class AB amlifier.

[yasir_ali]

Hello
i want to design a class AB amlifier for my little project and for that i've choosen TIP31C & TIP32C. and i want to biase it upto 1A. i am enclosing the datasheet. now the problem is how would i know which diode is gonna match with the transitors' be characteristics? which diode should i select?


Thanks

 

[Nigel Goodwin]

Don't use a diode, that's a very old and crude method, and usually required special diodes. Certainly from a servicing point of view (back in the 70's when diodes were used) you had to replace them with the exact correct diode, or the amps normally just blew again.

Use a simple Vbe multiplier where you can adjust the current with a simple preset resistor.

However, I don't really see the point of 1A?, you're going to need massive heatsinks, and even then it's likely to run VERY hot. Such amps usually only run at a few 10's of mA, there's little reason to run them any higher.

 

[audioguru]

I think he wants to make a room heater (a class-A amplifier). Maybe he doen't know that nearly all hi-fi amplifiers use class-AB with fairly low idle current.

 

[yasir_ali]

Hey guys you both made me confuse. ok let me tell you all the things. I've an Oscillator which is supplying an output of 8V(RMS) with 10mA. now i want to amplify only it's currnet to 1A. that's why, i thought that a class AB amplifier will do it. Am i right or going wrong? please help me.

Thanks

 

[Nigel Goodwin]

Hey guys you both made me confuse. ok let me tell you all the things. I've an Oscillator which is supplying an output of 8V(RMS) with 10mA. now i want to amplify only it's currnet to 1A. that's why, i thought that a class AB amplifier will do it. Am i right or going wrong? please help me.

You are wrong - VERY wrong.

EXACTLY what are you wanting to do? - not how you think it should be done, but what you actually want to do.

 

[audioguru]

Your oscillator has an idle current of 10mA or has a peak current of 10mA into a load of 1131 ohms. An 8W RMS class-AB audio amplifier has an idle current of 30mA but has an RMS current of 1A into an 8 ohm speaker.

You do not want to amplify the DC idle current, you want to amplify the AC current in the signal.

 

[ben7]

I have been wanting to make my own amp circuits, but don't know how, however they seem so complex and if one part is off value it won't work.

I prefer class AB, class A does sound good-and it is simple too-but it wastes too much energy. Class B is OK, but the sound isn't usually as warm.

For some reason I am HOOKED on amps that use a few transistors-I'd spend a long time staring at them in amazement,-instead of a boring chip with hundreds of transistors-simplicity maybe? Oh, I love the looks of them too.

Is anybody willing to teach me and yasir_ali?
LOL I've got experience with some electronic circuits. I am a quick learner too.

 

[audioguru]

I have been wanting to make my own amp circuits, but don't know how, however they seem so complex and if one part is off value it won't work.

The parts values are calculated from the required power and from the datasheets of the transistors. Resistors are common at 5% which is fairly accurate and is not "off value".

Class B is OK, but the sound isn't usually as warm.

No.
Class-B produces horrible crossover distortion because both output transistors are completely turned off for a good portion of the waveform.

For some reason I am HOOKED on amps that use a few transistors-I'd spend a long time staring at them in amazement,-instead of a boring chip with hundreds of transistors-simplicity maybe? Oh, I love the looks of them too.

Simple amplifiers with only a few transistors have very high distortion which sounds cheap and awful.

Is anybody willing to teach me and yasir_ali?

Many of us learned about electronics in university. There are a few teachers here.

 

[yasir_ali]

@ Ben7
I also think about electronics in the same way as you do that ICs make me bore and i can spend hours working with simple and small discreate and analog components.

@ Audioguru
i am including my oscilltaor alone. but in the circuit included the battery is of 12v but i will replace it with 9v. and also including second circuit which tells that i want to do as Nigel Goodwin asked me.

i want at about 3v or 4v (RMS) with 1A at final stage.
now i think every thing seems clear now please help me.

thanks

 

[audioguru]

Your phase-shift oscillator does not have a 12V battery. It has two 12V batteries so its total supply is 24V. It does not have anything to limit its output level so if you build then it its output will be clipping.
With a 25 ohm load and an RMS current of 1A then the output signal is 25V RMS or 70.7V peak-to-peak which is not possible from a 20V supply.

The values of your coupling capacitors in the amplifier are much too small to pass 45Hz. For example, the 1uf output capacitor has a reactance of 3555 ohms at 45Hz.

The two diodes should be replaced with a transistor and two resistors. The output transistors should have low-value emitter resistors.

 

[yasir_ali]

hey! brother. the class AB amplifier i post was not designed. i told you that i am just post it to show Nigel Goodwin that what i want to do, when he asked me. i just picked the components and arrenged them remdomly in a class AB fashion. sorry for that. now please tell me 2,3 things.
1- Is my idea right that i can amplify the current, coming from the previous stage to 1A or any required value?

2-what should be the reactance of the coupling capacitor at operating frequency?

3- i am always confuse to select the value of coupling,filter and bypass capacitor i don't know or i am lacking with some important topic. i have read the chapter in book named "Frequency response of Amplifiers" even then i dont know why or i did not understood it in a fine way.

