a question about OP design

Hi,

I'm designing an op amp using folded-cascode and CS as the 1st and 2nd stage, respectively. A 0.9V DC and 0.5V peak-to-peak swing is given as input and the spec requests to see a 1V peak-to-peak at the output.

I've already designed a CS with 30 dB. Here are my questions.

1) Since 1V peak is required at the output of 2nd stage, I think I have to leave a head room for the swing at the 1st stage, which is equal to 1V / 30dB=0.016V. Is this correct?

2) And then this is really confusing. Since I'm designing a cascode, I should be able to get a high gain from this stage. But, 0.016V at the output, and 0.25v swing at the input, it seems to me that this stage is decreasing the gain. how could that be?

Can anybody indicate the mistakes I make?

thank you.

airboss said:

2) And then this is really confusing. Since I'm designing a cascode, I should be able to get a high gain from this stage. But, 0.016V at the output, and 0.25v swing at the input, it seems to me that this stage is decreasing the gain. how could that be?

Click to expand...

Hi airboss,
I think a little explanation could clarify u and meet ur confusion.
When do we call the gain it means, generally, small signal gain. That means when u r giving very little signal at in put (say in mV or uV level, as an convention it is taken as <10mV), u will be getting the Vi x Av at the output. Now in general, the gain specified doesn't change much for large signal also. Although it may depend on different factor which is not the interest of this topic.

Now, think, the amplifier is working with a fixed power supply. It is having some input swing and some output swing. These are definitely bound to be lower than the power supply. Now what happens, when u give a pure sinusoid at the input with much smaller amplitude, u will be getting at output a pure sinusoid if the (Vi x Av) doesn't croos output swing (OS). When u starst increasing the input amplitude the output amplitude also starts increasing and once it touches OS and again try to cross it. AT this point the circuit becomes saturated. NOTE: the circuit ; not necessarily the devices / transistors. SO, if u give peak to peak input signal; u definitely cannot get (Vipp X Av) at the out put due to power supply and circuit saturation limitation. It doesn't mean the circuit is working as attenuator.

This is applicable for a signle stage; or double stage or what ever - those are apmlifier.

To add : A higher gain is preferable because; in general apmlifiers are used with some kind of feed-back configuration. Like for buffer; as much gain u will have , that much less will be ur offset.


sankudey

Hi, sankudey,

thanks for your reply. i don't fully understand so i'd like ask some questions. let's say the required input is vi,p_p and output is vo,p_p.

do you mean that there's actually an vip_max, which is the maximum input i can apply so that vip_max * Av = vo,p_p but vip_max < vi,p_p ? if so, when then vi,p_p is given?


thanks for your patience.