necessary to synchronize the CCDs to ensure the consistency of the video signals generated by each CCD, thus providing the possibility of stitching together the image signals from multiple CCDs. This article mainly introduces two implementation schemes for multi-CCD image sensor synchronization circuits. The circuit design concept uses the video output signal of one CCD as a reference source, serving as the external synchronization signal for the remaining CCD sensors, enabling all CCDs to work synchronously and capture images at the same moment, as shown in Figure 1. The video output signal of CCD image sensor #1 is connected to the input terminal of the multi-CCD image sensor synchronization circuit. After processing by the video distribution amplifier (taking four CCD image sensors as an example), four video signals completely consistent with the input signal are output. Three of these are used as external synchronization signals for CCD image sensors #2, #3, and #4, while the remaining one is directly used as the video output signal of CCD image sensor #1. 2. Requirements for the Video Distribution Amplifier The core of the multi-CCD image sensor synchronization circuit is the video distribution amplifier. The total gain of the video distribution amplifier is 1:1, which completes the function of one input and multiple outputs. It amplifies the video signal output from CCD #1 and outputs multiple video signals that are completely consistent with the original input signal, which are used as the external synchronization input signals for the other CCDs. From the perspective of circuit performance, the video distribution amplifier should meet the following requirements: (1) Sufficient bandwidth to ensure sufficient clarity. (2) Low output impedance. Since this amplifier sends the image signal to a 75Ω coaxial cable, the amplifier should have an internal output impedance of 75Ω and meet the output power required by the 75Ω load resistance. At the same time, it should also isolate the mutual influence between the loads to ensure that the output signals do not interfere with each other. (3) Large dynamic range to meet the requirements of various video signals. Figure 1 Multi-CCD image synchronization acquisition system Figure 2 Direct coupling amplifier Based on the above points, the author designed two implementation schemes for the synchronization circuit from the perspectives of discrete component circuits and integrated circuits. 3. Principle Circuit of Direct-Coupled Amplifier Discrete component circuits use transistors to achieve amplification. The main problems with using transistors in video amplifier circuits are low input resistance, large input capacitance, and current amplification factor that varies with frequency. To address these issues, the amplifier design uses a parallel-regulated single-ended push-pull amplifier circuit composed of NPN and PNP transistors. This circuit achieves approximately 2.5 times the output power with low distortion, making it ideal for 75Ω line amplifier circuits. Unlike typical video distribution amplifiers, this design first equally distributes the input signal energy, then amplifies it separately by four amplifier circuits. This ensures that each output signal is independent and does not interfere with others, and the amplifier's output power easily meets the requirements of the load circuit. The principle circuit is shown in Figure 2. The component within the dashed box represents one of the amplifier circuits; the components outside the dashed box are shared by multiple amplifier circuits. The amplifier operates as follows: the input video signal, after being blocked by DC by capacitor C1, is applied between the input terminals b and e of Q11. After being amplified by Q11, the voltage across collector resistor R12 is applied between the input terminals e and b of Q12. After being amplified by Q12, the voltage across collector resistor R16 is used as the output. Both transistors operate in common-emitter amplification mode, and the operating points of the two stages are coordinated to give the output voltage a wide range of variation. This amplifier circuit is characterized by low signal distortion, high amplification factor, good temperature stability, high input impedance, and low output impedance. R1 is an impedance matching resistor; since the video signal is transmitted through a cable, the video distribution amplifier must be a matching terminal. C1 and C13 are DC blocking capacitors, which isolate the DC path between the amplifier's input terminals and the signal source, and between the output terminals and the load, ensuring that the amplifier's quiescent operating point does not change due to input and output connections. R2, R3, R12, R13, and Q11 form a voltage divider current negative feedback bias circuit, which is characterized by providing appropriate bias current and stabilizing the quiescent operating point. R2, R3, R12, and R13 are bias resistors, placing the emitter of Q11 forward biased and the collector reverse biased, enabling it to operate in the linear amplification region. R2 and R3 form a series voltage divider circuit, and the voltage division relationship between R2 and R3 fixes the base potential of Q11, satisfying the condition for stable operating point. R13 is used to implement current negative feedback. If the collector current IC of Q11 increases with increasing temperature, the voltage drop UE generated by the emitter current IE across the feedback resistor R13 will increase simultaneously. This will