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Oops schematic mistake on first message. Correction made --> Help. Transistor circuit that connect or disconnect 75ohm terminator for video application

Hi Jasen

Attached is you suggested circuit redrawn in Workbench. When I looked at it, I like the idea how D1 and D2 are used to terminate through the voltage divider powered by -5v and +5v (R1||R2= 75ohms) and then reverse D1 and D2 to no termination. Very clever. I ran this circuit on the simulator and toggle J1 between 5v and 0v. Unfortunately, I saw no change on the right side of R11. Checking on the circuit I notice that Q5 and Q1 can never turn on regardless if the right side of R8 is 0v or 5v. The BE (Base- emitter) of Q1 never completely gets forward bais to turn the transistor. The same is Q5. I can't seem to see any bipolar configuration for Q2 to switch Q5 and Q1 on or off. Can you see if I redrew your schematic correct. I hope to get your schematic to work on Workbench first before I try to bread-board it. Your idea for the diode to switch on or off the termination make sense. I just have to get the logic controls to work proper. Please send me your feedback.

Caesar

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Reply to
LRCR
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yes the FET is better for AC coupled signals

make sure you don't leave the gate OPEN when it is off, it is VERY high Z and will pick up 60 Hz hum, that may be what you are seeing... connect the gate to the source (ground) when you want the FET off..

if the 75 Ohm video cable is not temrinated, thee WILL be peaks and valleys in the frequency respsonse due to the relfections adding constructivley and descructivley,,,thats why we need terminations....

an N ch JFET with the source at ground, the drain to 75 Ohms to the signal, and you control the gate voltage + for on and 0 for off should work well.

Mark

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Mark

Reply to
LRCR

Set whatever you are using for a news reader to use Courier font.

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JosephKK
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Joseph2k

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