I built the below video amp & it worked nicely.But it is One in One out video amp. http://www.learningelectronics.net/circuits/video-amplifier.html

Now I want One in TWO out Video distributor.I added the red colored capacitor coupled but did not worked.Without the capacitors it is working nicely.

Any Ideas?enter image description here

  • \$\begingroup\$ Even if the target device should (or may) have coupling cap at its input, you may need to use a coupling cap at the output of your amplifier circuit since it has DC offset. I think you can put only one coupling cap right at the output (i.e. collector) then use 75R split resistors. \$\endgroup\$ – Rohat Kılıç Dec 4 '18 at 5:24
  • \$\begingroup\$ Hi I did as you suggested, Added a single 100uF to the COllector output & split into two via two 75R resistors but that didn't work either.Also lowered the capacitor down to 10uF but nothing helped :(. \$\endgroup\$ – Joseph143 Dec 4 '18 at 6:21
  • \$\begingroup\$ The reason I like to put the capacitors is because, by mistake if the output short circuit, the transistor will blow up :( \$\endgroup\$ – Joseph143 Dec 4 '18 at 6:22
  • \$\begingroup\$ What, exactly, does "not worked" mean? Amplitude low? Amplitude zero? Something else? \$\endgroup\$ – WhatRoughBeast Dec 4 '18 at 7:01
  • \$\begingroup\$ Not worked means, Video is not coming.It shows Video loss in the monitor.But when I short the capacitor it is working fine. \$\endgroup\$ – Joseph143 Dec 4 '18 at 7:11

This is a very interesting circuit. I've used similar, for very fast (10nanosecond pulses) amplification with 2N3904/3906, where I needed better dynamic response than opamps could provide and still work within 0/5volt rails (this was many years ago). This circuit is actually 1/2 of a current mode opamp, and the emitter of the NPN is the summing node.

The input circuit, especially with that diode, looks like a sync-tip clamping circuit, a peak-detector, that controls the biasing of the entire amplifier. Thus the black and the white levels depend on how you set P1.

The feedback circuit is P2, and setting the pot wiper in the middle should provide a gain of TWO. This gain of TWO is needed to overcome the 2:1 voltage division of the 75ohm Rout and the termination resistor, also 75 ohms, at end of the video coax cable.

Now ---- why do your modifications not work?

Knowing this circuit rather well, my first thought is "That 5Kohm pot P2 is the ONLY DC pulldown. That will not work."

The 75 ohms is also the DC_pulldown (actually 75 + 75 = 150 ohms).

I suggest you remove the output capacitors.

The extra 14 milliamp may overload your +5 power supply. Probably not. But check that.

  • \$\begingroup\$ Hi, Thanks for the detailed explanation.If I remove the output capacitors will it burn the transistor if I short circuit the output to Ground by mistake? \$\endgroup\$ – Joseph143 Dec 4 '18 at 9:20
  • \$\begingroup\$ You still have the local 75 ohm resistors to limit the current. But do not short the collector to ground. \$\endgroup\$ – analogsystemsrf Dec 4 '18 at 14:06
  • \$\begingroup\$ Hi, Thanks for the details.I tested this circuit on a Vero Board using CAT5E Twisted cables on workbench. Regarding the video quality, I like if it has a little bit of sharpness on video.Do not not know the lack of sharpness is due to the poor board quality or cable issues...!!! Is there anyway to improve little bit of sharpness...!! \$\endgroup\$ – Joseph143 Dec 6 '18 at 11:28
  • \$\begingroup\$ With the amplifier connected in the video path as you look at the video monitor, use scope probes in 10x mode, and look for ringing or slow settling (needs 0.1uf on emitter of the pnp) or slow rise or fall time. Maybe reduce r1 and r4; for low inductance tie coax gnds to bottom of c3 and c4. \$\endgroup\$ – analogsystemsrf Dec 7 '18 at 3:11
  • \$\begingroup\$ Hi, you mean to add another 0.1uF capacitor in parallel with 1uF (C3)? \$\endgroup\$ – Joseph143 Dec 7 '18 at 3:39

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