analysis for 100w amplifier ciruit

[pippone1987]

I think so. These Toshiba's are widely use and sound good!

 

[mr_byte31]

I couldn't find any of those transistors on Orcad 10.5
I don't know should i try multisim ?!!!
is it good in simulation ?

 

[pippone1987]

Yes, of course...but i don't know if in that software there are them...

 

[Audioguru]

I can see the input sin signal very clear
i think the crossover distortion of the darlington is minimum as possible because of the negative feedback removes it
am I right?

No.
Your amplifier is also missing frequency compensation which rolls-off the high frequencies so that phase-shift does not cause the amplifier to oscillate when it has negative feedback. Then at high frequencies where crossover distortion is bad the open loop gain is too low to reduce the crossover distortion.

LM358 and LM324 opamps have 3% crossover distortion (when they have negative feedback) because their output transistors do not have enough bias current so that their power supply current is low.

 

[mr_byte31]

then how can I design the frequency compensation circuit?

 

[Audioguru]

You need to know about electronic circuits to design an audio amplifier with frequency compensation.

All audio amplifiers and opamps have frequency compensation.

 

[mr_byte31]

is it as simple as RC ?

Added after 10 minutes:

i think a coil on the output can stop oscillation
right?

 

[Audioguru]

A capacitor with its value properly calculated in the correct position is used for frequency compensation on an amplifier circuit.

Your inductor feeding the speaker simply reduces high frequencies going to the speaker but the amplifier will still oscillate due to phase-shift and too much gain at high frequencies.

 

[mr_byte31]

do you mean a capacitor connected to the load and the other part to the collector of the diff pair?

 

[mr_byte31]

am I right?

 

[Audioguru]

One transistor in an opamp or in an amplifier circuit has the most voltage gain. A capacitor from its collector to its base is amplified (Miller compensation) and reduces high frequencies which are less likely to cause oscillation due to phase-shift in the amplifier.

 

[mr_byte31]

thanks for the reply
then how could I calculate the capacitance?
should I but the capacitor on the transistor connected to the input signal?

 

[Audioguru]

thanks for the reply
then how could I calculate the capacitance?
should I but the capacitor on the transistor connected to the input signal?

The amplifier circuit is extremely simple so maybe it does not need a compensation capacitor.

To calculate the capacitor's value and where it should connect you must analyse the gains of the transistors and their phase shifts.

 

[mr_byte31]

Of course, but they are not so cheap. For the drivers you can use 2SC2238 and 2SA968 (respectively for pull-up an pull-down net) while for the outputs you can use 2SA1216 and 2SC2922.

I couldn't find any of those transistors
how could i simulate them ?

 

[mr_byte31]

In my opinion the distortion is minimum when you use a couple of Sziklai pair in the output stage (or CFP complementary feedback pair) which guarantee a 100% feedback

I used TIP41 and TIP42 I know that it isn't logic to use them because they can't supply much current but it will be in simulation only till i find any substitutions
but the distortion is the same and i can't see any difference using Sziklai pair
any suggestions?

 

[mr_byte31]

ops
Sziklai pair and a capacitor(to stop oscillation)made the distortion reaches 0.15%
is there any way to improve more?

 

[Audioguru]

Good opamps and good audio amps have low distortion circuits. They are careful to control the amount of current in their output transistors (for class AB) to avoid crossover distortion (class B) and avoid an idle current that is too high (class A).

Good opamps have an open-loop gain of 200,000 to one million at low frequencies. Then when negative feedback is added the distortion at low frequencies is extremely low.

Good quality audio amplifiers are the same.

The compensation capacitor (to stop oscillation) reduces the open-loop gain at high frequencies which results in less negative feedback at high frequencies which increases the distortion at high frequencies.

 

[mr_byte31]

I tried to change the gain
by changing the resistance R4 and capacitor C2
the gain can reach 4500 but the distortion increases !!!

 

[FvM]

the gain can reach 4500 but the distortion increases

Yes, understandable. The suggestion was to increase the open loop gain, not the closed loop gain.
As also said, it wouldn't help much anyway without getting rid of the class B output behaviour.

 

[mr_byte31]

do u mean the open loop gain of the diff pair?