This is the video amplifier circuit or the video splitter circuit, it is designed to take video signal is stronger. Compensate for the loss of signal, and is video splitter video, which can connect up to three outputs. so is suitable for display on several television screens, or video tapes recording at the same time too. (Now may will no video tape.)
The advantage of this circuit is cheap, easy and the results are detailed in the circuit below.
How it works
– Increasing rate Typically, the circuit video amplifier is designed to have low amplifier rates. (it different with the common pre-amp circuit) For our the circuit has the maximum gain ratio is only 4 times.
– Impedance is 75 ohms on both the input and output.
– Bandwidth of this circuit wide is 5 Mhz.
It can be seen that the circuit contains only three transistors. By Q1 (BC547), Q2 (BC557) acts as a signal amplifier, and Q3 (BD139 or BD137) serves as the output circuit of common emitter follower.
The input impedance of circuit is determined by the value of R1. The input signal entered through C2 to the base of Q1. By the VR1 is the set the level of the signal swing.
The output of Q1 is connected directly to the base of Q2. The amplified gain of the circuit is determined by adjusting VR2. Which is in the feedback circuit, between collector pin of Q2 and emitter pin of Q1.
However rate of increase depends on the ratio between the resistance of R5 and R6, R8, VR2, C3. The based on the components used in the circuit is 1.95 to 8.7 times the rate of increase. Thus, the output 75 ohm load to the rate of increase of the last, reduce the signal to half will be output signal is only 1-4 times.
The R9 that may change to suit the application. For example, if only a single channel networks that use the 150 ohm. If the second channel is used for network 82 ohms, the current in the circuit will decrease.
and The LM7812 is main of Dc voltage Regulator circuit in this project that I think you still to know about it very well for popular ICs.
All equipment can be assembled on a PCB as shown in the figure below. You should put the device in the circuit to be completed. Points of caution is that capacitors, diodes and transistors put always the correct way. Otherwise, the circuit does not work. And damage.
The detail parts
R1, R10, R11, R12__________75 ohm____ 1/4W Resistor
R2_____________________10K_______ 1/4W Resistor
R3_____________________8.2K_______ 1/4W Resistor
R4_____________________1K_________ 1/4W Resistor
R5, R8__________________180 ohm____ 1/4W Resistor
R6_____________________3.3K_______ 1/4W Resistor
R7_____________________470 ohm___ 1/4W Resistor
R9_____________________56 ohm____ 5W Resistor
C1, C3__________________ 10uF 16V__ Electrolytic Capacitors
C2, C8, C9_______________ 0.1uF 50V_Polyester Capacitor
C4, C5, C6_______________ 100uF 16V Electrolytic Capacitors
C7______________________ 470uF 25V Electrolytic Capacitors
D1_______________________ 1N4148___ 75V 150mA Diodes
D2, D3, D4, D5_____________ 1N4001___100V 1A Diodes
Q1___________________ BC547b ,BC546 45V 100mA NPN Transistor
Q2___________________ BC557b, BC556 45V 100mA PNP Transistor
Q3______________________ BD137 ???? BD139 __NPN transistor
IC1______________________ LM7812__12VDC voltage regulator IC
The customization of circuit
When certain that circuit is complete, then Enter the power to the circuit to be tested by the following method.
Adjusting VR1 done two ways.
1. Adjust VR1 until the voltage at the base of Q1 is equal to 1 volt. And has voltage drop across R7. (no input signal.) approximately 0.75 volts.
2. Adjust VR1 in the middle, Enter the input signal is 1Vp-p then adjust VR2 to have the lowest rate of increase.
Then connect the TV or monitor to output, and then adjust VR1 until the test signal shall be applied without distortion.