Bootstrapping R2 in my amplifier circuit allows the bootstrap capacitor to pull the base of Q2 to above the power supply voltage so the output swing and power is higher. It also increases the impedasnce of R2 so that the closed-loop gain is higher which allows more negative feedback and less distortion.

A transistor plus two resistors can replace the two diodes in my amplifier circuit.
It is called a VBE Multiplier and it has the same negative temperature coefficient of diodes and its voltage drop is adjusted with the resistor values.

I copied this one from the datasheet of a 741 opamp:

 

How the value of resistance R7 & R8 are choose, the transistor in amplifier are npn & pnp if we take BC548 & BC558, on place of the transistor shown in your circuit will they work??

 

My guess is yes but as with all things Rritesh the learning comes from finding out.

 

How the value of resistance R7 & R8 are choose, the transistor in amplifier are npn & pnp if we take BC548 & BC558, on place of the transistor shown in your circuit will they work??

R7 and R8 are a voltage divider with negative feedback. The beta of the transistor and how much voltage you need the transistor to have between its collector and emitter determine their values.
Usually the values of the voltage divider for biasing a transistor uses 10 times the base bias current of a typical transistor.

Please do not ask questions about low power transistors (BC548 and BC558) without looking at the spec's on their datasheets.
My amplifier circuit has a 9V supply. If R2 is bootstrapped then the output voltage swing into an 8 ohm load is about 7.3V. Then the peak current in each output transistor is 456ma.
But the max allowed current in a little low power BC548 or BC558 is only 100mA so they cannot be used.
I selected 2N4401 and 2N4403 transistors for the output transistors because their max allowed current is 600mA.

 

I think all he wants it to do is work with what little he has to experiment with, power output is not a factor here.

 

I think all he wants it to do is work with what little he has to experiment with, power output is not a factor here.

He must understand that his low power transistors will blow up and my higher power transistors were selected for the amount of current and will survive the power.

 

I understand that, he doesn't yet.

Can you draw him an equivalent using what he has available?

 

If i want to change supply voltage to 12V, Then what changes to do??

 

If i want to change supply voltage to 12V, Then what changes to do??

With a 12V supply and bootstrapping for R2 then the max output voltage swing will be about 10.5V then the peak output voltage will be about 5.25V. Then the peak output current into 8 ohms and in each output transistor is 5.25V/8= 656mA which is too high for the little transistors. They will also get too hot.

Power transistors can be used for the output transistors but they will have less current gain. Sziklai pairs of output transistors will have more current gain and less voltage loss than darlington transistors. All high power amplifier circuits use emitter resistors to equalize current in parallel output transistors that could also be done on this little amplifier.

 

Hi, see this I find in Google.

 

Hi, see this I find in Google.

It is a circuit with a high resistance load. It might be used for the phase-shift oscillator transistor circuit on the other website.
It cannot drive a speaker.

 

The circuit of transistor amplifier with diode, the transistor shown are not available in market, can i use like 2n3005,... transistor of high saturation current??

 

The circuit of transistor amplifier with diode, the transistor shown are not available in market, can i use like 2n3005,... transistor of high saturation current??

Why are you studying electronics when common ordinary parts are not available in your market??

A 2N3005 is an old SCR, not a high current transistor.

The NPN and PNP output transistors should match each other.
I used 2N4401 and 2N4403 because their spec's are suitable, they match, they are inexpensive and they are common because they are made by at least 20 semiconductor manufacturers including a few in India.

 

I suppose the OP meant 2N3055.

 

Why would a 115W 2N3055 transistor be used in a 2W audio amplifier circuit?
Its current gain is much less than the 2N4401 that the circuit is designed to use so the output power will be much less or the circuit needs to be re-designed.

 

Why would a 115W 2N3055 transistor be used in a 2W audio amplifier circuit?
Its current gain is much less than the 2N4401 that the circuit is designed to use so the output power will be much less or the circuit needs to be re-designed.

I wasn't supporting his idea. Just trying to guess what he meant.

 

Why are you studying electronics when common ordinary parts are not available in your market??

A 2N3005 is an old SCR, not a high current transistor.

Hi, AG the place where i live is very far from main electronic city (DELHI ASIA NO. 1 ELECTRONICS MARKET).

ok, that mean 2N3055 is an SCR. i don't know this.

 

Hi, AG the place where i live is very far from main electronic city (DELHI ASIA NO. 1 ELECTRONICS MARKET).

I order parts online from Digikey or Newark in America and they are delivered to me the next morning here in Canada which is a different country.
They stock all electronic parts.

[/B]ok, that mean 2N3055 is an SCR. i don't know this.

You said 2N3005 which is an old SCR.
You did not say 2N3055 which is a power transistor.
A 2N3055 is not suitable for my low power circuit.

 

ok, that mean 2N3055 is an SCR. i don't know this.

RRITESH - You have INTERNET access why don't you try looking up some of the part numbers yourself? You don't know how lucky you are. Audioguru does not ever spend this much time helping someone that can't help themselves.

 

RRITESH - You have INTERNET access why don't you try looking up some of the part numbers yourself? You don't know how lucky you are. Audioguru does not ever spend this much time helping someone that can't help themselves.

Yaa,....!!