Understanding Transistor Amplifier circuit design

[FuzzJourney50]

Hi folks
I need some help understanding BJT and JFET transistors in simple amplifier circuits, specifically for input and output buffers.

I can't seem to get my head around understanding how you control the amount of gain you will get, and I'd like to understand the topic properly.

I do understand the basics of how the gate/base of the device affects the flow of current between drain/source, or collector/emitter.

I understand that the Hfe affects the amount of gain the device can achieve.

And in the JFET circuit (Fetzer valve) I've been playing with I've seen that setting the source resistor allows you to control the amount of gain.

And I've seen that it's common for 4.5v biased unity gain type buffers to have a collector/source resistor value of 3.3k.

But how do I put all this together to be able to work out what level of amplification I would get when choosing a particular transistor, and the reasons why?

Do I need to refer to those datasheet graphs? or measure the component?

Or is it always as simple as adjusting the collector/source resistor?

I've read lots of books, watched youtube tutorials, and I just seem to be missing the big picture.

So if someone could explain in simple language it would be hugely grateful....

All the best
John

[antonis]

In brief:
BJT buffers are biased on their Base which stands about 650mV higher than Emitter..
So, to have 4.5V on Emitter (mid-supply - not necessarily but preferable), Base should be biased at 5.1V or so..
Now, for Emitter biased at 4.5V, Collector-Emitter current should be 4.5/Emitter resistor value..
(4.5/3300 -> 1.3mA)
Emitter resistor value and Emitter voltage level set working current..

P.S.
Probably, next query should be "What is the optimum working current..??"

[r080]

I can't seem to get my head around understanding how you control the amount of gain you will get, and I'd like to understand the topic properly.
...
I understand that the Hfe affects the amount of gain the device can achieve.

And in the JFET circuit (Fetzer valve) I've been playing with I've seen that setting the source resistor allows you to control the amount of gain.

Are you interested in buffers only, or voltage amplifiers? Buffers, by definition, will have a voltage gain of around 1.

[marcelomd]

I took an analog circuit design course in college. I learned that there's the "proper" way to design things, with datasheets, lots of math, etc. and that there's the "usual" way, where you take a circuit you know and adjust it, possibly using simulation.

So, my advice is to play around with SPICE simulation and then run some experiments with a breadboard. You'll get a felling for how things interact.

[soggybag]

This article here was the best thing I have read that helped me understand transistors.

http://moosapotamus.net/files/stompboxology-going-discrete.pdf

Here is an article I wrote that reprises the ideas from the article above:

http://www.super-freq.com/stompbox-studies-class-1/

[antonis]

This article here was the best thing I have read that helped me understand transistors.
http://moosapotamus.net/files/stompboxology-going-discrete.pdf

Excellent brief approach but ruther confusing for niewbes..
(I'm talking about positive and negative swing output impedance..)

[PRR]

> how you control the amount of gain

Voltage gain? Current gain? Power gain?

Do you understand resistors THOROUGHLY? Your meaningless observation about 4.5V and 3.3k suggests a superficial grasp.

> I understand that the Hfe affects the amount of gain the device can achieve.

Not really. May limit the maximum current gain. Has no direct effect on the voltage gain, which is what most audio-folks think of as "gain".

> ..setting the source resistor allows you to control the amount of gain.

Totally, dude! In BJT design you do NOT care what transistor you use (unless it is extremely lame, 1951 era). Gain relationships depend mostly on resistors.

[FuzzJourney50]

Well, the problem is I don't know what I don't know! I'm on a journey and I've only got so far....

I've built circuits in Spice, and I've experimented on breadboards.

And I can build a buffer and a gain stage with no problem, and I can adjust them to create more or less gain, but I've learnt that parrot fashion.

but, I have doubts and would like to understand how to know how a chosen transistor is going to respond, and thus how it needs to be setup.

I was hoping someone could help and provide an overview explanation.

If there is anyone that would be up for a Zoom conversation I'd really appreciate it.

[antonis]

IMHO (ask me why.. ), if you want to UNDERSTAND you have to STUDY..

By trial and error, you might result into proper conclusions but you'll have to work countless variants to establish lab test rules of thumb..
(and you'll always wonder about the "theory" you've experimentally documented..)

As Paul said above, resistors play the main role 'cause "everything" exhibits resistance, hence it's a potential voltage divider..
(as far as we deal with small signal amplifiers, we don't want to lose signal amplitude - at least, not without a good reason..)

Transistor buffers serve for impedance matching (for the less achievable voltage loss)..
"Voltage" is one of the main parameters involved in Ohm's Law and is in direct dependency with the other two parameters, which are  "Current" and "Resistance"..

So, for 1% voltage loss (say), signal source output impedance needs to face a next effect input impedance 101 times bigger..
(1% should be lost and 99% should go into next effect - so far so good as for buffer input impedance..)
Of course, buffer's output impedance should also be much smaller than next effect input one 'cause buffer is now the driver configuration..
(it took delivery of signal source and delivered it to next effect..)
Buffer's output impedance is determined by its current and its Emitter resistor..
(here you'll have to study about "intrinsic emitter resistance", which is effectively set in parallel with  Emitter concrete resistor..)

What we have to take into account for a simple amplifier, is output impedance (set by Collector resistor for CE amp and practically intrinsic emitter resistance for CC amp), input impedance (set by bias configuration and h_FE X Emitter resistor(*) for both CE/CC amps) and gain (voltage for CE amp and current for CC amp)

(*)In the absence of Emitter resistor (grounded Emitter), intrinsic emitter resistance comes into bussines..

[GibsonGM]

"Parrot fashion" works pretty good to get things running, to repair many things, and to enable one to get working with switches and amplifiers. 

Learning to set up a BJT for switch or amplifying - thinking in terms of CURRENT...seems to be another step up the staircase of understanding...

Really understanding the relationships between the equations and how they work with external resistances and such - that's entirely another!

I think you'll find many of us are on steps 1 and 2, and still scratch our heads on the hard math and physics part... 

[Clint Eastwood]

Hi,

Playing around with a simulator and breadboarding are actually very good ways to learn is my experience. But I also recognize getting stuck because of lack of basic knowledge. Here are two sources that helped me a lot over time: Electronics tutorials https://www.electronics-tutorials.ws/ explains very clear the questions you have about amplifier stages,
and also Rod Elliott's site is great https://sound-au.com/ here you find plenty practical circuits and articles, and contrary to many similar sites, things are explained quite detailed.
And if you have more specific questions, forums like this are invaluable.
Hope this can help you as it did me,

Clint

[Vivek]

I got into electronics because I found a copy of National Semiconductor Audio handbook when I was 14

Absolutely great book for basic principles of Audio, using  Opamp

It's available for free download now

Highly recommended