# simple BJT audio amp experiment (need help)

#### Wendy

Joined Mar 24, 2008
23,189
Nothing to work out, I gave a reference. I have a lot of books on theory that state what I showed, never seen your work. So as to which I believe, I'll stick with 30+ years experience and college. It's a no brainer guy.

I have never seen thermal effects used for feedback, but in this case it is regenerative (ie, positive), as temp goes up the collector current goes up. Not a valid arguement. The real increase in current swamps all your assumptions.

This equation has several holes.

2ndly, the base current is tied to the collector voltage, Ib=(Vc-Vbe)/Rb; and the collector voltage is tied to, among other things, the collector current: Vc=Vcc-Ic*Rload.
False assumption there.

Ic = β Ib

states otherwise. There is no connection between collector voltage controlling base current.

The correct equation is
Ib=(Vcc-Vbe)/Rb

I suspect you are thinking of this design:

I don't have the time or inclination to look up its bias formula, but it starts with Ic = β Ib, and adds the resistors effects. The emitter swamping resistor and base resistor divider are much more effective though.

#### Attachments

• 1.3 KB Views: 147
Last edited:

#### millwood

Joined Dec 31, 1969
0
Originally Posted by Bill_Marsden
And the base current is in no way tied to the collector current. The base BE drop will vary a little according to temperature, between .6 and .7V, but the resistors set the bulk of the current, and this doesn't vary. Temperature variation is not considered negitive feedback either.
here is a chart of Ic vs. Vbe from ON Semi 3281/1302 datasheet. I just picked one curve to demonstrate the point for you: that even if there is Re resistor, negative feedback exists in the 1st circuit (where the base is tied to the rail).

as you can see, the Ic/Vbe curve is positively sloped. meaning that as Ic goes up, Vbe has to go up - they follow a logarithmic function, if you follow semiconductor physics.

so if there is a disturbance to the circuit that causes Ic to go from 1amp to 10amp, Vbe will go up 0.7v to about 1v. This will cause a decrease in Ib, even in the first scehmatic, as Ib=(Vcc-Vbe)/Rb, so dIb/dVbe=-1/Rb<0.

whether that is strong enough to counter the thermal runaway is another discussion.

#### Attachments

• 25.5 KB Views: 22
Last edited by a moderator:

#### millwood

Joined Dec 31, 1969
0
I have never seen thermal effects used for feedback,
check out lateral mosfets.

This equation has several holes.

False assumption there.

Ic = β Ib

states otherwise.
no idea what you are talking about.

The correct equation is
Ib=(Vcc-Vbe)/Rb
that's correct for the 1st schematic the original poster had where the base is tied to rail.

try for the 2nd one where the base is tied to the collector via a resistor, which is what we are talking about here. you work it out and if it differs from what I had, i would have been surprised.

#### Wendy

Joined Mar 24, 2008
23,189
Guy, I've shown the math, I've shown the reference, we're going nowhere. Where ever you got your info you have some unlearning to do.

We're talking BJT's, so don't change the subject. FETs have their own rules, and have no place in this discussion.

It doesn't matter what power supply the base is connected to, as long as it's stable. If anything, it will further reduce any possibility of feedback. Vc is Voltage of Collector, not power supply voltage, so you're being disingenuous.

You want to get away from it, but you can't. A BJT first equation is the Ic = β Ib , all else follows. Any assumption that doesn't use this equation I will consider invalid. You can't even add to it, since it is the derivative for all other bias equations.

Last edited:

#### millwood

Joined Dec 31, 1969
0
I suspect you are thinking of this design:
I was talking about the 2nd schematic that the original poster posted.

I don't have the time or inclination to look up its bias formula, but it starts with Ic = β Ib, and adds the resistors effects.

work it out, you will be surprised how wrong you were about the circuit the original poster posted and the one he built and the one we are talking about here.

The emitter swamping resistor and base resistor divider are much more effective though.
we are NOT talking about there effectiveness. we are talking about IF there is a negative feedback. how effective that negative feedback is is another discussion for another day.

#### millwood

Joined Dec 31, 1969
0
Guy, I've shown the math, I've shown the reference, we're going nowhere. Where ever you got your info you have some unlearning to do.
i've shown the math and you refuse to follow.

point out which part of my math is wrong and I am happy to go with you.

We're talking BJT's, so don't change the subject. FETs have their own rules, and have no place in this discussion.
we are talking about bjts too.

