How to speed up a single-ended comparator?

[benchen]

I use a single-ended tow stage amplifier as a comparator, but I found that when the input is small ( such as 0.5mv), the output changed very slowy( almost 4~5us from the lowest to highest voltage). I wonder how to speed up a single-ended compararator?

 

[pthoppay]

Hi,

Increase the gain of the amplifier, (i.e increase gm beware dont increase W as capacitance would increase and in turn the effct will slow down) Try decreasing the biasing current and thereby increasing the gain.

Regards,
Prakash.

 

[cretu]

depending of the voltages that you would like to compare, you can chose nmos or pmos. Nmos for higher then 1/2 Vdd and pmos for closer to gnd

 

[leomecma]

Use a comparator to do comparation, and opamp for do amplification. It isn't a good design use opamp for do comparation.

But if you need use it try use a little gain, because:

const = W*GAIN then if you use it in open loop it has a very little W, how can you see isn't a good choice for this type of application.


leomecma

 

[chency]

Hi,

Increase the gain of the amplifier, (i.e increase gm beware dont increase W as capacitance would increase and in turn the effct will slow down) Try decreasing the biasing current and thereby increasing the gain.

Regards,
Prakash.

I don't think only decreasing the biasing current can do more help for the comparator, the slew rate is down, the delay because of the offset of the opa, we should trade off the gain and gm and the slew rate , and we don't confirm the offset of the opa with hspice .

 

[cretu]

for the offset you should try to use a monte carlo simulation. It will give you the standard distribution and the spread. As a rule of thumb, W*L is inverse proportional with the offset

 

[zenisle]

Use a comparator to do comparation, and opamp for do amplification

 

[artem]

You can speed up opamp slew rate by using feedforwarding .
Some info (applicable to amplifiers):
https://www.analog.com/UploadedFiles/Application_Notes/135208865AN-202.pdf

but not sure about its applicability to comparators and also there is stability issues

 

[shadoweek]

try using a buffer before the load but make your calculations to determine its size first

Cg=((Cg+1)(Cg-1))^(.5)

where Cg is the buffer input capacitance
Cg+1 is the load cap
Cg-1 is the previous stage input cap

 

[Iouri]

Try to build transistor comparator using HF transistors