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John |
#17
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That means that the circuit is designed for a 75 ohm input 1 volt video source (standard).
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#18
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*Except for FET transistors which are functionally and behaviorally identical to triode tubes.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#19
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Ok.... so it's not an actual part, just a way of noting (in dummy terms) "video is supposed to go there"
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John |
#20
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You probably want to actually install the 75 ohm resistor, so that your DVD player or other source is seeing the proper load impedance.
The exception would be if multiple inputs are "daisy chained" from a single video source. Then you install the 75 ohm terminator across the last input in the chain. |
Audiokarma |
#21
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Transmission line theory 101 terminate your line with it's characteristic impedance to prevent reflections, ringing, reduced signal at terminals etc...Might be a good idea to brush up on (or look up the basics of) transmission line theory. If you are ever running a wired signal more than a few feet (especially if it is more than a tiny fraction of wavelength), or transmitting, T-line theory is crucial.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#22
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Question.
What is the polarity at the CRT? Are you looking at the schematic or a scope?
The CRT should be positive. Which means the driver tube input should be negative. The output of the detector is usually negative, because it is transmitted that way. (Sync is peak power.) Some of those detector circuits produce 5V of video.
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#23
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*In fact color sets used both at once to effectively mix the color difference and monochrome signals into RGB in the electron gun to save a few signal tubes in the design.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#24
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John |
#25
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Progress! I've got output with the correct polarity using that circuit. I'll have to do some tweaking, but it's a victory for now. One question, is there a way to adjust gain in that circuit?
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John |
Audiokarma |
#26
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2)Are you using the collector output (which is inverted)? In that case, the gain is equal to the ratio of the collector resistor divided by the emitter resistor. Without the emitter resistor, the gain is very non-linear and depends on the amount of current in the transistor, which varies the effective emitter impedance. So, to get the gain you want, divide the collector resistor by the gain needed and use that value for the emitter resistor. 3) a fine point to add: if the following circuit (connected to the collector) has a high impedance (like a tube grid), all is well, but if it has a lower impedance, closer to the collector resistor, it will reduce the gain because it reduces the total effective resistance at the collector. |
#27
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So by all means sub some resistances in the emitter leg per the more experienced advice given. (In the early days of transistors the "triode" analogy was indeed used as a visualization aid for newcomers. For the purpose at hand, it disregarded the input-impedance disparity. Then when FETs finally came along, the input impedance truly mimicked a vacuum triode.) Last edited by old_coot88; 12-14-2016 at 12:21 AM. |
#28
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A common deliberate use for the variable emitter impedance is in an IF stage with Automatic Gain Control (AGC). Even then, an emitter resistor is generally used, but the emitter also has an AC bypass capacitor directly to ground. The AGC feedback voltage is applied to the base of the transistor to vary the current in the transistor. This works fine as long as the IF signal is small compared to the DC bias, so the signal itself doesn't cause a variation in gain (which results in distortion). So, you may see AGC applied to the early IF stages and not to the last stage.
Because the video you want to amplify (1 volt) is large compared to the base-emitter drop (0.7 volts) it definitely could cause distortion; but when applied to the combination of the base-emitter junction and the emitter resistor, the emitter resistor keeps things linear. |
#29
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Yeah, the emitter resistor all makes perfect sense now. The stuff I built decades ago was all small-signal devices, and low supply voltage (like 3 to 6 V). I was locked into the 'no emitter resistor' pavlovian habit. Moral of story: Dont be stoompid
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#30
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I was using the circuit with the emitter tied directly to ground. I will experiment with an emitter resistor next.
By the way I have to say this is really fun to experiment with transistors. I've never done anything with them before...They're a lot smaller than tubes .
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John |
Audiokarma |
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