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0.1uF bypass cap for high freq bypassing

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zesla

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Hi,

I have a question regarding 0.1uF bypass ceramic caps which are used in power supplys and amplifiers so that eliminate the noise in high frequncies.

I want to know if it is enugh to use them in power supply unit (in parallel to big electrolyte caps) or they must be used near the power amplifier chip (LM3875) too? My circuit uses an op-amp (TL072) too, so is it enough to just use 2 of 0.1uF caps in power supply unit or I should use 2 on power supply and 2 on poer amplifier and then 2 on op-amp power rails? (if I use 6 of them then all are in parallel but in differnt places on the circuit). So what is the best method to go with?

Thanks
 
I forgot to tell that the whole of my circuit uses a rail to rail power supply like below:
 

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If in doubt, put them next to everything.
 
The reason that they should be DIRECTLY between the Vcc and Vss pins of each IC (with <10mm leads) is because with modern IC processes, the transistors inside the ICs have Gain*Bandwidth products exceeding 100's of MHz, which means that if the bypass is more than a few cm distant, the Vdd/Vss wiring can act as an RF transmission line, which when coupled with the high frequency gain, will create a "tuned-line" oscillator...
 
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In general, ANY device that has power pins (Vcc, Vee, Vdd, Vss, etc) gets a 0.1uF ceramic cap right at the lead terminals. It's way more important to have them at the device than in the power supply itself for a power amplifier, although you should put them at both places generally. Also at every opamp or anything else that has power pins.

If in doubt, look at the data sheet and app notes. Some devices (regulators, high speed devices) have stricter requirements for bypass caps (may require multiple ceramics in parallel, eg 0.1uf and 1nf may be recommended).
 
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MikeMl hits the nail on the head. To link this back to basic theory (which is sometimes helpful) consider all of the copper that brings Vcc and GND up to each IC as part of the equivalent circuit of the power supply from the point of view of that IC (review theory of Thevenin equivalent circuit). These traces and planes add series impedance that is negligeable at DC, but become more significant as frequencies increase.

If the IC is a switching device, as are all logic ICs for example, it requires a broadband source of power in order that those switching functions perform as fast as they should. Recall from theory that a fast rise or fall time has a spectrum equivalent (which can be analyzed via fourier transform) rich in high frequency energy, the shorter the switching time, the higher frequency energy is required to power the switch. Another way of saying this is that switching IC requires a very brief surge of current when it switches. So a logic or switching IC needs a source of power that has low source impedance at a broad range of frequencies.

For linear ICs like amplifiers, indeed the amplifier may require a low impedance power supply throughout its gain bandwidth in order to maintain stability or to achieve its best performance.

So, it is all about providing a low source impedance for the power supply to the IC over a broad range of frequencies. The ceramic bypass capacitor placed right at the IC gives you a point of local charge storage from which the switching/logic IC, for example, can draw coulombs of charge quickly to power the high frequency needs of its switching function without having to pull that current through all the higher impedance copper traces from your power supply regulator. In the same way, the broadband linear IC sees a very low RF impedance right at its power supply leads, free of all the copper trace impedances, which allows it to perform at its best and the way the IC designer intended.
 
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