BJT biasing formulas

[walid1]

Hi my best friend Alun,
I'm never ignore your replies, I read it carefully every time and I'm waiting for your next instalment that you will add Re in it to the previous common emitter amp and then finally the resistor below the base and you will explain everything, I wait for this tell now.
The problems are that:
(1) Your answers are direct, simple and easily understood compared to Audioguru's
(2) Your answers in most are similar to what I already read in many textbooks. It is true that there are some of helpful additives.
(3) It seems to me that the more hot discussion is that with Audioguru, and you instead of helping me to stand together face to face with Audioguru, you a rise subjects (only to read) not pour into the point of the hot subject.
Please don't admonish at (or to or from) me. I looked at this as there are many points in Audioguru's replies were hard to understand and I expect from you and others to help me to over this, then if any of you have an addition welcome, but not to ignore yours. I respect you and others from all my heart. I feel you are my electronic family and if I meet any of you someday in a road, I'll be very pleasure and I'll help him as I could.
Yours, Walid

 

[walid1]

Hi AUDIOGURU

Audioguru:"The 1st thing was to choose a voltage gain of 10"
Walid: Voltage gain Av = Vo/Vin, how this affect my calculations of resistors, tell me please.

Audioguru:"so leave the collector resistor as 10k."
Walid:" Why you insistent to take it 10K, 8.2K is a close value to not to recalculate it from the beginning!"

Audioguru:"I didn't use a reverse approach. The gain of 10 and requirement for fairly low current demanded that R3= 10k and R4= 1k. Since idle current through R4 would raise the emitter voltage, I knew that the operating point must be with the collector voltage about 5.5V for max output swing. HFE and Ohm's Law calculated the remaining parts."
Walid:" Please Audioguru I don't understand this. You can calculate it as I do, save your time. The best way to make someone easily understand is to show him in form of calculated example, so why they do this in any technical book. Numbers solve the problem, thank you."

please Audioguru I want not to repeat my questions back and forth, with numbers and examples you make me understand fast and saving your time to others who need your experience.
yours, walid.

 

[audioguru2]

Hi Walid,
The voltage gain of a common emitter transistor amplifier stage is RC/(Re+RE). Re is inside the transistor and is small, so the ratio of RC/RE is 10 for a gain of 10.
For a fairly low current and a 10V supply, making RC=10k is fine. Therefore RE=1k. ;D

 

[autir]

I wonder why Autir doesn't learn this basic stuff in school. ???

Because the electronics I have learnt is that of the computer scientist, that is very few and almost all of it digital.
Plus my theory books were meant for physicists and electrical engineers. They are too complicated.
Regarding electronics I am a hobbyist, not a professional.

@walid:
Thank you for your reply. I have visited this link and found it to be horrible - the guy just throws in the Rb values and implies that Vce=0! No, thanks

 

[walid1]

To AUTIr

Autir:"why did you choose a ratio of 10"

I think that there is a rule: If you want a voltage gain say 100 don't make one stage to avoid distortion, make it two stages (10 * 10) or if you can more.

 

[walid1]

Hi audioguru
u told me before that IE= 0.55mA was wrong and IE actually = 0.45mA
please, make another figure like the first one marked "s transistor.png " with the wright numbers.
thank you.

 

[audioguru2]

Hi audioguru
u told me before that IE= 0.55mA was wrong and IE actually = 0.45mA
please, make another figure like the first one marked "s transistor.png " with the wright numbers.

I made a minor mistake near the beginning of my calculations so IE is still 0.55mA but the collector voltage is roughly 4.5V instead of the desired 5.5V. I tried changing the 330k bias resistor to 360k, changing the 47k bias resistor to 43k and other combinations when I realised that when the base current is included in IE and the numbers aren't rounded off, then the actual collector voltage will be near 5.0V which will be fine.
I will leave re-calculation for you to do if you want since 5% resistors will change the voltage about 0.5V anyway. ;D

 

[walid1]

Hi
If Zin=R1//R2//(hfe*(re+R4)), where re=25mV/IE, then
re = 25/0.55=45.45
Zin = 330k//47k//(230*(45.45+1000)
Zin = 330k//47k//240.45k = 35.13K as audioguru said 34.9K

Augioguru, why u think this slight difference. I tryed all combinations of 26 instead of 25 and 0.45mA instead of 0.55mA and the results are more difference.

 

[audioguru2]

Forget about only less than a 1% difference, the tolerance of the resistors and coupling cap will be 5% won't they?
Years ago I was taught 26mV, but new transistors' datasheets show it is wrong.

 

[walid1]

Dear Audioguru,
This may be the last question to complete this circle of questions, but i expect u also want to finish this.
you said this statement: "A 10:1 ratio in a voltage divider doesn't allow the divided voltage to change much when a low gain transistor causes a 5:1 ratio or a high gain transistor causes a 20:1 ratio.
The resistors could also be 360k for R1 with 51k for R2."
1)I understand from this that you are usually take a 10:1 ratio but you may go slightly up or down. Can I take this as a rule in my own design?
2) You said also:"doesn't allow the divided voltage to change much when..."
Are you mean by this "divided voltage" that the VB which now = 1.2V doesn't change much by the applied AC signal, and WHY?
3) if R1=330k and R2=47K, then, the divided voltage VB = 10 * R2/(R1+R2)= 1.25v and ID = 10/(47+330)=26.5uA and the ratio is 11, where
  if R1=360k and R2=51K, then, the divided voltage VB = 10 * R2/(R1+R2)= 1.24v and ID = 10/(51+360)=24.3uA and the ratio is 10.
  and from all that we can concude again that the raio must be around 10.
I love you Audioguru I feel that i'm very close to the truth.

