Question about Class B audio amplifier

Audioguru

Joined Dec 20, 2007
11,248
Do u have any simple quasi-complementary circuit that i could build and discover the output signal?
Copy one of the thousands shown and described in Google.

Don't you know about Google? Google knows everything and costs nothing.
Sometimes there is an article written by a nOOb who knows nothing and his article is wrong.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
hi guru,

If i didn't connect any input signal to the (IN) when i turn on the power. Is there any affect to the circuit? will the signal goes more negative level at the output of the opamp and some consequences happen afterward? or it will just run as normal which no audio is outputted from the speaker?

abc.JPG
thanks.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
could this be a good example to start with?
http://sound.westhost.com/project12.htm

Does the claim below correct?
Designers also sometimes use a "quasi-complementary" configuration, which uses a Darlington push pair (i.e., two NPN transistors) and a Sziklai pull pair (i.e., one PNP and one NPN transistor). This configuration, which uses three NPN transistors and one PNP transistor, is advantageous because:

  • Silicon PNP transistors have historically been more expensive than their NPN counterparts.[citation needed]
  • The performance of the lower pull pair, which uses a single NPN transistor, more closely matches the performance of the upper push pair, which consists of two NPN transistors.[citation needed]
thanks.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,248
If i didn't connect any input signal to the (IN) when i turn on the power. Is there any affect to the circuit? will the signal goes more negative level at the output of the opamp and some consequences happen afterward? or it will just run as normal which no audio is outputted from the speaker?
R12 is not needed and should be removed.
R11 makes the (+) input of the opamp at 0VDC and the (-) input has negative feedback from the output of the amplifier so the output of the amplifier will also be at almost 0VDC (plus or minus the input offset voltage of the opamp).

Is this claim correct?
Yes.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
If you disconnect the -12V going to the collector of the PNP transistor then the base signal of the NPN transistor could go more negative than -5V which is the max allowed reverse-bias emitter-base voltage causing avalanche breakdown of the junction and damaging the transistor. Look at the datasheet of the NPN transistor to see the Absolute Maximum Emitter-base Voltage.

If you disconnect the +12V to the collector of the NPN transistor then the PNP transistor might be damaged.
hi guru,
u mean that
If you disconnect the -12V going to the collector of the PNP transistor then the base signal of the NPN transistor could go more negative than -5V
how could this happen? isnt it the base is control by the output of the Op-Amp?

thank you.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
hi,
By using the power amplifier below.
abc.JPG

when i measure the collector pin of both Q3 & Q4 using a scope i get the waveform below. This waveform is outputted when the volume is low.
Q3 & Q4.jpg

when i tune the volume slightly higher.I get this.
Q3 & Q4 with high signal..jpg

And the pic below show the comparison between Q3(Collector) & Q1 (collector).
Q3(c) & Q1(c).jpg

From the first waveform, i can see that the Voltage level of Q3(C) is complement with Q4(C). Does this mean the push-pull characteristic?

Another ques is how could the voltage go up and rapidly down in such a way? which part is controlling the up and down.

When i tune the volume lower there is no signal inside the triangle waveform.But when i tune the volume higher it seems like the audio signal which i inputted carry by the triangle waveform. Can someone explain on this?

thank you.
 

Audioguru

Joined Dec 20, 2007
11,248
I think your main filter capacitors have values that are much too small which results in a lot of mains hum (triangle-waves) on your power supply voltages.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
How does the output of the Op-amp control the voltage level of the collector pin of Q3 & Q4 which result to the triangle waveform which seems like charging up and down.

when the signal at Q3(base) go high then transistor Q3 will saturated and voltage flow from Q3(c) to Q3(e).am i correct till this point?

thanks.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,248
How does the output of the Op-amp control the voltage level of the collector pin of Q3 & Q4 which result to the triangle waveform which seems like charging up and down.
Don't you understand? i think the low frequency triangle waveform is because your power supplies do not have big enough filter capacitors. The triangle wave at the power supplies goes through Q1 and Q2 to the output.

when the signal at Q3(base) go high then transistor Q3 will saturated and voltage flow from Q3(c) to Q3(e).
You never allow amplifier transistors to saturate. You want the transistors to always be linear for low distortion.

When the signal at the base of Q3 goes high a little then it conducts a little and causes Q1 to also conduct a little. Then the two transistors (and the other two transistors) have some voltage gain (set with negative feedback in R2 and R8) so that the output swing of the amplifier can be higher than the max allowed supply voltage of the opamp.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
Don't you understand? i think the low frequency triangle waveform is because your power supplies do not have big enough filter capacitors. The triangle wave at the power supplies goes through Q1 and Q2 to the output.
So if i change it to a bigger filter cap then i should get a 1kHz sine wave which is same as the 1Khz i inputted but just the Vp-p different which is more higher? Am i correct?

thanks.
 

Thread Starter

simpsonss

Joined Jul 8, 2008
173
hi, this would be the pos and neg rail waveform from the power supply part. As refer to this waveform, problem probably comes from the filter cap?
thanks.

Pos & neg rail.jpg
 
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