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Large signal CE amplifier

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I guess, it would be interesting to analyze the exact circuit.
 

Goldsmith show as example.
Because when we design power audio amplifier we also use the "small- signal" model.
I've never seen that anyone designing the amplifier using "Large signal".
 

Just one comment from my side regarding the term "gain".

The term "gain" is applicable only for systems that have input and output signals of the same shape - otherwise we cannot create the output-to-input ratio of both signals.
With other words: A voltage gain can be defined or computet for small signals only (without a remarkable distortion).
Large signal amplifiers are treated in most cases under the aspect of power gain, which can be defined also for different input resp. output signal forms.
 

Thanks for replies
See these pictures please:
amp.jpg
amp2.JPG
Thanks
 

the second stage will work as a large input amplifier!
when i was in vocation , we had to design with this types of amplifier. one of our designs was 5w amplifier with these arrangements . the work of these types of amps was good and efficiency was awful ! they was in this believe that when we feel the disadvantages of class A , we can learn higher class . we learned the higher class amplifiers in vocation ! ( about 3 years ago) .

---------- Post added at 18:22 ---------- Previous post was at 18:20 ----------

( at first step we designed a small signal amplifier , and then with randomly change of resistances we obtained our aim.)
 

All linear amplifiers work on the basis of "linear" elements.
Transistors are inherently exponential, ie non-linear.
They can be approximated to be linear only when the input signal is small and the transistor is operating in saturation, hence the need for a small-signal input biased to an appropriate DC level.
Most "large-signal" amplifiers essentially use some form of feedback network to
1. Shift the input to an appropriate DC level
2. Attenuate the input such that it is seen as a small signal at the input of the internal amplifier
Without this feedback network, any minor shifts in input signal, process/temp/supply variations will easily force your amplifier out of saturation.
 
View attachment 63899

Hmm, what can I say about this diagram? Maybe OMG will be appropriate world.
Simply you don't know nothing about amplifier design.
Engineers long time ago understood that the best way to drive the Push–pull stage, is to use the "current" (connect CE amplifier directly before the output stage) instead of voltages.
See the example diagram.


But to encourage you I show you my first headphone amplifier that I design and build, and it's work pretty well.
 
Referring to post #24. Your circuit is a good example about the limitations of transistor amplifiers without linearizing feedback. Although the 8 Vpp output looks almost O.K., you'll see already 2.7 % THD after the second stage. (I simulated the preamplifier stages with an equivalent load of 500 ohm). The distortions increase rapidly with slightly higher output level. Even if you consider several percent THD as acceptable, the amplifier is simply inappropriate to drive an output stage powered with +/- 22 V.

You'll find a rich choice of audio amplifiers with better designed driver stages in literature.
 
Hi my friends!
I think that for the large signal amplifiers , it is better that we use direct coupling between stages ( due to behavior of the capacitor at large AC signal per large DC signal). is it right?
Thanks
 

You would prefer DC coupling over input stage, pre-driver and output stage of an audio amplifier. It gives you the option to precisely set the bias point of all stages by suitable DC feedback. For stages with an input voltage below e.g. 100 mV, AC coupling is good as well or may be even preferred. Otherwise you get uncomfortable long bias point settling times, because the feedback time constant is multiplied with overall gain.
 
Dear FvM
Hi
Thanks for your helpful reply . How can i design DC coupling amplifiers? i know all things about cascode pair and its designing . but for other types for example , 3 stages CE amplifier , if i use DC coupling , all of equivalent circuit will change. is it possible that you guide me for designing a multistage DC coupling amplifier , please?
With Appreciate
Goldsmith
 


Hello all,
I just have a remark on the post #5 from jony130, does the C3 is correctly placed ? I suppose there is a mistake and it would be in parallel with R5... you don't think so ?
 

Hello FvM,
Thanks for your reply, well I thought that R9 was here to make the feedback not to provide the DC from the output of the amplifier...

Maybe I would have to study again transistor basics; does anyone know a good book(s) for polarisation/multiple stages design and feedback theory of transistor stages ?
 

glias said:
Thanks for your reply, well I thought that R9 was here to make the feedback not to provide the DC from the output of the amplifier...
Click to expand...
R9 provide the the DC path but also determines close loop voltage gain.

Av = 1 + R9/R5 in reality it will be closer to R9/R5.

So R9 is two in one :grin:

glias said:
Maybe I would have to study again transistor basics; does anyone know a good book(s) for polarisation/multiple stages design and feedback theory of transistor stages ?
Click to expand...
This book is good
Amazon.com: Fundamentals of Microelectronics (9780471478461): Behzad Razavi: Books
 
Dear Friends
Again Hi
Thanks for your replies
i'm professional to designing the amplifiers with RC coupling , but about DC coupling. is it possible give me , a simple methodology for DC coupling design,please? i know that when coupling become DC , the stages should design carefully because of that the equivalent circuit , will huge when the amplifier become multistage , and we should regard that the DC perform point , become in middle of load line. but , how?
With appreciate
Goldsmith
 

DC coupling omits a capacitor between transistor stages. It's better at passing very low frequencies.

However without the capacitor there is greater risk that an overload in one transistor will be conveyed to all transistors.

To use DC coupling it is more convenient to alternate NPN and PNP types. That way the bias voltage levels stay within a manageable range.
 
Dear BradtheRad
Hi
Thanks for your reply.
Is it possible that you say , how is DC coupled amplifier (multistage) calculation? for example 3 stage CB amplifier or CE amplifier.
With appreciate
Goldsmith
 

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