Questions about gain of a two stage comparator

[huangjw]

hi,
when i simulate a two stage comparator, its gain is only 7db,but the gain of the first stage is 40db.i don't know where is wrong?,

 

[ezt]

Re: gain of comparator

Hi huangjw.
First of all, I can't understand why you calculate the gain of a comparator. In recent years, nobody uses conventional opamps as comparators, it's a waste of power. Usually designers implement a preamp as the first stage and a positive feedback in the second stage. because the goal of comparator is to resolve a digital output which is vdd or vss. Even for reducing the DC power consumption, there are some structures called "Dynamic Comparators", I really recommend you to read some papers by Sumanen about this category (its available in IEEE explore). However, If you still want to use simple Opamps, with more power consumption and lower speed, you should use e.g. a 2-stage opamp with a large DC gain and use it in an open loop manner to have a comparator behavior.

Regards,
EZt

 

[huangjw]

gain of comparator

HI,ezt
Thanks first. I'm a biginner of designing analog circuit.and now i major in DC-DC convertor .I THINK the conventional opamps as comparator is popular in DC-DC convertor. the gain of a comparator often detemine the offset voltage and some other function .

 

[flatulent]

Re: gain of comparator

Comparator gain is lower than op amp gain. This is so they can switch faster. Generally, the gain is designed so that the input offset voltage will be amplified enough to put the output to one of the logic levels.

 

[Karthikeya]

Re: gain of comparator

hi
i hv a doubt...i the comparator needs to switch faster then the transition region is to be smaller tht means for a given o/p swing the gain is larger ...or i m wrong??
how r input offset voltage and min voltage(Vin-min ) related???

thnkx

 

[surianova]

Re: gain of comparator

hi,
when i simulate a two stage comparator, its gain is only 7db,but the gain of the first stage is 40db.i don't know where is wrong?,

I think for the second stage of comp, the transistors not in saturation. This
will cause the gain to be negative. If the negative gain X first stage gain (40db),
you will get a very low overall gain. Or you can check second stage gain is negative or positive.

Suria