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As much information as possible about this figure would be helpful

From what my professor said, "Q2 oscillates when we connect a Loud Speaker, this might generate high frequency signals, so we use a low pass filter to filter them out". I understand that we need low pass filter to filter out high frequencies but (why is high frequency bad for Loud speaker?). Some help please.


What the professor should have said is: "oscillations may occur, R6 and C3 help prevent this".

If there are oscillations, it's not only because of Q2. I see a feedback (?) line connecting to R5, feedback can cause oscillations if not done properly.

The network of R6 and C3 provide a defined impedance at high frequencies and that helps with the stability of the feedback loop. The loudspeaker might have a high impedance at the frequency where oscillations can occur. This high impedance increases the loopgain of the feedback loop and that helps oscillations. By adding R6 and C3 there is a low(er) impedance at the these frequencies which lowers the loopgain and that prevents oscillations.

So that R6 and C3 are not there to prevent high frequencies from getting to the loudspeaker ! They are there to prevent oscillations and to keep the amplifier well behaved.

  • \$\begingroup\$ I will re-upload the entire schematics, is it ok if you have a look and describe me few things? i have a defense coming up for my project. @FakeMoustache \$\endgroup\$ – Hilton Khadka May 10 '16 at 20:36
  • \$\begingroup\$ Hmm, a class B amplifier. Indeed there is feedback like I expected. I build a similar thing 30 years ago when I was a teenager ;-) \$\endgroup\$ – Bimpelrekkie May 10 '16 at 20:45
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    \$\begingroup\$ 1) Who prefers PWM input and why ? It depends on how you want to use this amplifier. As a linear amplifier we would input a sine wave. For class D operation (switching) indeed you could use a PWM signal as input but this PWM frequency cannot be so high because this amplifier is not very fast due to the opamp. This is a "slow" general purpose opamp. For a proper class D amplifier I would use a different design. \$\endgroup\$ – Bimpelrekkie May 11 '16 at 8:54
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    \$\begingroup\$ You mean the two transistors at the output ? It is possible using only one but then the other would need to be replaced by a resistor. For low power amplifiers (not driving a speaker) this is a common configuration. Google: "common collector circuit" and you see what I mean. The two transistors form a "push-pull" configuration, one pushes current into the speaker (here NPN Q1) and the other (PNP, Q2) pulls current from the speaker. This makes it possible that the current can be positive and negative. Which is what you need for audio signals. \$\endgroup\$ – Bimpelrekkie May 14 '16 at 14:08
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    \$\begingroup\$ Yes the circuit will still work if we replaced Q1 or Q2 with a resistor of for example 100 ohms. But then the current through that resistor (and also the current to the speaker flowing through that resistor) would be limited as it is always 100 ohms. \$\endgroup\$ – Bimpelrekkie May 14 '16 at 14:24

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