It doesn't matter what power supply the base is connected to, as long as it's stable.
that's just simply wrong. go through the math, as I did for you.

If anything, it will further reduce any possibility of feedback. Vc is Voltage of Collector, not power supply voltage, so you're being disingenuous.
what is "it" here?

you are being disingenuous by NOT following the math.

figure out the bias for the 2nd schematic that the original poster had posted. just two equations so you should be able to solve it.

if you cannot, I am happy to help.

#### Wendy

Joined Mar 24, 2008
23,189
OK, I was looking at the first one, I didn't notice the difference between the two.

plus the microphone.

Given that I took the time to draw everything I was talking about, do you really feel this discussion was necessary?

I get the feeling you weren't really looking at my posts.

#### millwood

Joined Dec 31, 1969
0
You want to get away from it, but you can't.
wrong.

A BJT first equation is the Ic = β Ib , all else follows.
true.

Any assumption that doesn't use this equation I will consider invalid.
Ib may or may not be determined by that.

You can't even add to it, since it is the derivative for all other bias equations.
here is a reworked equation, I just changed the sequence in which they follow, and I took out the tempco calculation as that might be too heady for you. and i also add a few explanations to help you understand it.

for the first schematic, where the base is tied to Vcc via a resistor, the equations are simpler:

Vcc=Ib*Rb+Vbe

Ib=(Vcc-Vbe)/Rb.

and Ic=hFE*Ib=(Vcc-Vbe)/(Rb/hFE)

for the second schematic:

between the collector and the rail, the following must be true:

and from the collector to ground via the base, the following must be true:

Vc=Ib*Rb+Vbe

and since Ic=Ib*hFE,

I hope you can work out the tempco and see the feedback (introduced by Rload - that's the difference between the two formula).

#### millwood

Joined Dec 31, 1969
0
OK, I was looking at the first one, I didn't notice the difference between the two.
well, you cannot blame me for your not seeing it, can you?

more to it.

Given that I took the time to draw everything I was talking about, do you really feel this discussion was necessary?
yeah. you learned something in the process and that's valuable to us all.

I get the feeling you weren't really looking at my posts.
if I didn't, why did I kept mentioning to you to look at the second schematic?

i was practically screaming out of the bottom of my lung, .

what I hope you understand by now is that there are many forms of negative feedback. some stronger than others.

but it is practically impossible to design a circuit without any negative feedback.

#### Wendy

Joined Mar 24, 2008
23,189
Dude, I've done this a while. I believe I know more than most, a lot more.

And yes, since you didn't bother to look at my diagrams, which I referred to in the same posts, I do have a problem here. But I'll leave it at that.

#### millwood

Joined Dec 31, 1969
0
Dude, I've done this a while. I believe I know more than most, a lot more.
unless you know most people, you cannot know that you know more than most. that's just fact.

And yes, since you didn't bother to look at my diagrams, which I referred to in the same posts, I do have a problem here. But I'll leave it at that.
just the opposite: i look at each and everyone of them. and it is you who didn't know which schematic we are talking about and falsely accused me for not knowing my math just because you were thinking about the wrong schematic, in spite of my repeated reminders to you.

#### hobbyist

Joined Aug 10, 2008
892
there is a form of negative feedback in this particular design: you will notice that the base resistor is tied to the junction of the speaker and the collector. so when the collector current goes up, the voltage at that junction will go down, which lowers the current going through the base, which lowers the collector current.

you CAN make this circuit work but it will be device dependent.
Are you talking about post #8 schematics,

Could you please point out where you see a base resistor connected to the collector terminal?

Thanks......

#### Wendy

Joined Mar 24, 2008
23,189
Second of the original OP schematics, post #1.

I'm not sure you could ever get that design to work very well, since the collector is 8Ω (and we're talking a complex impedance).

#### hobbyist

Joined Aug 10, 2008
892
OK, I see it
THANKS...

#### count_volta

Joined Feb 4, 2009
435
Oh wow it seems I have started a huge discussion. So much good info. Thank you.

Yes the problem is, nobody teaches the limitations of these devices in my courses, maybe they will in the future but they have not so far. Its always, we have so much to cover in so many weeks, and the student suffers as a result.

I remember asking a fellow classmate who is a senior in his last semester, "Do you think that what they teach us here can be used in real life, and how practical is it?" He said, "Not at all" That just about sums it up.

And yet I heard that when you try an intern engineering job they expect you to design real life circuits right away. There is a huge gap between the real world of the workforce and university. Do any of you agree?

