amplifier

To allow the AC gain to be set, separate from the DC bias point.

Jon

I sometimes wonder if there shouldn't be an s.e.b / s.e.d basic electronics fundamentals course you know !

Graham

Emitter resistor provides dynamic biasing (holds the current down as the transistor heats and tries to pull more) . The emitter resistor also would hold the signal down (degenerative feedback) without an AC bypass for signal.

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Could be audio frequencies too depending on the size of the cap.

Graham

Could be audio frequencies too depending on the size of the cap.

Graham

Yes. The definition of "high frequencies" is strictly dependent on the relative reactance of the capacitor to the resistance of the resistor.

1/(2*pi*R*C) Hz is the arbitrary boundary between low frequencies and high frequencies, in this case.

If there were such a thing, I would gladly partake in it myself. :-)

But for now... google-google-google, AoE-Mims-ARRL, allaboutcircuits.com, google google google, datasheet datasheet datasheet.

*sigh*

-phaeton

But that's the point. People are asking questions that should be covered when they read the books, but they aren't bothering reading books. I'm not sure if that has changed since when I was a kid, but the newsgroups make it easy for someone to show that off.

Questions should be to 1) get a clear idea internally of what the problem is so the right answer can be found and 2) to get clarification based on some experience or reading.

In the first case, it's a question one asks oneself, and likely is immediately followed with the internal "and where can I find it".

The second case, someone has read something and can't grasp a bit of it, either because it's not well written or because the poster has missed some other step. But in their question, they can convey that they have done some work, and just need clarification.

The books exist to help beginners. They benefit from someone likely getting some return for the writing, and include a process where the material is edited and looked over before being published. They likely are more comprehensive than anything "on the web". And nobody should have to rewrite them simply because someone is too lazy to buy a book or go to a library.

Michael

I am trying to splice some RG6 Dual Shield and RG6 Quad Shield coaxial cable.

I have some questions:

A). For stripping RG6 Quad Shield, one web site says to remove the outer two layers of aluminum foil and braid, then fold back the inner layers of foil and braid.

Another web site says to cut off the outer 3 layers of foil and braid, leaving just the inner aluminum foil.

Would either of these work? Does folding back the aluminum foil and braid make much of a difference?

B) The Radioshack "Heavy-Duty" F-crimp connectors I bought are supposed to be for RG6 Dual Shield only, not Quad Shield, but they look exactly the same as those for QS.

Can I use F connectors made for RG6 Dual Shield for RG6/QS?

C) Would there be much signal degradation if I connected a stretch of RG6 Dual Shield to RG6 Quad Shield?

If they're both rated 75 ohms, there wouldn't be any wave reflections, right?

D) For coaxial cable, are there any safety hazards I should be aware of?

If I short the inner conductor to the outer layers of shielding, how many amps would flow? Enough to damage appliances like a cable modem or TV, etc.?

Thanks.

I never fully understood the idea of increasing the gain with a bypass cap across the emitter resistor. The problem you have is discharging the cap after it has charged through the emitter circuit. Suppose the emitter resistor is large at maybe 1 Megohm and the cap is 100uF. The transistor can charge the cap very quickly as it turns on and therefore the gain is increased at the collector due to the extra current. But now the capacitor must discharge through the 1 Meg parallel emitter resistor which takes RC time to fall 63% or about 100 seconds in this case. If the frequency is high, the cap will not have time to discharge much and cannot charge much on the next cycle.

So why does it work?

-Bill

You're thinking essentially DC. Think instead about the impedance of the capacitor at the frequencies of the signal in question. If it's audio at, say, 2KHz for example, your 100uF capacitor would have an impedance of 0.8 Ohms. Compare that with your 1 Meg that's setting the bias point.

Emitter resistors are typically rather low - think 100 ohms or so. The current flowing through the transistor drops voltage across the resistor - a volt for the sake of discussion - that voltage moves the emitter away from zero while the base resistors are maintaining a more or less fixed voltage on the base. More current = greater drop on the emitter resistor = less "on" bias = less current.

That is the DC theory - but it works with a signal imposed on the emitter current as well - the bias, sans capacitor, will try to track the signal - and will serve to decrease the amplification - bypass the resistor with a sufficiently large cap and the DC bias is maintained while the (higher frequency) signal is not affected.

You seem to be trying to think in terms of a static switch - this is a transistor biased into its linear region to amplify a linear signal. We aren't concerned with it turning on quickly - it is never fully on or off (for class A operation). The cap isn't really charging and discharging - it is just providing a low impedance path for AC. (imposed on the DC)

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It normally works because the high frequency content is much smaller than the DC bias current. In the extreme case you have described, the rectification of the AC signal and the distortion of the DC operating point do occur.

It's exactly because the capacitor can't charge and discharge that fast that the voltage on both sides of the capacitor is forced to be equal, which makes the capacitor a short circuit at those frequencies. If the capacitor is smaller, or the frequency lower, then the capacitor has time to charge to the voltage that would have been across the resistor even if you didn't have a cap, and it's as if only the resistor is there.

-- John

Sorry, but you're thinking of this in the wrong terms. You're really thinking about how the capacitor behaves in DC terms, when the important consideration here is the impedance seen at various frequencies. At sufficiently high frequencies (depending on the capacitor value and that of the emitter resistor, etc.), the cap is "bypassing" the emitter resistor - it is essentially a short to ground for AC at those frequencies. Work through the gain calculations (this is assuming that you're taking the output from the collector, of course - a typical CE amp configuration) and you'll see that impedance in the emitter-to-ground leg actually REDUCES the gain.

That's the whole point - a large enough cap CANNOT change its charge state rapidly enough to respond to rapidly-changing AC currents in this manner. That is really what makes it a low impedance to AC - the fact that you CAN'T change the voltage across the cap very quickly.

Bob M.

Yes, the gain is well defined without the capacitor since you can work out all the changes across the resistors without concern of the transistor gain. But as you say, it's fairly low around 10 or less without the capacitor. Bypassing the emitter raises the gain to about

40 times the drop on the collector resistor, or 10 to 20 times higher. That's just a rule of thumb I found somewhere.

No, you can't change it fast, but it still changes faster one way than the other, and that was the confusion. I suppose you get a fast rising edge and slow falling edge with a square wave input? But it doesn't matter how fast it changes, as long as the total change is not too much?

-Bill

where's the bias point of this setup? 100uA with the emitter +100V ?

because they use smaller resistors on the emitter.

typically 0.1R to 1K depending on the application

--

Bye.
   Jasen

No, it really doesn't, esp. not if the signals of interest fall within the range where the impedance presented by the cap is very small compared to the resistance of the emitter resistor - which, if you've chosen the capacitor correctly, is always the case.

Bob M.