 

[audioguru2]

you said this statement: "A 10:1 ratio in a voltage divider doesn't allow the divided voltage to change much when a low gain transistor causes a 5:1 ratio or a high gain transistor causes a 20:1 ratio.
The resistors could also be 360k for R1 with 51k for R2."
1)I understand from this that you are usually take a 10:1 ratio but you may go slightly up or down. Can I take this as a rule in my own design?

My rule always works, the rule in your tutorial fails when the transistor's gain is low like in power transistors. Take your pick which rule to use.

2) You said also:"doesn't allow the divided voltage to change much when..."
Are you mean by this "divided voltage" that the VB which now = 1.2V doesn't change much by the applied AC signal, and WHY?

You don't understand that the bias voltage divider is for the transistor's DC operating point. AC won't be affected if the coupling cap is big enough and the transistor has its DC operating point correct.
When the transistor's gain is low at half, its bias current is doubled which reduces the base voltage due to the extra current subtracting from the current in the lower divider resistor and adding in the upper divider resistor. Since the base voltage is lower, then the emitter voltage and current are also lower and therefore the reduced current in the collector resistor increases its voltage.

 

[autir]

I love you Audioguru

Ehmmm.... guys?!?  ;D

 

[audioguru2]

Ehmmm.... guys?!?

 

[autir]

I have designed a class A preamp from scratch and written the procedure in a Word document. I would feel very obliged to all that can read it and point out any mistakes/ misconceptions etc.

class_A_preamp_v.0.1.zip

 

[walid1]

The voltage gain of this amp = RC/RE, where Re not bypassed.
What would this gain be if RE bypassed?
thanks.

 

[walid1]

Hi Autir
I downloded this ZIP file and when opening it in WORD97, the document looks like this:

EMBED Equation.3 
14,28 Ohm.
R3=20
EMBED Equation.3 
EMBED Equation.3 
EMBED Equation.3 
285 Ohm.
By Kirchoff's second law, the voltage drop across R2 will be VBB=VBE+IC
EMBED Equation.3 

please help me to view his document.
thanx.

 

[Alun]

This is exactly why I strongly discourage the use of the word document format as it isn't a proper oficial industry supported standard.

autir,
Exchanging documents in word format is generally a bad idea as they can transmit macro viruses, often not read reliably on other computers, and can be hard to open on non-Windows machines.

Try converting it in a differant format like .rtf or .pdf

 

[walid1]

Hi Alun

I miss u ....
I this have petium 1 133MHz PC since march 1997, it is 1300$ that days, today it is less than 100$.
I can't buy new one pentium 4, not only for money but because this PC satisfies all my requirements with less problems.
Audioguru, have 486 PC
So i agree with u that AUTIR replace his file with .pdf
thank you.

 

[Alun]

walid,
What operating system are you running? if it's Windows 2000 SP 4 or later then MS Word 2003 view should work.

How much RAM do you have? OpenOffice 2 requires 128MB and is more modern than Word 97 or go for OpenOffice 1.1.5 if you've only go 64MB and if you've don't even have that then why not give AbiWord a go? All of the aforementioned are at least 99% MS compatable, this only things that normally cause problems are macros and some complex layout formatting and be aware also that macros can spread viruses so it's not big loss them not being supported.

Most people don't give much thought to what productivity software they use, they normally just stick with MS Office as it's what "everybody uses" and it's what they're are used and they are blissfully unarware of the alternatives which are often cheaper and better in some cases. I think this is a shame as there's lots of money to be saved and they should choose productivity software carefully as it's what they're going to be using most of the time.

 

[audioguru2]

The hard drive on my 486/100MHz pc broke on Dec 26/04. On Dec 27/04 I bought a Pentium4/2.93GHz name-brand pc and 19" LCD monitor on sale with many manufacurers' rebates. On Dec 28/04 I got most of my money returned to me because I spotted an ad with a better price and my store guaranteed to beat any price. They were shocked to see that their own ad on the internet had a much better price! When the rebate cheques didn't arrive, I complained and got them 2 days later in the mail. I gave the very nice "photo" printer it came with to my daughter but I could have sold it. The amplified speakers it came with are so good that I haven't bothered to connect the pc to my stereo only 3m away. The CD and dual-layer DVD burners work well and are very fast.
I bought this very good pc nearly for nothing! I was lucky to have good timing.

The accurate voltage gain calculation of a common-emitter transistor stage is RC/(Re+RE). Re is inside the transistor and usually limits the max voltage gain available (with RE bypassed) at a certain supply voltage and with a collector resistor. The voltage gain can be increased with a high supply voltage because then a higher value can be used for the collector resistor. The voltage gain can be increased a lot by using a current-source or bootstrapped collector resistor to replace the collector resistor and an emitter-follower to buffer its very high impedance. Audio amplifier and opamp ICs use these methods.

My MS Office XP Professional program messes-up when I open docs with schematics so I didn't bother trying to see yours.