Please take a minute to look at the audio amplifier diagram below:

enter image description here

In case it helps, the diagram comes from a do it yourself Velleman 7W Mono Aplifier (K4001) kit. The amplifier is based around the TDA2003 amplifier.

My question is: Are R1, R2, R3, C1 and C5 forming some sort of negative feedback network? If yes, how does this setup work?

The reason I ask is because so far, I have only seen negative feedback on op-amps and the setup always consists of two resistor that can be configured to provide negative feedback and adjust the op-amp gain. This particular audio amplifier looks like an op-amp to me but I don't see the usual two resistors... heck, I don't even see one of the input biased to half the voltage supply.

Whats going on here? Am I missing something obvious?


  • 1
    \$\begingroup\$ The function of those components is in the data sheet. Question should be closed. \$\endgroup\$ Commented Nov 8, 2015 at 19:07
  • 2
    \$\begingroup\$ TDA2003 is not an op-amp. You need to read its datasheet to see how it's intended to be used. \$\endgroup\$
    – The Photon
    Commented Nov 8, 2015 at 19:11
  • \$\begingroup\$ Many audio amp ICs that are designed for single supply have internal biasing.If you did not have C4 C5 then your feedback circuit would look more familiar but your amp probably would not go properly. \$\endgroup\$
    – Autistic
    Commented Nov 8, 2015 at 19:23
  • \$\begingroup\$ @Leon Heller: I am sure that all the answers are in the datasheet. I actually tried to understand the datasheet but quickly got lost. My hope is that help provided here will help me get started on understanding this a little better. I am a beginner, perhaps this was not the most appropriate project to try to understand as a beginner. \$\endgroup\$
    – T555
    Commented Nov 8, 2015 at 19:23
  • \$\begingroup\$ I think R1/R2/C5 is for of bass boost, R3C1 to suppress highest frequencies. Or rather (from low to high along the frequency axis) open loop gain defines low frequencies, then R1/R2/C5 defines gain for mid frequencies until R3/C1 kicks in to attennuate high frequencies. \$\endgroup\$
    – jippie
    Commented Nov 8, 2015 at 19:26

3 Answers 3


It's like an op-amp but it has the output DC set-point control mechanism already inside the chip: -

enter image description here

R7 connects the output to pin 2 which is regarded generally as the "inverting input". So, any external negative feedback you apply should avoid upsetting the internallt set dc bias, hence C1 and C5 in your circuit.

At high frequencies (maybe above 10kHz or 20kHz) I expect that C1 starts to become significant and this effectively places R3 in parallel with R1 and the gain reduces.


R1 and R2 are the usual resistors, and control the gain.

R3/C1 reduce the gain above (???work out the frequency yourself, since you didn't give the values) probably to improve HF stability.

C4 and C5, in a change from opamp practice to audio amp practice, lets the input pins take care of their own DC biassing.


The resistors R1 and R2 for determine the gain of the input.

The branch R3 and C1 which reduce the gain for maximize more bandwidth as audio from 20Hz to 20kHz for more stabitiy in the overall spectrum

Capacitor in the input is for remove DC baising (which I general the capacitor that connected in series configuration)

and Capacitor which is in parallel configuration blocks AC which reduce the noise.

  • \$\begingroup\$ 'Gain of the input' and 'reduce the gain for maximize more bandwidth' are meaningless. Capacitors do not block AC. \$\endgroup\$
    – user207421
    Commented Nov 8, 2015 at 22:17
  • \$\begingroup\$ @EJP there is a gain-bandwidth product for opamp which Is constant \$\endgroup\$ Commented Nov 8, 2015 at 22:19
  • \$\begingroup\$ @EJP and capacitors I mean coupling & decoupling capcitor \$\endgroup\$ Commented Nov 8, 2015 at 22:28

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