In CMOS design, we always use two inverters as a buffer, but at some point, I dont quite understand the functions or importances of the buffer.

As I was told before, 1) the buffer could smooth the output, but why ? It is two inverters, for example, if I design a ring voltage oscillator consisting of many inversters, why do i still need two inverters to the output of the VCO ? How can this buffer to smooth the ouput of VCO. 2) Also, the buffer usually can drive the big load (cap or res ), this is totally blurry for me, I dont understand it.

I just know these two buffer's function, and I even dont understand it why it has such functions, and for other use, I am not sure...

Hope get more help to understand this small but important block...

  • This question is difficult to answer without a circuit example. – Andy aka Oct 13 '13 at 14:26
  • Suppose you have a controller with open-collector outputs, which mean its 1 values are weak. You want both 1 and 0 to be strong. You need a buffer. I was once a case for me that clock failed because it was weak and buffer has rescued the PCB. But, I am not aware of other cases and why special clock buffer are better than others. So, I join your question. – Val Oct 13 '13 at 18:14
  • @Andyaka at some point I agree with u, hard to answer without a circuit, but also, this is what I want to know, I want to conclude the uses of a buffer, for IC design. then, if you dont know how to say, can I just understand that you never thought about my question before ? – alan Oct 15 '13 at 6:58
  • There are so many uses of a buffer and what a buffer does and that's just low frequency analogue stuff let alone digital. – Andy aka Oct 15 '13 at 7:10
  • @Andyaka many uses ? maybe i am not an expertise, but as I know, usually people just use buffer to separate one circuit from another circuit, so that they can drive the load without effecting the source circuit, both applied analog and digital, except this, I dont see where they often use buffer for, could you tell me more, thanks. – alan Oct 15 '13 at 7:25

Fundamentally, a buffer is an amplifier. It takes a small signal (lightly loading the source of the signal) and provides a copy of that signal that can drive a heavy (e.g., capacitive) load.

They are used in places where connecting the heavy load directly to the source would adversely affect the signal. Such effects arise because the signal source has a nonzero ouptut impedance, and the output may also used for feedback (or to feed other loads) in some way. Loading the output also affects the feedback, which then changes the behavior of the source in undesirable ways.

  • 1
    not to forget they also serve as means of introducing delay in the circuit which is of potential use – Raghunath V Jul 10 '14 at 9:54

Buffer could smooth the output, but why?
The CMOS resistive inverters can act like a buffer smoothing out the output, if you do analysis on the circuit you will find the noise-margins for the input/output help cancel the perturbed noise from external influences.
Consider the following circuit :

Ring oscillator.

enter image description here
The output from various inverters is being fed to the next stage of the inverter, any noise between the poly-silicon connection can cause the values of output of the inverter making it as a corrupt input to next cycle, having a high noise margin can help prevent this. Hence the greater the value of the Noise margin the better is the tendency of circuit to reject noise,CMOS inverter have sufficient noise margin that help cancel noise and make the output smoother.

Also, the buffer usually can drive the big load (cap or res ), this is totally blurry for me, I dont understand it.
Voltage buffer:

A Voltage buffer.
It reasonably depends on the type of the load you are driving and what you expect from it, A voltage buffer amplifier has large output resistance, hence prevents your circuit from loading the buffer circuit unacceptably, and a current buffer has low impedance output.

In CMOS inverter can drive any load, by supplying the current to the load, or by sinking the current from the load.

A CMOS buffer can also act as a level shifter. For example, when you have a sine wave with 400mV Vpp, and you want that sine wave to be a square wave swinging from rail-rail (from Vdd to Gnd), you can have a output buffer. Also, buffers are used as delay elements in logic circuits (for example reset), where you want an operation to happen after some time. (For higher delay times, I would recommend using a Voltage controlled Delay element). But for lesser delay times, we can use 2 cascaded inverters.

As rightly said by dave, you can drive a capacitive load with a buffer. Hope this helps :)

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