help bulding BJT amp

Hero999 · Mar 10, 2010

audioguru said:
Now I think your circuit will have a very wide operating point for its transistors since its emitter voltages are so low.
One transistor has a Vbe of 0.7V and it will be almost cutoff. Another transistor has a Vbe 0f 0.6V and it will be almost saturated.
Temperature will also affect the operating points. The transistors will be thermometers.

Sorry I didn't bother to label my components, sat the lef transistor is Tr1 and the right is Tr2.

What you're saying doesn't make any sense. I know simulations aren't always accurte but still.
Hfe = 50
Vc(Tr1) = 12V
Vc(Tr2) = 9.75V

Hfe = 500
Vc(Tr1) = 6.7V
Vc(Tr2) = 6.8V

If the output voltage is 3Vp then so what?

Hero999 · Mar 10, 2010

eng.samir said:
so you mean i shouldn't use it??

Just ignore me, do what audioguru says.

He tends to set very strict rules to follow when designing circuits which is good practise but sometimes it makes things harder than it needs to be. I'm just more relaxed and am willing to sacrifice one parameter for another and will take the odd risk of something not working as well as it should.

The chances are we will disagree but me arguing with audioguru is probably not to help you and might even confuse you so I'm not going to bother.

audioguru · Mar 10, 2010

Some people design a circuit and one might work due to luck. If many circuits are made then many might not work.

I design circuits so all of them work whether the resistors are 5% low or 5% high or whether the transistors have high gain or low gain or high Vbe or low Vbe. Even when all the variables add together, all my circuits work perfectly.

eng.samir · Mar 11, 2010

audioguru said:
Some people design a circuit and one might work due to luck. If many circuits are made then many might not work.

I design circuits so all of them work whether the resistors are 5% low or 5% high or whether the transistors have high gain or low gain or high Vbe or low Vbe. Even when all the variables add together, all my circuits work perfectly.

i still didnt understand what should i do ???i cant use your circuit becouse am using higher input .

Hero999 · Mar 11, 2010

What is the impedance of the load you want to drive?

eng.samir · Mar 11, 2010

speakers 4 ohm

Nigel Goodwin · Mar 11, 2010

eng.samir said:
speakers 4 ohm

Sorry, but your circuit is a preamp only - and a very poor preamp at that.

You need a power amplifier to feed a speaker.

Hero999 · Mar 11, 2010

eng.samir said:
speakers 4 ohm

Why didn't you say so before?

You'll need many more transistors than two to drive a 4Ω speaker.

Use a TDA2003, the circuit is on the datasheet.
**broken link removed**

What's the input voltage?

You might need a pre-amplfier. You could play around with a common emitter amplifier or save yourself the trouble and use a TL071 op-amp.

audioguru · Mar 11, 2010

In the beginning he should have told us the input signal level and the 4 ohm speaker as a load.
I should have simulated the rising gain at high frequencies for his preamp circuit.

eng.samir · Mar 11, 2010

i have to use transistors i cant use opamf..i have to come out with my own cirucit...
why i cant use my own my circuit the output is 3.5v isn't it enough???

Hero999 · Mar 11, 2010

eng.samir said:
i have to use transistors i cant use opamf..i have to come out with my own cirucit...
why i cant use my own my circuit the output is 3.5v isn't it enough???

Sounds like an assignment to me.

If you have to design your own circuit then why are you asking us to do it for you?

Don't get me wrong, if you design it yourself, we will be willing to help you get it working but we won't do all the work for you.

Nigel Goodwin · Mar 11, 2010

eng.samir said:
i have to use transistors i cant use opamf..i have to come out with my own cirucit...
why i cant use my own my circuit the output is 3.5v isn't it enough???

Sorry, but what have you done on your course?, you don't have the slightest clue what you're doing - have you not been paying attention, or missed most of the lessons?.

eng.samir · Mar 11, 2010

i didn't ask you to do it for me. i got a circuit and i was asking why its not working, i want to understand not just do it.. all i wanted is to know why its not working...
if the circuit is totally wrong and not sutable do drive speakers i would like to know what are my mistakes and i would try to fix them.

eng.samir · Mar 11, 2010

Hero999 said:
Sounds like an assignment to me.

If you have to design your own circuit then why are you asking us to do it for you?

Don't get me wrong, if you design it yourself, we will be willing to help you get it working but we won't do all the work for you.

the circuit in the beginning is mine and i was trying to do it by my self , but it is obviously that it is not working so i would appreciate to get some help that's it.

Hero999 · Mar 11, 2010

The mistake is the power output is too low.

There is no simple fix, you need to redesign the whole circuit.

The circuit you posted is a class A amplifier, you need to add a class AB power amplifier to drive a speaker.

Look at the internal schematic for a small audio power amplifier IC such as the LM386 and it might give you an clue of how to design one. There are also many other audio amplifier circuits on the Internet.

eng.samir · Mar 11, 2010

Hero999 said:
The mistake is the power output is too low.

There is no simple fix, you need to redesign the whole circuit.

The circuit you posted is a class A amplifier, you need to add a class AB power amplifier to drive a speaker.

Look at the internal schematic for a small audio power amplifier IC such as the LM386 and it might give you an clue of how to design one. There are also many other audio amplifier circuits on the Internet.

thank you a lot for your help and your time. ill try to look for a new circuit then discuss it here.
but i don't understand why is my output is low, 3.5V should be enough to drive the speakers.even when i went to my lecturer he didn't say the circuit is wrong he said i just have to get the right values.

Hero999 · Mar 11, 2010

eng.samir said:
thank you a lot for your help and your time. ill try to look for a new circuit then discuss it here.
but i don't understand why is my output is low, 3.5V should be enough to drive the speakers.even when i went to my lecturer he didn't say the circuit is wrong he said i just have to get the right values.

Sorry but you don't stand a chance if your lecturer said that.

Your lecturer obviously doesn't understand that the output impedance is too high so the voltage will drop to a mV level as soon as a 4R speaker is connected.

If your lecturer does not know this, then what chance do you have of learning anything?#

You might as well drop the course and work at Maplins (or a similar store in your area), you'll probably learn more about electronics there and you'll get paid for the privilege.

eng.samir · Mar 12, 2010

Hero999 said:
Sorry but you don't stand a chance if your lecturer said that.

Your lecturer obviously doesn't understand that the output impedance is too high so the voltage will drop to a mV level as soon as a 4R speaker is connected.

If your lecturer does not know this, then what chance do you have of learning anything?#

You might as well drop the course and work at Maplins (or a similar store in your area), you'll probably learn more about electronics there and you'll get paid for the privilege.

thank you for the heads up...
i tried to use the LM386 circuit but its to complicated for me ..isn't there is any simpler circuit that suite my load . i want to do something simple then ill try it to improve it .

Nigel Goodwin · Mar 12, 2010

eng.samir said:
thank you for the heads up...
i tried to use the LM386 circuit but its to complicated for me ..isn't there is any simpler circuit that suite my load . i want to do something simple then ill try it to improve it .

What EXACTLY, word for word, is your assignment?.

eng.samir · Mar 12, 2010

Nigel Goodwin said:
What EXACTLY, word for word, is your assignment?.

i have to design an amplifier using BJT's only , the amp should have similar ratings to IC HA13118. Then i have to compare their outputs.i finished with the IC but am struguling with the BJT circuit.

help bulding BJT amp

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