Help me with designing a push-pull amplifier

[sec]

Hi everybody, I would like to build a push-pull amplifier (minimum 100W) and I miss few thing (almost everything) to achieve my project. I will need a lost of help from all of you.

My first question is, mosfet or darlington ?

Second, how do I desing the circuit to avoid the crossover-distortion (flat)

and finally, can someone send me a good circuit to control and limit the current, I do not want my two transistor to be open at the same time!!

 

[A.Anand Srinivasan]

Re: Push-pull amplifier

first check your power value of amplifier because 100W is too high for any amplifier.... to avoid crossover distortion use diode i.e i mean class AB amplifier.... MOSFET is slow and has lot of nonlinearity.... darlington is also slow(not as much as MOSFET) but can be linearised and less noisy... so you decide based on your need....

 

[raghuram_msc]

Re: Push-pull amplifier

Hi Sec..!

as a second part to anand's reply i would suggest you to consider BandWidth also... If you r working on narrow bands Class-C would be a better option...Do let us know..All the best..!

Bye,
Sai.

 

[sec]

Re: Push-pull amplifier

I'll go with the Class AB for my design !!! I am presently working on the current control/limiter and the power supply

Added after 3 hours 42 minutes:

I finally finished my power supply, (it is the easiest part I think). By the way, no need to have high frequency transistor. It is an audio amplifier, I try to design a bass amplifier. with all of my experimentations, I think I will go with the mosfet, there is less ( practically nothing) crossover distortion because it is not an "diode based transistor"

I think I found good schema of current limiter, but it would be appreciate if someone would explain me how it work.

 

[A.Anand Srinivasan]

Re: Push-pull amplifier

please post the current limiter schematic that you have got.... i would try to work out how it works....

 

[sec]

Re: Push-pull amplifier

this is the current limiter I found, but I work to design a different one

 

[A.Anand Srinivasan]

Re: Push-pull amplifier

it goes something like this... the Q1 Q2 configuration makes Vce of Q2 around 1.4V... Ve of Q1 is about .7V less than its Vb... Vb of Q1 is decided by current through R1 which is same as output current.... now when output current increases the Vbe of Q1 starts decreasing due to current through R1 as well as R2 and at one stage Q1 doesn't have enough base drive and hence Q2 doesn't have base drive due to this.... at this point current limiting is effected....

 

[sec]

Re: Push-pull amplifier

What about my design, will it work, there a JFET current limit, a transistor to amplify and a current mirror.

 

[Borber]

Re: Push-pull amplifier

Circuit is nonsense. Q2 has no bias.

 

[A.Anand Srinivasan]

Re: Push-pull amplifier

i think Vgs of Q1 is too low for it to bias Q2 to work... also even if you modify and make it work it is just gonna be a constant current source....

 

[sec]

Re: Push-pull amplifier

Question of anatomy of mosfet, why is there a crossover distortion?

 

[A.Anand Srinivasan]

Re: Push-pull amplifier

it is not something related to the anatomy of the MOSFET... it is just that the input has a reach a particular voltage before turning the MOSFET on... this results in crossover distortion at the zero crossing....

 

[sec]

Push-pull amplifier

Is there something I can do to avoid that ?

 

[Ave_Rapina]

Re: Push-pull amplifier

You need an bias circuit to allow the power transistor to be in conduction and avoid crossover distorcion

 

[sec]

Re: Push-pull amplifier

I work on my current limit a bit, and here is what I made. I suppose that this circuit is not perfect, but when I open and test with my oscilloscope, I was greatly surprise. It is perfect for a AB class amplifier! How can I reproduce this with a negative peak. Does I need to use the complementary of each device (JFET and Transistor) or simply make connection to the Vcc instead of the ground ?

 

[sec]

Re: Push-pull amplifier

I made it, but I will be pleased to read your suggestions too.

 

[sec]

Re: Push-pull amplifier

I will check later on the day to see if it is a current limiter. is it right to think that the limit current is defined by Ib*Hfe=Ic ?

 

[Borber]

Re: Push-pull amplifier

It is not a current limit, it is simply calculation of collector DC current.

 

[A.Anand Srinivasan]

Re: Push-pull amplifier

use two diodes to bias the mosfets and to avoid crossover distortion... that is a simple way...

 

[sec]

Push-pull amplifier

what are the trick to match transistor stages I am a beginner in this subject. The thing I learn is to use a capacitor to block the D.C. current, but the problem is, it cut too much the lower frequency. So what are the basic to match transistor stages ?