cause the emitter potential UE to rise. Since the base potential Ub remains unchanged, the voltage UBE between the base and emitter decreases accordingly, thereby causing the base current IB to decrease, which in turn causes the collector current IC to decrease, bringing the operating point closer to its original position and achieving the goal of stabilizing the operating point of Q11. Q11 is an NPN transistor, and Q12 is a PNP transistor, forming a complementary direct-coupled circuit. Because in a direct-coupled amplifier circuit, the base voltage of the second stage is the collector voltage of the first stage. If the same type (e.g., NPN type) transistors are used, the collector potential of each stage of the transistors will gradually increase, which will limit the amplification stage. However, by using NPN and PNP transistors in combination in the front and back stages, the collector potential of the back stage can be reduced. R12 and R16 are the bias resistors of Q12, which put the emitter of Q12 on the positive bias and the collector on the negative bias, so that it works in the linear amplification region. R14 and R15 are the feedback resistors, which together with R13 form a two-stage voltage series negative feedback network. The closed-loop amplification factor Af of this circuit depends only on the ratio of the feedback resistor (R14+R15) to R13, and is independent of the transistor, so the static operating point of the circuit is stable. Moreover, it can stabilize the output signal voltage, so that the output resistance of the amplifier is reduced, the driving capability is improved, and the amplification factor is stable. At the input end, the input impedance of the amplifier is increased due to the series connection of the feedback signal. The following functions are achieved: (1) Improve the stability of the gain. During the use of the amplifier, the gain of the amplifier often changes due to factors such as power supply voltage fluctuations, temperature and load changes, which affects the stability of the working performance. If negative feedback is used, the gain change of the amplifier can be reduced relatively. (2) Reduce frequency distortion and nonlinear distortion. This feedback network is made of pure resistive elements, so the gain of the amplifier is basically independent of the frequency. (3) Expand the bandwidth. Due to the presence of transistor junction capacitance in the amplifier circuit, the amplification factor will decrease in both low and high frequency bands. Introducing negative feedback can make the closed-loop gain tend to be stable, so the rate of decrease of the closed-loop amplitude-frequency characteristic is slowed down. Compared with the open-loop amplitude-frequency characteristic, the lower cutoff frequency of the closed loop is smaller than that of the open loop, while the upper cutoff frequency of the closed loop is larger than that of the open loop, so the bandwidth of the closed loop is larger than that of the open loop. (4) Increase the input resistance. The smaller the input resistance, the larger the signal current that the amplifier draws from the signal source, and the smaller the input voltage obtained by the amplifier circuit. This not only increases the burden on the signal source, but also the output voltage decreases as the input voltage decreases. The series negative feedback weakens the effect of the input voltage of the amplifier circuit, and the net input voltage actually applied to the input terminal of the amplifier circuit decreases. Therefore, under the same input voltage, the input current will decrease, which is equivalent to the input resistance increasing. (5) Reduce the output resistance. When the load resistance decreases, the output signal voltage tends to decrease. Due to the effect of negative feedback, the effective input signal voltage increases, thus increasing the output signal voltage and keeping the voltage constant; the reverse is also true. For the load, the effect of negative feedback is equivalent to reducing the output resistance of the amplifier. At the same time, it improves the load-carrying capacity of the amplifier. In order to ensure that the amplifier works more reliably, damping resistors R11 and R17 are connected in series at the base of Q11 and the collector of Q12, respectively, to prevent parasitic oscillations. Since the collector current is large, the damping resistor R17 should not be too large (generally tens of ohms to hundreds of ohms), otherwise the energy consumption will be too large. The base damping resistor R11 should also not be too large (generally tens of ohms to hundreds of ohms), otherwise it will also affect the frequency response too much. At the same time, R17 also has the function of matching the load resistance. C11 and R15 form a correction network to eliminate self-oscillation, which lowers the main pole frequency of the amplifier circuit, thereby destroying the conditions for self-oscillation. The capacitance of C11 should not be too large, otherwise the high-frequency response of the circuit will deteriorate. C12 is a power supply decoupling capacitor to reduce the ripple of the power supply voltage. In order to reduce "zero-point drift", the following measures are adopted. (1) The circuit design adopts negative feedback, which realizes the stability of the DC operating point by feeding back the change of the collector potential of the Q12 transistor to the emitter of Q11. The principle is: when the temperature changes, if the collector current of Q12 