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a problem with 2sc1815 to use as a video switch

hi, I want use the (2s1815)or( 2n2222) as a switch for a video signal when I bias it with (~4.5v) it connect the signal I know that the input & output must connect to an cap withabout 33uf & aresistor of 75 ohm but when I try to do this the picture on the monitor become bad but when I remove the input resistor (75ohm) the picture become good what is my problem now because I will connect the outputs of 4 of this circuit together& each time the the control pulse the desired transistorit will connect the desired video signal & want to ask is the better way that I bias the transistor from asource & power it from another source like fig.2

my reistors & caps values which I used +4.5v I / \ r1=1k r2=4.7k / --\/\/\---I c1=33uf I c input----II---I------b signal e c2=33uf r3=75 ohm I------II----\/\/\----outputsignal \ /r4=1k \ _I_ gnd fig.1

+4.5(from +5v(from power supply) I the I / control / \ circiut) \ r1=1k / / \ r2=4.7k I c1=33uf I c input----II---I------b signal e c2=33uf r3=75ohm I------II----\/\/\----outputsignal \ /r4=1k \ _I_ gnd fig.2

thanks for any help yar

Reply to
st_yar
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Just from the seat of my pants, fig. 2 is better, because the loads on the outputs are supplied from the power supply, The switching source shouldn't be used as this power source.

Good Luck! Rich

Reply to
Rich Grise

Well, you're not terminating the input signal properly (at all) so depending on the length of the cable sourcing the video, you will have a nice little notch filter. Get the length just right and there will be no color. Then, you put an output terminating resistor but don't make up for the insertion loss. Whatever your video was, half of it is gone amplitude-wise. Unless you're trying to do pixel time switching, which this configuration would never come close to doing, why don't you use a simple DIP relay? If you insist on having electonic switching, go check Elantec, Maxim, Analog Devices, Gennum or TI. Maxim in particular has easy-to-implement chips that work very well. GG

Reply to
Glenn Gundlach

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