Newbie question about Biasing transistor in class A with voltage divider metod.

[Olegarich]

Don't know where to ask, hope it's a right place.

Dear friends.  I'm trying to learn electronics by reading and watching videos.
Now I'm trying to understand transistors. I stoped in section of Biasing transistor in Class A. Not sure if I understand it correctly. If you have time to comment on it, would be much thankful.

On picture below, here is typical schematics for amplifier in class A.

As I understand, R1 and R2 sets Dc value that sets BASE bias voltage. Without this, transistor on output (Collector)would give us only possitive part of waveform. By biasing it, we put waveform beetwen cutoff (now working) and saturation. That way we here possitive and negative swing of it.  Am I right?

What is RL needed? Is it keeping transistor always "ON"? If yes why, dc applied to base not doing that?

As I understand RL  is called load resistor, what would be if there would be straight connection to PSU?
What exactly difference in job of R1-R2 and RL???????????????

Last question is about RE, why we need this resistor here? What would it  be if it would be plugged straight to the ground?

Thank a lot for your help and time.

 

[JohnRoberts]

Or you could google "common-emmiter" gain stage and read that...

Short version is that your transistor is a high impedance at the base, and low impedance at the emitter, basically following the voltage changes at the base 1:1. Whatever current flows out the emitter must get sucked into the collector. The collector load resistor changes that current back to a voltage. 

So very simply the voltage gain will be -R(load)/R(emitter ). Of course with simple transistor gain stages you must DC bias them up considering optimal signal swing.

Nominal DC operating point at the output can be calculated first as a simple resistor divider to feed the base, then subtract one Vbe voltage drop (roughly 0.5V) to figure the emitter voltage.  DIvide this by the emitter resistance to compute the collector current. That collector currunt times the load resistor will tell you how much of drop below the + supply it will sit at for 0V AC in.

The collector will saturate when it hits the emitter or cut off when it hit + supply so your output AC signal swing will be limited by the DC operating points.

Gee that was easy  8) ... but of course in the real world there are many interactions and limitations , while that simple circuit forms the basis of many cheap tape decks and transistor radio audio paths.

JR

 

[PRR]

> RL  is called load resistor, what would be if there would be straight connection to PSU?

Then your output would be the power supply. Do you want to listen to the power supply? It has no talent and only knows one note (at most).

There are whole books and websites which discuss what you are asking (and probably easier to digest than movies). Don't make us type-up private lessons.

Transistor Circuit Approximations by Malvino is good, and you can get it for a penny.

 

[Olegarich]

Thank you guys. Defenitely i don't want private lessons here.  Now I know where to start looking. Thanks.

 

[PRR]

> trying to understand transistors. ...As I understand, R1 and R2 sets Dc value .... What is RL needed? .... As I understand RL is called load resistor, what would be if there would be straight connection to PSU? ... What exactly difference in job of R1-R2 and RL????? .... Last question is about RE, why we need this resistor here?

You are actually pounding around the KEY point that I think most folks who "want to learn transistors" SKIP.

One transistor can't do anything by itself.

We "always" set-it-up with resistors, to tell it what to do, and to "fight against" to extract an output.

Most self-learners skip the first 4 chapters about resistors, and go into the transistor chapter practically un-armed, soon get befuddled.

I admit that many books (including Malvino's) make the resistor chapters as un-useful as possible. Round-number resistor problems with no goal or usefulness. It may help to pose yerself some practical problems.

The power wire from the street is 1 Ohm resistance. You connect a 100 Ohm load. What voltage do you get? How about a 10 Ohm load? (This is a real problem at my house: I have 0.4 Ohms in a long wire and my loads run down to 6 Ohms.)

 

[PRR]

Here's a resistor-oriented 1-2-3 cookbook for transistor bias.

There is much unsaid. He pulls goal-numbers from thin air; not wrong, but not right for all situations.

At step 9 he pulls a fast one. He secretly assumes that 0.5 is the same as 1-0.5. It is; but if at step 8 he had needed a ratio other than 0.5, then step 9 is wrong as written.

Also note that he does not compute the *amplifier* performance; indeed does not even show any signal connections.

The scan is fuzzy in fine subscripts. I have clarified them, also noted some numbers I like.

 

[PRR]

The unloaded voltage gain is 5. This is for zero impedance source and infinite impedance load. This is not a practical situation. If you really had zero/infinity, you could use a transformer to give any voltage gain you wanted.

The input impedance is (about) twice the output resistance.

You "can" get any larger voltage gain or impedance change by cascading this amplifier with maybe a few voltage-dividers (resistors again!).

A test-case for cascaded amplifiers is the "iterated gain". If you had an infinite chain of these, what is the gain per stage? The ideal gain is 5, but the 2:1 impedance radio drops that by 2/3, so the iterated gain is 3.333 per stage. If you wanted a 60dB mike-amp, you would need 6 such stages. If you wanted a 600 Ohm output, you could get a 40K Ohm input.

THD at maximum output is about 0.2% per stage (his 10% rule gives significant NFB). In an iterated chain, all on the same supply, only the last stage hits full output, so the iterated THD approaches 0.3%, with a distortion profile unlike a high-gain amp with overall NFB, because each stage distorts the distortion of all the stages before it. 

It is possible to increase the voltage gain (to nearly Vs/0.025V) but THD will rise, and past some point the input impedance falls.

While you have to work-through and thoroughly digest 1-tranny amplifiers, in many practical cases transistor pairs are more than twice as good.

I believe there is much merit in pagiarizing, then analyzing what you plagiarize. Don't re-invent the most invented wheel in history.

 

[JohnRoberts]

I believe there is much merit in pagiarizing, then analyzing what you plagiarize. Don't re-invent the most invented wheel in history.

When I was just a junior technician I would study any schematic i could get my hands on and try to figure out what all the different components were doing there.  Especially with discrete component designs, these small building blocks become like a vocabulary that you assemble to make sentences (actual functional circuits).  But first you need this basic vocabulary.

JR

 

[PRR]

> study any schematic i could get my hands on and try to figure out what all the different components were doing

+1

 

[buildafriend]

> study any schematic i could get my hands on and try to figure out what all the different components were doing

+1

I went to tech school for electronics and honestly I've learned more building stuff and ordering parts than I did in school. Start putting things together and take real world measurements with a multimeter.. but don't scrap the theory because it helps. The art of electronics is a good book to pick up.

 

[Olegarich]

thank you guys, I will try this metod.