Thanks Audioguru!!

Why does R10=R9?

R11=375/5
R11=75

When you say Vc=6 should it be Vce=6?
Does this not hold true for emitter follower?
Icq4=(Vcc-Vc)/R10=(12-6)/750
Icq4=.008
Vr11=Icq4xR11=.6

VBQ4=VR11+Vbe=6.7
R12=20R11=20x75=1500

shouldn't IR12= beVbq4/R12
if so IR12=6.7/1500
IR12=.0045
R13=(Vcc-Vbq4)/IR12=5.3/.0045
R13=1177

Can you send a schmatic with the correct values, just so I can compare.

 

I will post my full schematic I did yesterday, this will NOT work because the current is to great.

This schematic you could look over to see if you can understand how I got the values.

 

Here is the heavy current schematic.

For learning purposes only.




I'll put another bigger to read.

 

So I will need to add another CE stage to get the gain of 100? what things should I consider to get the final gain? Thanks so much for you time, I certainly wasn't expecting that much, thanks again. Did I get the values right?

 

The emitter resistor of the last transistor does not need to be as low as 7.5 ohms.
With a base resistor of 750 ohms then its output impedance is 750/beta= 5 ohms.
Then the emitter resistor can have a much higher reasonable value so the current is low and the output voltage can be much higher for a much greater output voltage swing.

 

R10 = R9 so I can use a bigger emitter resistor Normally we like to make R10 at least around 10 times smaller, as small as possible to stop severe signal attenuation, however I had to trade off this so to get the hiogher emitter resistor, so I can get a higher Zin ultimately. This first CE stage is to start getting some gain while trying to keep the Zin climbing higher.

 

Listen to what he's (audioguru) saying he has the expertise in this field, youll learn a lot from him......

And take notes he will lead you in the right direction.....

 

I'll post them in short while here.

 

Here are the schematics..

 

I would make it a class-AB amplifier with complementary emitter followers at the output for a low output impedance and low current but a very high output voltage swing.
Almost all audio amplifiers and most opamps use this type of output circuit.

 

So I will need to add another CE stage to get the gain of 100? what things should I consider to get the final gain? Thanks so much for you time, I certainly wasn't expecting that much, thanks again. Did I get the values right?

I posted all the schematics up above a few posts up.

Your welcome, I hope I was of some help in some way.

Like I said I can only explain things at the basic level.

Your doing real good in learning this stuff.
Again keep up the good work...

 

Was I wrong on Vce to calculate Icq4?
Setting R10=R9, so you can get a bigger parallel combination?
Why is R12=20xR11?

 

How did you get R13=12k?

Icq2=(Vcc-Vce)/R10 =(12-6)/750=.008A

VR11=Icq2xR11=.008x75=.6v
Vbq2=VR11+Vbe=.6+.7=1.3
Make R12=20xR11=20x75=1500
IR12=(Vcc-Vbq2)/R12=(12-1.3)/1500=.0071A

IR12=IR13=.0071
R13=(Vcc-Vbq2)/IR12=(12-1.3)/.0071=1407

I got 1.4k you got 12k for R13

 

Because IR12 is not equal IR13.
IR13 = Ibq4 + IR12.
And IR12=Vbq4/R12=1.3V/1.5K=866uA
IR13=(Vcc-Vbq4)/R13=891uA.
And rule of thumb for voltage divider current is that.
Voltage divider current must be greater than the base current.
In practice 5 to 30 times larger then Ib (typical 10 times larger).

So for β=hfe=Ic/Ib=100---> Ib=8mA/100=80uA

R12=Vbq/(10*Ib)=1.3V/(10*80uA)=1.3V/800uA=1.6KΩ

R13=(Vcc-Vbq)/(11*Ib)=10.7V/880uA=12KΩ

As for Re=R11
Ve must be large or equal Vbe to improve temperature stability.
Re=(0.1...0.3)Vcc/Ic


hobbyist
Why you always choose such a low resistors value?
This is waste of energy.

 

Jony,
I'm a little confused on your question,
are you asking about all the resistors in this whole circuit, or the base bias resistors.

The reason I choose the base bias resistors, is I try to keep the base to ground resistor, any where from 10 - 20 times larger than RE, only to not have base current load the divider, and dropping the voltage.

My attempt on this entire circuit, was to try to get the input impedance at the output stage high enough to be able to use large enough collector resistors, for the CE stages to follow, so as to start obtaining gain, while at the same time keep the RE value high enough (which sacrifices gain), to be able to use larger base bias resistors, to obtain the Zin greater than 600 ohms as was given in the parameters, as I progress to the input stage.

If it was up to me I would have added another CC stage to the first 2, to get the Zin higher yet, but then it would require more tedious calculations and stuff, or use a different config. alltogether, (using NPN's ONLY), but I think I was already making this more confusing for the OP, as it was, BUT the real clincher was the 20mA. total supplied current,

I am going to take my time and see if I can come up with a design that meets all the criteria, just for the sake of knowledge of what I can learn from this.

Please Remember I only understand this at a somewhat, advanced BASIC level, these threads also help me to be challenged in learning more about this.

Original poster: LAGUNA

Was any of this any help at all,?
or did I waste your and my time trying to teach this basic level design...

Please be honest,
I need this feedback, so I'll know if it is worth jumping into threads like this to try to help.

I seen no one else was answering your request at the time, of your first post, so I tried to get the ball rolling.

TO EVERY ONE on the forum who was following this thread,

THANKYOU, for showing the courtesy, of allowing me to have the floor, so to speak, but I do not take any offense, if anyone could jump in and speak there own mind on the subject at hand, I see this as an OPEN forum, that's why I do NOT accept PM's either, so if you can better clarify for the OP to understand than please feel FREE to do so, But I do appreciate the effort of the courtiousness that was shown here.

Thankyou..

 

Your work and effort were of tremendous help to me. I learn best by examples. In some of these forums, you're told to go read something, but I have read something already and didn't understand it and I'm made to feel stupid. That is why I've asked a question on the forum to begin with. In a lot of cases it is even tough to frame the question to ask. I was very impressed with your knowledge and willingness to work through it with me. Reading and understanding theory is one thing but seeing it in practice is extremely helpful, I loved it. For me as a newbie just knowing where to start is tough. But you gave me a real practical approach to working through a design, that was invaluable, plus I had to get this thing done. I'm sure it was old hat to a lot of folks on the forum, but to me it was great. Thanks again and I hope some day I will have the patience and concern to walk someone else through the process as meticulously as you did me.

Thanks again

PS is R13 1.4k or 12k?

 

Laguna,

Thankyou very much for replying back, and the kind words of encouragement.

Here is how I got 12K ohms for R13.


volt at the collector = 1/2 VCC
so the volt drop across the R10 is 6v.

IC = 6v. / R10 = 8mA.

Volt across R11 = 8mA x R11 = 600mV.

Vbe = 0.7v.

Voltage across R12 = (600mV + 0.7v) = 1.3v.

current through divider = (1.3V / R12) = 867uA.

R13 = {( VCC - 1.3v.) / 867uA. } ,where, VCC = 12v.

which gave 12,346 ohms

nearest standard value = 12K ohms.

Thanks...

 

The simplest way to achieve this spec
Av=100
RL=10kΩ
Rin=>600Ω
Rout=<8Ω
Is to use opamp or BJT amplifier with negative feedback.

Zin≈R1||R6=10KΩ; Zout=<16Ω and Av≈R8/R3;

Or use bootstrap to achieve Av=100 and Rin>600Ω

And remember that Rout of a emitter follower is equal:
Rout = Re||(re + Rsource/(hfe+1) )