How many parameters can speed of compare determine?

[watersky]

about speed of compare

i want to know how many parameters can determine speed of compare?
fT,fc,gain,current of the last stage, are there other parameters?
if we use a amplifier and two inverters as a compare, speed of compare is perhaps faster,but at this time which parameters will determine speed?

thanks a lot!

 

[suraj]

Re: about speed of compare

Hello Friend,
I didn't get your question.Particularly last sentecnce, can you please explain in more detail, i can try .

Suraj

 

[watersky]

Re: about speed of compare

suppose power supply is 5V, inp(in+) of comp is step sigal, which is 2.498V at first, using 1ns step changes 2.502V, and inn(in-) of comp is 2.5V, out will change from 0V to 5V if comp is perfect,but how many is time of change from 0V to 5V? which parameters will determine this time? otherwise comp is not so perfect, when 2.498V and 2.5V compare, out is perhaps 1V; when 2.502V and 2.5V compare, out is 3V. so as to realize logic supply,perhaps two inverters will be added.

 

[suraj]

Re: about speed of compare

Hello,

I think there in no clock in your comparator.
In this case if you have a differential amplifier in first stage.
out put waveform's rise and fall time depends upon the output cap.,gain of the Diff. pair.
Gain also depends upon the gm, baised current, W/L. So you have to see these parameters.

Secondly you are using the inverters in the output to get the required gain.

I think this will help, as I am also student of Analog IC design, if some one has better answer please suggest us and correct me if I am wrong.


Suraj

 

[suraj]

Re: about speed of compare

Hello watersky,
if you get any better ansere please let me know
Suraj

 

[terryssw]

Re: about speed of compare

The comparator speed is determined by the W/L ratios of the transistors in comparator, especially in the ones used as positive feedback (the two transistors with cross connection).

Also, comparator will have difficulties to determine differences of around several mV unless you have preamp. This can be referred to the offset of the comparator. If your applications can tolerate this offset than don't bother to such error, if not, then add pre-amps. I hope this can help.

 

[nkchenliy]

Re: about speed of compare

The comparator speed is determined by Gain, Bandwidth, tail current. In fact, if you use positive feedback (as terryssw said), comparator speed will faster. You can see "Circuit Design, Layout, and Simulation" by R. J.baker in Page 691.