Personally I want to be the best in my profession and hence why I'm not going to wait until next semester but I want to get a head start in the summer and build about 5 projects I have planned, and learn all the limitations of these devices in real life.

Project 1. This audio amp.
Project 2. make a continuous electronic beep with an oscillator.
Project 3. Mess around with my FPGA and make some logic circuits.
And the other 2 projects have yet to be defined.

I will go read all your posts and then ask questions, of which you will be sure I have hundreds. So be ready for that.

Umm is it allright if I take one fo you guys home with me, so you can live in my house and be my own personal tutor? I will not only get straight A's but be the best in my field.

Last edited:

#### millwood

Joined Dec 31, 1969
0
Second of the original OP schematics, post #1.

I'm not sure you could ever get that design to work very well, since the collector is 8Ω (and we're talking a complex impedance).
it depends.

the basic topology: feedback from the collector, is what Pass used in tons of his amps.

whether that works, or works very well, obviously depends on individual preference.

#### millwood

Joined Dec 31, 1969
0
Do any of you agree?
most of the texbooks are useless when it comes to their applicability to life. however, they teach you a structured way to solve problems. you don't learn particular knowledge from those books. you learn the principles and how to solve real life problems using those princeples.

Personally I want to be the best in my profession and hence why I'm not going to wait until next semester but I want to get a head start in the summer and build about 5 projects I have planned, and learn all the limitations of these devices in real life.
you are on to a good start. keep up the good work.

#### Wendy

Joined Mar 24, 2008
23,189
it depends.

the basic topology: feedback from the collector, is what Pass used in tons of his amps.

whether that works, or works very well, obviously depends on individual preference.
Not really, that speaker might as well be a dead short. You do know they aren't 8Ω at DC levels, right? There is reactance there.

Since a transistor can be current only I used the first drawing, which is actually almost practical. I remember a clorox bottle megaphone design using a GE3 transistor (TO3 case style), the carbon mike, and a speaker. I build it, it worked, but not well enough to keep.

Count Volta: Keep the questions coming, they are pretty good ones from what I've seen.

Side note, there are whole classes of configurations for audio (and RF) amps you haven't touched on. All of them attempt to answer the various problems with transistors in different ways. Things like Class A amps, Class B, etc are worth looking into for more information.

#### count_volta

Joined Feb 4, 2009
435
Making it work

You can make this design work, but it will never be any good. So lets go through the exercise.

First of all, get that power supply down, way down. Try 3V instead. This will reduce the amount of heat generated by the transistor. It will still get warm, maybe hot.

Since the exact gain the transistor has is unpredictable, use a 1 KΩ pot, maybe a 10KΩ for the base bias resistor, with a 470Ω resistor.

Start with the resistor high, then slowly adjust it down. You will hear something, but the gain of a single transistor is also pretty low overall, so it won't be overwhelming.

If you have a DVM measure the voltage on the collector. It will be somewhere around 2.5V, more than that the currents through the speaker and collector of the transistor start getting too high. As is they will be pretty large, if you're using a battery it won't last.

As has been mentioned, a carbon mic works best. This is because it is basically a variable resistor, which is exactly what this circuit needs. It is also much more sensitive to audio, which means it will give the transistor a larger signal to amplify, but it has a very limited frequency response. Telephones didn't need fidelity, so it was perfect for the application.

The reason I showed a speaker for the mic is basically that is what a dynamic mic is. You can also use a conventional speaker for a mic with good results.

Most circuits are more complex for audio amps. They incorporate negative feedback to suppress as many of the undesirable characteristics as they can, plus adding the gain of several transistors means you will have enough gain, while keeping the current to the circuit and the heat it generates strictly under control.

Pesky editor dropped a lot of typing during the edit, so you will note changes as I worked on it.
Bill what is the point of the 470Ω resistor if you already have a variable resistor there? Or is it the resistance of the microphone?

Also I take it I need a variable resistor because I don't know the max output of the microphone and hence don't know exactly what bios I need to make the transistor pass AC right?

I am using a speaker I bought from radio shack. This one. http://www.radioshack.com/product/index.jsp?productId=2062406

Can I get another one and use it as a microphone? Or is it better to get the carbon mic?

I think that what is different between a speaker and a mic is the type of diaphragm that is used. A mic diaphragm is designed for picking up vibrations in the air, while a speaker diaphragm is designed for moving due to magnetic fields caused by the signal. Am I right?