increases, the feedback voltage will also increase, so that the potential fed back to the emitter of Q11 will also increase. Since the voltage division relationship of R2 and R3 fixes the base potential of Q11, the feedback effect reduces the potential difference between the base potential and the emitter potential of Q11, thereby reducing the base current of Q11. Consequently, the collector current of Q11 also decreases. The collector current of Q11 is the base current of Q12. Its reduction restrains the increase of the collector current of Q12 and suppresses zero-point drift. (2) Silicon transistors are selected because their iceo value is very small and has little impact on the operating point. (3) Reduce power supply fluctuations. Select a switching power supply module and connect a damping capacitor at the power output terminal to reduce the fluctuation of the power supply voltage to within the allowable range. When designing the PCB circuit of a video amplifier, attention must be paid to the arrangement of components. Only with a reasonable arrangement can the parasitic parameters (parasitic capacitance, lead inductance) and the mutual influence between various parameters (including real lumped parameters and parasitic parameters) in the circuit be reduced to the minimum, thereby reducing many troubles (such as insufficient frequency, parasitic oscillation, etc.) when debugging the video amplifier. To minimize parasitic parameters, components operating under video voltage should be connected as close as possible and ideally as small as possible. Components should be located as far away from the metal ground potential as possible; connections between components should follow the shortest path; leads should be kept far apart and not parallel. In particular, the collector circuit must be kept far from the base circuit to prevent parasitic coupling between them. The amplifier's ground wire should preferably use relatively thick bare copper wire (approximately 1mm in diameter) and should be connected sequentially from the final stage, grounded at a single point, avoiding using the amplifier chassis as the ground wire. Figure 3 shows the internal structure of the MAX4137. Figure 4 shows a typical application circuit. 4. MAX4137 Video Amplifier The MAX4137 from Maxim Integrated (USA) is a 1-input, 4-output voltage feedback video amplifier suitable for high-speed video distribution and conversion systems. Its gain is 2V/V, -3dB bandwidth is 185MHz, slew rate (SR) is 1000V/μs, output channel signal conversion time is 25ns, output amplitude is 2V, output current is 65mA, and it can drive a 150Ω load. The power supply voltage is ±5V, with high input impedance and low capacitive reactance. Its internal circuitry (see Figure 3) consists of one input amplifier and four output amplifiers. The output selection terminals SEL1～SEL4 (active low) control whether the output amplifier outputs a signal. The input amplifier is constructed from a non-inverting operational amplifier using voltage series negative feedback. When a video signal is applied from the non-inverting input of the op-amp, a large non-inverting voltage signal is generated at the output. Its voltage amplitude is [value missing], exhibiting high impedance and low capacitive reactance input characteristics. The output amplifiers are composed of voltage followers, providing isolation, buffering, and high-impedance matching to low impedance to improve driving capability. They also have short-circuit protection. The basic working principle is: after the input signal is amplified by the voltage series negative feedback amplifier, it is evenly distributed to 4 voltage followers. Each voltage follower determines the output state of each output terminal according to the corresponding SEL signal level. MAX4137 can be used for video signal conversion and distribution, high-resolution RGBCRT monitoring, high-speed analog bus driving, RF signal processing, composite video preamplifier and other fields. MAX4137 has few external components. C1 to C5 are used for power supply damping, and R1 to R5 are matching resistors. Typical applications are shown in Figure 4. This circuit is easy to debug and has high reliability. Table 1 Functions of MAX4137 pins To obtain a frequency band of 185MHz, the following principles should be followed when designing the PCB board. (1) Closed loop circuits are prohibited on the PCB board to reduce electromagnetic induction. All connections should be as short as possible. 90° corners should not be used. Arc corners should be selected instead. (2) IC sockets should not be used to reduce capacitive and inductive reactance. Surface mount components should be used. Shortcuts should be taken between components to obtain higher frequencies. (3) At least two layers should be used: one signal layer and one ground layer. 5. Conclusion Both of these circuits have been used in practical applications with excellent results. Cascading discrete component circuits is relatively convenient; simply connect the parts within the dotted frame in parallel. The required number of paths can be flexibly designed according to the number of CCDs. As long as the PCB circuit design is reasonable, success can be achieved in one go. Integrated chip circuits are small in size, occupying only 2.5cm × 3cm (double-sided board), effectively improving space utilization. These two video distribution amplifiers can also be applied to closed-circuit television monitoring systems, providing video signals from one camera to multiple monitors or other devices.