Tutorial 6: Complete the Op Amp IC

In this Tutorial, we will put 3 Op Amps and 3 inverters into your padframe.

Let's finalize the Op Amp design first.

All the DC operating points are set up when the off-chip resistor is around 200k. The open loop gain can be simulated use the following circuit and Spice code:

The results can be presented like the following:

The second diagraph shows the open loop gain is around -400.

However, if the CMOS transistors are not biased in the saturation region, the gain will decrease and the Op Amp cannot work correctly. I changed the resistor to 10k, you will see the drop of the grain:

Now let's add a capacitor from the output of the second stage to the output of the differential pair to improve the stability of the Op Amp. Here is a new concept - Phase Margin. In frequency response of the Op Amp, when the gain of the Op Amp is 0 dB, the phase difference from this point to -180 degree is the Phase margin. 90 degree of phase margin is usually ideal (CMOS Book Chapter 24). However, if we don't add the 10 pF capacitor, the phase margin is around 40 degree (not that stable): (note that the 100 MEG resistor is used to conduct the DC bias but blcok any AC component and the 10 uF capacitor is used to remove any AC noise).

When we have the 10 pF 'compensating capacitor' added, the phase margin is pretty close to 90 degree!

Now let's add the 10 pF capacitor to your layout:

Create a new cell called 'cap_10pF'. In the artwork editor mode, add a 'filled box' as the P1 plate, and a 'opened Polygon' as the P2 plate (dimension: 0.5 x 3). Press 'Y' to edit the shape.
Then create two exports and name them as 'P2' and 'P1' respectively. Make sure they are DRC-clean before you leave this page.

Create a layout view of the capacitor. The total area of the capacitor should be 130,000 by scale. So measure the distance between the vdd rail and the gnd rail before you the length of the material is determined.

Start with P2 material. Grab a poly-2 pin and change the dimenion to your desired dimension but a little bit longer (to fit the metal 1 - poly 2 contact to it). Then select the pin and press 'c' to change it into a poly-2 node:

Don't forget to add the poly - metal 1 contacts to the poly layers. Use the ruler to double check if the dimension of the overlapped area is the one you expected.

DRC and LVS the schematic and the layout, make sure they match.

Go to your Op Amp layout view, move the last stage to the right to fit the capacitor in between. Connect all the circuits.

Drag the schematic of the capacitor into the schematic of your Op Amp:

!!! Make sure you DRC, LVS, and NCC your circuits after every step before you move on, otherwise, you will find hundreds of errors in your final circuit.

Now, put three copies of the Op Amp and three copies of the inverters into your pad frame:

Delete the 'Skyhawks' logo in your pad frame if you still have it there.

Complete the schematic of the chip:

Again, DRC/LVS/NCC your final design for credit.

Finally, select your final chip layout view, go to file - export - GDS II (stream) to create the GDS II file, send it to yiyanli185@gmail.com before March 30, Friday 5 pm.

Requirements for the final report:
1. Have the DC operating point simulation data organized in a table. Show the correct resistor value that can make all the transistors in saturation region.
2. Show the open-loop gain of the Op Amp.
3. Show the schematic and layout view of every component and the final chip view.
4. Show the DRC clean, LVS clean, and NCC clean report (screen shot).

The report is due April 9, Monday, 10 pm.