This is a video amplifier splitter circuit or the video splitter circuit. It is designed to take video signal is stronger and compensate for the loss of signal. Also, it is video splitter up to three outputs.
So, it is suitable for display on several television screens, or videotapes recording at the same time, too. (Now, may no a videotape.) The advantage of this circuit is inexpensive, easy, and more detail in the circuit below.
- Usually, the video amplifier circuit always has rate is a low amplifier. This circuit has the maximum gain ratio is only 4 times. Which it is different from the common pre-amplifier circuit, high gain.
- Impedance is 75 ohms on both the input and output.
- The bandwidth of this circuit is wide of 5 MHz.
How Video amplifier works
The circuit contains only three transistors.
- Q1 (BC547) and Q2 (BC557) act as a signal amplifier.
- Q3 (BD139 or BD137) serves as the output circuit of the common emitter follower.
The input impedance of the circuit is determined by the value of R1. The input signal enters through C2 to the base of Q1. And, the VR1 adjusts 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 it is in the feedback circuit, between collector pin of Q2 and emitter pin of Q1.
However, the rate of gain depends on the ratio between the resistance of R5 and R6, R8, VR2, C3. The based on the used components in the circuit. We can get the gain of 1.95 to 8.7 times.
Thus, if the load is 75 ohms. The load will reduce the gain of the final amplifier section is half. The output signal is only 1-4 times.
You can change the resistance of R9 to suit the application. For example, if only single channel networks. You use R9 is 150 ohms. But if we the second channel is used for the network. The resistor R9 should be 82 ohms, the current in the circuit will decrease.
You can assemble all components on a perforated board. You should put carefully the device in the circuit. Points of caution are capacitors, diodes, and transistors. You put always the correct way. Otherwise, the circuit does not work and damage.
If you want the PCB layout image of this project. Please contact email to me.
The detail parts
R1, R10, R11, R12—75 ohms
R5, R8—180 ohms
VR1, VR2—2.2K, Potentiometer
C2, C8, C9—0.1uF 50V_Polyester Capacitor
C1, C3—10uF 16V,
C4, C5, C6—100uF
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 setting of video amplifier circuit
When sure that the circuit is complete. Then, enter the power to the circuit to test by following method. You can adjust VR1 in two ways.
First, adjust VR1 until the voltage at the base of Q1 is equal to 1V. And there is a voltage drop across R7. (while no input signal.) approximately 0.75V.
Second, adjust VR1 in the middle, enter the input signal is 1Vp-p. Then, adjust VR2 to the lowest rate of gain. Next, connect the TV or monitor to output. And, adjust VR1 until the test signal without distortion.
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