Video Polarity and Direct Injection
 

I have a set into which I'd like to directly inject video. My problem is the set only has one video amplifier tube so the video polarity at the grid of the video amp is positive. Injection after the video amp isn't doable as I don't have enough gain, and injection before the video amp isn't doable because the polarity is backwards. Can anyone tell me how flipping the polarity on composite video might be accomplished so that I may inject positive video into the grid of the video amp?

A phase inverter using a single triode shouldn't be too hard to slap together. No doubt the collective wisdom on here can ace a quick design.

A factoid worth bookmarking is that a tube's plate is opposite phase from the grid, while the cathode (if unbypassed) is in-phase with the grid.

Or with a single transistor:

Video Inverter near the bottom of the list.....

http://www.hobbyprojects.com/A/video.html

I built a crosshatcht generator using an EDN magazine "Design Idea" and tried the video out with a salvaged Astec modulator - no joy, so I rigged up a 2N2222A as an inverter, and voila....


BTW, it still works, and is more stable than my RCA portable unit...

http://oh3tr.ele.tut.fi/~ftp/video/videoinv.gif




Would a 2N4401 work in that circuit?

Sure - any low gain (hfe<500) NPN should work. If you get any ringing, try ferrite beads on the base and collector leads.

I built my 2N2222A inverter on a double sided PCB, with the bottom as a ground plane, and "Manhattan" style construction on the top. The circuit I used was in "Circuits Cookbook" from 1967....

The whole circuit takes advantage of the common emitter configuration for inversion - and - the common collector configuration gives you the non-inverted video on the emitter. Two in one!

https://en.wikipedia.org/wiki/Common_emitter
https://en.wikipedia.org/wiki/Common_collector

Probably need some gain
 

Can anyone verify that the circuit I posted should work? I can't get anything out of it.

You mean you've built it already? If it's not working, the transistor may not be biased correctly. First measure the emitter/base voltage, which should be about 0.7 V. If the voltage is too low or missing, you need to increase value of the base-to-ground resistor.

Or, if the emitter/base voltage is too high, the transistor will overconduct and saturate. So lower the voltage by increasing value of the base-to-12V resistor.

This is all assuming the transistor is NPN silicon type and supply is +12V.

BTW, the circuit shown will deliver only half the output level that it would if the non-inverting feature (which you don't need) were eliminated. The mod would be easy to do if the output level turns out to be inadequate. Here's a neat little tutorial on transistor biasing..
http://www.electronics-tutorials.ws/...r-biasing.html

How would I accomplish that?

By the way, I'm using a C1815. I got the emitter/base voltage to .7VDC. I have output now, but just a minuscule amount.

The problem is that the base is biased at about 1/2 of B+, which makes the emitter that (6 v) minus 0.7 volts = 5.3 v, and the collector drops to 12-5.3 = 6.7 v, so the transistor is almost saturated at one end of the circuit's output range. You need to reduce the lower base resistor or increase the top one to get about 3.7 volts at the base. Then the emitter will be at 3 volts and be able to swing almost from ground to 6 volts, while the collector will be at 9 volts and be able to swing almost from 6 to 12 volts.

You could try leaving the lower base resistor as 4.7k and increasing the upper one to 15k or 18k.

I always use a pot...
 

say 100K or so and adjust it till I get what I want with a scope on the output. Then measure the pot and substitute fixed resistors.

Since the non-inverting output feature is not needed, it could be eliminated. This would allow the output level to take advantage of the full supply voltage (rather than half as it is).

It's always useful to think of a transistor as a triode analog - with the emitter the cathode, the base the grid, and the collector the plate.

In the sketch shown, start out with R2 as 4.7K, and R3 as 1K. Then for R1, first try 100K and work down in value till you get 0.7 V on the base. Once a usable output is obtained, then play with values for R2 and R3 and see if the output level can be improved any, all the while tweaking R1 to maintain 0.7V on the base.

Edit.. Error in sketch, 1K resistor "R2" should be R3. Dumb, dumb.:o:screwy:

As a follow-on to the triode analog thingy, the analogy breaks down a bit since the base is biased positive to the emitter to set the operating point, while in a vacuum triode, the grid is biased negative to the cathode.

This circuit will not work as a decent video amplifier because there is no emitter resistor. The gain will be very large and very dependent on bias - in other words, without an emitter resistor, you have a switching circuit rather than a linear amplifier. EDIT: this has sometimes been used as a sync separator circuit - a very non-linear switch that changes state on sync pulses.

If you want to have both inverted and non-inverted outputs (although with gain = 1), go back to the circuit with equal emitter and collector resistors and make the upper base resistor about 15k or 18k as I suggested. If you want non-inverting with gain, use the feedback circuit suggested by kf4rca. If you need inverting with gain, you can used the single transistor circuit, but the emitter resistor needs to be smaller and the base bias voltage needs to be reduced so the collector voltage is at the middle of its possible range.

Second edit: I forgot to add that the gain for the inverted signal on the collector is close to the ratio of the collector resistor divided by the emitter resistor. The gain of the non-inverted signal on the emitter is always (very) slightly less than 1.