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Here is a CMOS two stage amplifier. As you can see there is no decoupling capacitor at all. Also I don't see decoupling capacitors that are used in CMOS integrated circuits. Could you explain why? Thank you for help. enter image description here

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    \$\begingroup\$ Capacitors are very expensive in terms of area, and therefore large capacitors are usually left outside the chip. \$\endgroup\$
    – John D
    Feb 1 '16 at 16:39
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    \$\begingroup\$ Decoupling/bypass capacitors are connected between power supply and ground. They wouldn't necessarily be shown on the amplifier schematic, particularly not on the internal schematic of an IC. \$\endgroup\$ Feb 1 '16 at 16:42
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Decoupling capacitors are used in digital circuits to suppress the high-frequency noise generated on the supply line (Vcc or Vdd) as a result of the rapid switching inside the chip. The signals that are switching are usually square waves, which generate a lot of harmonics.

These capacitors fulfill dual roles -- they provide a little energy reserve to suppress voltage spikes inside an IC, and they help suppress any voltage spikes on the supply line outisde the chip from getting in.

Analog circuitry, on the other hand, usually doesn't switch as fast as digital IC's do, and even when it does, the signals usually aren't square waves so they don't have as many harmonics. Still, it is recommended to put decoupling IC's on all analog chips like op-amps. They are especially needed for analog devices that have a poor power supply rejection ratio (PSSR) to avoid power supply fluctuations from affecting the output.

In either case, decoupling caps are almost never found inside the IC's; as others have pointed out, they are usually separate components placed next to a chip between the power and ground pins. Values of 100 nf (0.1 µF) and 10 nf (0.01 µF) are often used.

Decoupling caps are required to not only make your circuit behave more reliably, but also to reduce emissions to enable it to pass FCC certification.

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  • \$\begingroup\$ Apparently, the question was related to missing COUPLING capacitors (see my answer and the latest comment from the questioner). \$\endgroup\$
    – LvW
    Apr 18 '16 at 6:23
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As many people could and have pointed out, capacitors are large. If left outside the chip, it makes IC design easier to manage. This means we need them outside the IC. It does not mean, however, that we need a capacitor package on the surface. By keeping them outside the package we have allowed for the layout specialists to get creative.

The layout specialists will sometimes create layers with a given surface area, distance between them, material, etc. in order to create a capacitance between them. You can then use the internal layers as a capacitor. This means that by keeping them on the outside of the IC we can give designers more flexibility. They can choose to place the caps next to the part, a little further away, on the opposite side of the board, as internal layers (whatever gives us better results). This is a good thing in the ever shrinking HDI world.

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Very large cap are placed outside of the chip, however, it is usually good practice when you do the IC layout, to use any spare area, and fill it with decoupling cap, sometime the designer will have those cap already in the schematic but not necessarily, this will help with not only the performance of the amp/block but also with other process requirement like metal/poly density. It is also not unusual in RF part, to see the design term specify a minimum decoupling cap to be inserted - as many said this has a cost in term of area, that actually lead to creation of MOM cap which can be layout over circuitry.

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I think, the most obvious reason is that the shown circuit (with a diff. amplifier at its input) is intended to work for very low frequencies down to DC. Such a DC amplifier must not contain any coupling capacitors because it must allow DC negative feedback.

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    \$\begingroup\$ Because, continuously, I try to learn it would be very important (and interestingly) to read, why somebody felt the necessity to downvote this answer. Please, if something is wrong, dont hide your identity and try to improve my knowledge. Dont you think, that - because of the diff. amplifier at a first stage - the whole circuit is prepared for DC feedback? \$\endgroup\$
    – LvW
    Apr 15 '16 at 9:29
  • \$\begingroup\$ Looking at the question, it appears to be discussing the power supply decoupling caps, not stage to stage caps. \$\endgroup\$
    – W5VO
    Apr 15 '16 at 12:48
  • \$\begingroup\$ OK - I must admit that I have overlooked the prefix "de". I really could not imagine that somebody is surprised to see no power supply decoupling capacitors located on the chip. On the other hand - what is the working principle of capacitors in parallel to emitter resp. source resistors? Coupling or decoupling? \$\endgroup\$
    – LvW
    Apr 15 '16 at 14:17
  • \$\begingroup\$ I'll be honest, the term decoupling with capacitors is used inconsistently by a lot of people in different ways. Not sure what circuit you're referring to - in the question, capacitor Cc is a Miller compensation capacitor for stabilizing the frequency response of the amplifier. For a common-emitter/source amplifier with a source degeneration resistor for DC biasing, and a capacitor in parallel to the source resistor, I would probably call it a "DC Bypass" capacitor. \$\endgroup\$
    – W5VO
    Apr 15 '16 at 15:05
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    \$\begingroup\$ Unfortunately, the questioner (user 3126592) did not react upon the answers up to now. I would be not surprised if his question was initially related to the missing coupling capacitors between the stages (although he wrote "decoupling"). \$\endgroup\$
    – LvW
    Apr 15 '16 at 15:40

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