[SOLVED] power amplifer design

[fareen]

audio amplifer design for speaker

hi
im required to design a amplifer for the speaker with the rating of 4ohm ,10w
and input signal is 5mV
i decided that input stage could be common emitter and gain stage can be common emitter or bjt cascode and output stage of class AB.
well im having problem with deigning of circuit,can sum one can help??
can any one can outline main steps??

 

[FvM]

I assume that you'll find masses of amplifier circuits on the internet. For the principle circuits, you can refer to the below LTSpice example circuit. It has to be modified for output power (stronger output transistors, increased supply voltage) and possibly gain. For 5 mV sensitivity, you should better add a separate preamp stage.

 

[fareen]

thank you FVM for looking into the matter
the design you have suggested is not my requiremnet, i have to you power transistor for the output stage tip31,tip32
im trying to design discrete three stages and combing them
so starting with the common emitter input stage.
take a look at this pdf,i want to increase the gain to 20-30 for the input stage and as well high input resistance what changes do u suggest???

 

[FvM]

I don't see the paper directly related to your design intentions. For the output stage of a power amplifier, different circuit concepts are needed. The PSpice exmaple gives at least a basic idea, there are much more options of course. Some essential points that are common to all power amplifiers:

- sufficient current gain, typically provided by a common collector output stage or a similar circuit to drive the low impedance load
- bias circuit for class AB operation
- DC feedback to set a suitable operation point

Additional gain stages in fort of the PA would be preferably AC coupled not to disturb the operation point of th eoutput stage. Under these prerequisites, the discussed common emitter amplifier circuit can be used as a preamplifier.

 

[fareen]

yes u are right i intended to use class ab output stage amplifer
the required gain is 1264v/v thats quite highh,in designing common emitter peramplifer stage what points are needed to be considered like maxium resistances connected to base?,gain?,etc

 

[fareen]

i have design the following circuit,my output signal is getting clipped.as shown in the diagram whats the best way to get the full signal swing??

 

[FvM]

Adjust the bias network (base voltage divider).

 

[fareen]

i decrease the voltage at base but the clipping still exist

 

[FvM]

There are two points:
- the asymmetrical clipping shows, that the bias point isn't optimal and should be adjusted
- nevertheless, the output will be clipped with 200 mV AC input in any case. You have to reduce the AC input voltage or gain.

You should preferably plot collector and emitter voltages of the circuit to understand how clipping occurs. I don't think that it's useful to "spoon feed" an explanation of circuit operation. Everything is at your fingertips, you just need to look at the right circuit nodes.

Two hints, however:
- The "unclipped" output voltage can't be above about 10 Vpp, you have to consider the voltage drop across R6/C2.
- Please also look at the waveform. Even without explicite clipping, it's rather distorted a high output levels, with THD of e.g. 10%. That's normal behaviour for simple amplifiers like this. Reducing the gain will also increase linearity.

Don't get discouraged. You're on the right track to explore transistor amplifiers.

 

[fareen]

hi
im here again....
i understand the logic of clipping and unequal clipping
and the maxium swing can be vce+vcr
actually the problem is that i need a maxium swing of 18 v,although i have increase the supply voltage to 20v and 1v at the base but still not grtting the maxium output what else can i do without further increasing supply voltage???

 

[goldsmith]

Hey buddy!
How much gain do you need?
and what is the maximum value of your input signal?
Best wishes
Goldsmith

 

[fareen]

the maxium value of input signal is 2.5mvp
and we required out put signal to run the speaker of rating 10w,4ohm
meaning we required a gain of 3577.8v/v
i decided to cascade two common emitter stages first stage has a gain of 56v/v and output signal of 140mv
attaching the input amplifer to have a look.
thanks

 

[goldsmith]

Dear friend
Again hi
You should avoid to obtain this high gain with two stage! the gains that are more than 1000 , will obtain with more than three stage amplifier( for have a good band width and frequency response) and this type of amplifiers are small signal . the transistor at large signal has a magic nonlinear behavior! and its design need more experience( voltage amp) . if you want a simple way , ican suggest you to use a single stage common emitter amplifier as a pre amplifier and then use a good op amp as a large signal voltage amplifier and for current amplifier use a class AB complementary amplifier . and if you don't want to do this you can use a simple class D amplifier as a ultra high gain amplifier. but linear amplifiers are more easier than discrete amplifiers.
All the best
Goldsmith

 

[fareen]

u are quite right but the problem is i cant use opamp(teacher say so)
and class d amplifier not read yet so i im left with few options
common emitter and and class ab
so u saying that cascading more than common emitter will help against clpping???

 

[fareen]

hey any one knows where to find tip31 and tip32 in ltspice??
i couldnt find it
if its library does not have it then whre to gets its model???