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I was reading about op-amps and found this schematic.

What is the role of those capacitors?

Lets call them:

  • 10uF - Cap A
  • 250uF - Cap B
  • 0.05uF - Cap C

As of now, capacitor positioning and capacitance values choice seem so random to me. What am I missing?

Thanks!

enter image description here

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    \$\begingroup\$ That's not an opamp, it's an audio amplifier. A good start would be to look at the datasheet. \$\endgroup\$ – Finbarr Jun 27 '17 at 15:20
  • \$\begingroup\$ Cap B & C seems to be a band-pass filter to remove DC component and high frequencies \$\endgroup\$ – PDuarte Jun 27 '17 at 15:22
  • \$\begingroup\$ Ah, I see, I searched for op-amp schematics and this showed up for some reason. Anyway the capacitors' disposition is very similar to other schematics I've seen thus I think the question about each one's role is still valid. \$\endgroup\$ – nemewsys Jun 27 '17 at 15:22
  • \$\begingroup\$ Hmmmm, still an audio amplifier isn't an op-amp? \$\endgroup\$ – nemewsys Jun 27 '17 at 15:26
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    \$\begingroup\$ @nemewsys No, there are many differences, for example: An opamp has a virtual infinite gain whereas an audio amplifier has a limited gain. \$\endgroup\$ – PDuarte Jun 27 '17 at 15:28
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This is not an op-amp, not even an approximation to one, it's a fixed gain power amplifier. That image is figure 16 from the TI datasheet for LM386. Unfortunately it only explains the function of cap A, which is peculiar to the detailed circuit of the 386.

Cap A short circuits a gain setting resistor to change the amplifier gain. It has to be large enough to couple the internal resistor, in the order of 1k, down to bass frequencies.

Cap B is an AC coupling (aka DC blocking) capacitor always found on the output of a power amplifier that has a DC bias on its output. It has to be large enough to couple the speaker's impedance, typically 8 ohms, down to bass frequencies.

Cap C is part of a so called Zobel network frequently found across the loudspeaker terminals of power amplifiers, designed to compensate for the inductance of the speaker, to keep the amplifier stable. It is sized assuming a typical low power speaker that would be appropriate for that size of amplifier.

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Cap A does whatever the datasheet says about why you need to put it there. Or, if you don't always need it, the datasheet tells you what the purpose of pins 1 and 8 are.

Cap B is for AC coupling the output of the amp to the speaker. The output of the amp has some DC bias. That's not good for the speaker, and would cause extra current in both the amp and the speaker.

Cap C along with the 10 Ω resistor are most likely to keep the amp stable, which again the datasheet should explain.

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  • \$\begingroup\$ Ok. Thanks. I don't know what AC coupling is, nor DC bias. I'm going to search for those terms, any recommendations? I really need some basic oscillo to test and learn all of these things, I'm very visual. :( \$\endgroup\$ – nemewsys Jun 27 '17 at 15:28
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Added

The purpose of these capacitors is to provide a relatively low impedance above a designated frequency. The impedance at this designated breakpoint frequency is key to its function. But only the designer knows for sure, so unless you can figure out the criteria for selection, it will remain a mystery.

Starting with Cap "C" we see from my nomograph below that 0.05uF = 10 ohms the value in series with "C" occurs @ ~40kHz so we know it is not to provide an audio filter but rather to provide an AC coupled dummy load of 10 Ohms oscillations when the speaker impedance rises sharply due to inductance. This technique is common to most power amps because there is some instability from lack of phase margin at the higher frequency at unity gain. This dummy load improves that phase margin enough to prevent oscillations which can thermally damage the power output stage.

Cap "B" Must be large enough to pass bass frequencies into speakers. It appears to have a high pass breakpoint around 50 Hz into 8 Ohms. It could be made larger at the expense of size or perhaps more likely it was intended for 60 Ohm headphones then the cutoff is a lower frequency ~ 10 Hz.

Cap "A" bypasses and internal emitter resistor of 1.25k to boost gain which appears to be above 10Hz which is the impedance of 10uF.

There may be slightly different values which give better performance if you know what to look for.

--- end edit

You should learn how to use impedance to analyze Capacitors for ESR and breakpoint for different types ( e-cap, ceramic, low ESR types.) then apply this to a circuit where graphs are given for PSSR, (250uF) Gain ( 50uF) and spurious resonance f loading since speakers are inductive and RC of 10 ohms and 0.05uF provides 10 Ohm load at some breakpoint where Amp tends to resonate with no load. Hence all these types use this "snubber"

See if you can learn how to analyze graphs and evaluate the function of a capacitor in each type. ( eg. 250uF where ESR*C ranges from 1 to 250 us from quality) thus 250us/250uF= 1 Ohm ( poor) & 1us/250uF = 40 milliohms (good) and compare with 4 Om speaker that is 2 Ohms at DC for distortion 1 Ohm (Cap)/2 Ohms (DCR) is 50% then with bass maybe 25% and PSSR reduces 25% by only ~40dB or 1% of 50% is 0.5% distortion from PS noise from heavy bass current on supply ripple. So a mid range cap quality may be adequate for ESR along with 0.1uF.

enter image description here

It may take some concentration at first, then later, quick answers from practice to any question about RLC values using impedance ratios. Z1/(Z1+Z2) attenuation or simply looking up Breakpoint for HPF or LPF or BPF.

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    \$\begingroup\$ You can easily find "RLC nomograph" on web I have used this since my 1st day as an EE designer. for quick value selection and comprehension of the limits of capacitors due to ESR and time constants in each chemistry. Plastic is bigger but 1000x lower ESR*C then ceramic is more then electrolytic, then batteries. mayhave 10 second time constant with short circuit ESR * Cequiv. for Ah ratings. This is universal to all dielectrics including caps, batteries and snubbers for PS and drivers and filters. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 27 '17 at 16:49
  • \$\begingroup\$ So basically this graph shows how impedance varies per capacitance and frequency correct? \$\endgroup\$ – nemewsys Jun 27 '17 at 17:33
  • \$\begingroup\$ i.e. How each capacitor should be ranked (low pass, high pass, etc)? \$\endgroup\$ – nemewsys Jun 27 '17 at 17:40
  • \$\begingroup\$ So coils (high reactance -- Henries) act as low-pass filters, and capacitors (high capacitance -- F) act as high-pass filters? Using that nomograph I can pick up the best fit right? So cool. \$\endgroup\$ – nemewsys Jun 27 '17 at 20:41
  • \$\begingroup\$ L increases impedance with f while C decreases impedance with f. Depending on series or shunt, both can be used with series or shunt R to make either HPF or LPF based on impedance ratios while intersection determins breakpoint.e.g. if series L to shunt R it is a HPF or series R to shunt C same thing except differences of input and output impedance with f. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 28 '17 at 1:49
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Cap A is connecting the two gain pins on the OP Amp, from the data sheet:

enter image description here

So this is specific to the gain setting on this configuration, flick through the datasheet for other options.

Cap B is acting as a coupling capacitor, it removes any DC bias from the signal going to the speaker.

Cap C is acting as a bypass capacitor, it provides a low impedance path for signals of a certain frequency to go to ground. It essentially acting as a filter to stop unwanted signals going to the speaker.

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