I was reading about op-amps and found this schematic.
What is the role of those capacitors?
Lets call them:
As of now, capacitor positioning and capacitance values choice seem so random to me. What am I missing?
Thanks!
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.
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.
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.
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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.
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.
Cap A is connecting the two gain pins on the OP Amp, from the data sheet:
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.
