Collector to Base Resistor - Why?

It's providing bias current for the transistor. Without it, it wouldn't be an amplifier ! It's a failry crappy way of providing bias btw since the operating point wil be heavily affected by beta/hfe.

The resistor will be the last of your problems. That will also stop it being an amplifier too. Nor do you need 'back to back' diodes either. Since when will the collector ever be negative wrt to the base ?

I think you need to go learn some circuit basics.

Graham

That's why I came here. To gain knowledge from those willing to help a novice.

I'd be happy to exclude you from that pool of knowledge if you insist on being such an asshole.

Lumpy

--
You Played on Lawrence Welk?
Yes but no blue notes. Just blue hairs.
www.lumpyguitar.net

The resistor, R provides base current as has been mentioned but it also provides negative feedback and helps stabilize the operating point of the transistor. The operating point is establised by the Beta of the transistor and the voltage drop across R. If the collector voltage gets too high, the transistor turns on harder reducing the base drive which lowers the collector voltage. Likewise, if the collector voltage is too low, the collector rises turning the transistor on harder. This is a stabilizing action. The local negative feedback also reduces distortion. Think of "R" as the feedback resistor in an op-amp circuit where the transistor is a "poor man's" op-amp. This circuit was very common years ago in simple transistor amplifiers like in radios, etc. Bob

I think you ought to read a decent book on the subject first. A half-decent working knowledge of simple circuitry is a reasonable prerequisite before asking question as basic as this one !

Graham

This was posted to SEB. The B is for BASIC.

"Lumpy"....

When someone advices you to go out and learn something about basics, it isn't meant to piss you off. Your reaction is absolutely out of proportion. The only thing you have assured is no help in the future and that is a bad way to start your knowledge build-up. The question you asked is part of one of the first lessons in Transistor technology.

Go out, buy a book about basic electronics (Any), read it, try it, read it again and design your own. After that.. Maybe, someone is willing to help you again. (Ever heard of kill-files? They work like a black hole).

One pointer for your next question: It is bad practice to build your question like : "I have a problem, someone has the answer, Gimme, gimme, gimme and if you don't you're an asshole."

Try it in the following format the next time: - Tell us what you want to achieve. - What you read already about the subject. - Your approach so far. >

- Where are you stuck..

Don't forget that people on Usenet (Some excluded) are here to share knowledge to those who are willing to put some effort in the subject themselves. And they are all volunteers. Putting a question on the internet doesn't give you the right to demand an answer. You'll just have to wait if someone is willing to spend some of his or hers *spare* time in your problem. If you're here expecting direct answers to your problem, you're in the wrong place. You'll only find pointers here to (maybe) a solution. There's always an effort required from your part.

For now your only achievement was insulting someone who was willing to help you point you in the right direction and the only problem you have at the moment is your attitude.

Kind regards,

Peter

That sure doesn't mean this is an educational institute, right? People here are allowed to expect some effort from the one who's asking questions. After all, I went to 8 years of study and almost 25 years of experimenting to earn my Electronics Engineering Degree.

Kind regards,

Peter

Sure but not not s.e.k ( sci.electronics.kiddies )

Graham

It provides a bias current to get the transistor conducting a little, so that it can respond to the AC signal arriving through the base capacitor. Since it delivers a current roughly proportional to the collector voltage, the bias point is somewhat stabilized for different transistor gains (if a high gain transistor over reacts to the bias current by turning on too much, the low output collector voltage lowers the bias, reducing the effect of higher current gain).

Unfortunately for high impedance guitar source, the negative feedback does not only apply to DC bias current, but to the signal frequencies as well. The net effect is that the feedback tries to hold the input node at a more fixed voltage, lowering the input impedance of the amplifier. Guitar signals work best into high (relative to the guitar pickup) impedance amplifiers.

The feedback resistor will limit the gain to some maximum value when neither diode is conducting. You also need to think about the DC voltage across this resistor, and what that will do to the diodes. For instance, you may need to add a capacitor in series with the diode pair, so that only AC signals are altered by those diodes, not the DC bias point.

Ignore him. He's an asshole.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

You're right. We didn't see any need to create a newsgroup just for you and the other ignorant trolls.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

I guess maybe I don't understand what USENET is for then.

Jim

Darn, that must have been one heck of a tough school. That is 29 years longer than most four-year degrees take to complete ;)

Richard

Bob Eld wrote:

Thanks very much, Bob. The op-amp FB analogy helps very much.

Lumpy

--
You were the Tidy Bowl Guy?
Yes. I\'m cleaning your bathroom bowl.
www.lumpyvoice.org

Pot, kettle, black.

Graham

Whatever your personal fantasy is about what this NG is, it's a public, unmoderated forum with the words electronics and basics in the title. So if your nose is so far up your own ass the the purpose of YOUR being here is to look down on people asking basic electronic questions, that's your own problem.

Congrats on that 8 year electronic degree. Isn't there a newsgroup for people of your self perceived importance that doesn't lower you to basic status?

Lumpy

--
You Played on Lawrence Welk?
Yes but no blue notes. Just blue hairs.
www.lumpyguitar.net

John P> The feedback resistor will limit the gain to some maximum value when

Thanks John.

Here's a real world example -

Disregarding the diodes at the output for now, I'm concentrading on the D1 D2 pair. No cap. It clips the audio signal, but way too hard. So I'm assuming, perhaps incorrectly, that what I want to do is put some R in series with the diode pair. I don't have a scope, but the sound I hear at the output is that of a really hard clipped square wave.

Thanks -

Lumpy

--
Can you do that FM disc jockey voice?
Yes, but it doesn\'t translate well in ascii.
www.lumpyvoice.net

Without a series DC blocking capacitor, one of the diodes provides enough average bias current to pull the collector all the way down to

1 diode drop above the base voltage at no signal. and the other one is not used at all, because the collector to base junction would do its job.

Try it with a fairly large blocking capacitor (a few microfarads, with the positive end toward the collector, if electrolytic) and also a resistor in series with the diodes, to soften the clipping. Something around R/4 to R/10 might work better. Then adjust the value of R to get an average collector voltage a little above half of the supply voltage, so the diodes control the clipping, not the transistor saturating.

Hi Sr.! Your primary school math is perfect;^). But how many years it took, after the degree, to admit that on some subject you know nothing, and approach someone more experienced to ask? "Engineer knows everything about nothing, merchant knows nothing about everything."

Have fun

Stanislaw Slack user from Ulladulla