Thanks

 

[audioguru]

1- Is my idea right that i can amplify the current, coming from the previous stage to 1A or any required value?

No.
The datasheet for the LM741 shows that its max output voltage loss is 10V p-p when it drives 20V p-p into 2k ohms which is a peak current of only 5mA.
The datasheets for the TIP31 and TIP32 transistors show a minimum current gain of only 25 at 1A, so they need an input of at least 40mA.

You might be lucky and find an LM741 opamp with more output current and transistors with more gain but the worst case is that the max output current will be 5mA x 25= 125mA peak.

2-what should be the reactance of the coupling capacitor at operating frequency?

if the reactance of a coupling capacitor is the same as its load resistance then the output is -3dB which is 0.707 times the voltage or half the power.

3- i am always confuse to select the value of coupling,filter and bypass capacitor i don't know or i am lacking with some important topic. i have read the chapter in book named "Frequency response of Amplifiers" even then i dont know why or i did not understood it in a fine way.

The reactance of a capacitor is 1 divided by 2 pi R f.

 

[yasir_ali]

Hello.
Now i'll go sequentially. I found a link to creat a virtual ground which can help us to use a single battery to bahave like dual supply to the op-amp.
Virtual Ground Circuits
I have tried all the virtual groung creator circuits. All the circuits are working fine except one which i have show the screen shot of that while simulating and showed the error. i don't know why this is having the problem. look at that please.
And the Amp shown down to it is LT1206. i did not understand the write wants to say that it can handle upto 250mA. Am i correct? in the datasheet (i've included) written that 250mA minimun output drive current. what does that. can't it drive less than this current.?

 

[audioguru]

You do not need a virtual ground circuit that usually powers a complete high current circuit.
You just need to bias the high resistance low current (+) input of the opamp at half the supply voltage with two resistors and a filter capacitor.

The LT1206 is a high frequency current-feedback amplifier that will not work in your circuit that needs an ordinary voltage-feedback opamp.
Besides, the LT1206 needs a supply that is 10V minimum.

The lousy old 741 opamp also might not work with a 9V supply that drops to 6V when the battery runs down because it is spec'd with only a 30V supply. I would use a TL071 opamp.

The resistor values in the oscillator are too low which overloads the opamp and then the capacitor values are too high. Instead of 120 ohms you could use 12k ohms and instead of 10uf you could use 100nF (0.1uF).
The feedback resistor could then be 330k or 360k (I think the gain should be 29).

The output of the oscillator does not have anything to limit its output so it will be clipped with obvious distortion.

 

[MrAl]

I have been wanting to make my own amp circuits, but don't know how, however they seem so complex and if one part is off value it won't work.

I prefer class AB, class A does sound good-and it is simple too-but it wastes too much energy. Class B is OK, but the sound isn't usually as warm.

For some reason I am HOOKED on amps that use a few transistors-I'd spend a long time staring at them in amazement,-instead of a boring chip with hundreds of transistors-simplicity maybe? Oh, I love the looks of them too.

Is anybody willing to teach me and yasir_ali?
LOL I've got experience with some electronic circuits. I am a quick learner too.

Hello there,

I've taught quite a few people through emails and in forums like this but if you just ask questions here you'll get lots of answers too that way. Usually it helps if you ask a specific question rather than a very broad, general one, but heck any question will get you some kind of answer and then you can go from there. If your interest is that great you'll stay with it for a long time and find it very rewarding.

 

[ben7]

Hello there,

I've taught quite a few people through emails and in forums like this but if you just ask questions here you'll get lots of answers too that way. Usually it helps if you ask a specific question rather than a very broad, general one, but heck any question will get you some kind of answer and then you can go from there. If your interest is that great you'll stay with it for a long time and find it very rewarding.

I'd love to design and build my own amplifiers after seeing the inside of the beastly Sansui 9090DB!

 

[yasir_ali]

@ audioguru

Hey! i understand what you said But i did not understand the circuit you posted to make an op-amp to operate with single supply.
And sorry! to say that i am unable to simulate it. can please explaine the circuit you designed? or give the any link that i can read about it.
Thanks

 

[audioguru]

@ audioguru
i did not understand the circuit you posted to make an op-amp to operate with single supply.

I am not a teacher so I don't know sites that explain simple basic electronics.

The (+) input of the opamp needs a DC reference voltage. The two equal resistors and filter capacitor make a DC reference voltage that is half the supply voltage. Then the output idles at half the supply voltage so it can swing equally up and down to produce the AC signal.

 

[yasir_ali]

@ audioguru

Hello. I have understood and designed it single supply topology, you posted in last post. i am posting mine. But i am using multisim 10 and i don't know why the "TL071" Ic is not there in it. so today i'll download the new version.
ok. tell about this biasing circuit that as you told me that we do not use the virtual ground circuit for high input low current input and the virtual ground ciruits are for only high current circuits. is it so.?
i am posting the circuit.

 

[audioguru]

The input resistance of an old 741 opamp is typically 2M ohms and for the TL071 is 10 gigaohms so two equal value resistors can bias it at half the supply voltage perfectly without using a virtual ground circuit.