Specifications problem

Hello,

I've been designing a Two-stage CMOS OTA folded cascode amplifier.

The schematic for the simulation is presented below:

The schematic of the symbol Amplifier2 is presented below:

I must get the following specifications:

• Small Signal Gain (Av) greater than 70dB;
• Bandwidth (Bw) greater than 60kHz;
• Phase Margin (PM) around the value of 60 degrees;
• Slew-rate (SR) bigger than 200V/us;
• Current Budget of 400uA (the current used on the circuit must be below 400uA)

I have used the following dimensions for the transistors and values for the DC current sources of the biasing circuits used on the simulation:

• IBIAS2=125uA;
• IBIAS=75uA;
• L=0.55um (I’ve used the same length for all the transistors, I don’t know if this is a good method);
• W12=11.25um (this value is for the width and width stripe of the transistor M12 – the same logic is applied for the other transistors);
• W1=W2=50um;
• W11=27um;
• W3=W4=W5=W6=W7=W8=13.5um;
• W9=W10=8.9um;
• W13=11.25um;
• W14=55um

Using the values presented before, the specifications of interest obtained by the calculator functions, are the following (and also, some comments of what need to be improved):

• Av: 70.82dB -> I would need to increase the Small Signal Gain (Av), because it is near the minimum required value of 70dB;

• SR: 201.5M -> I would need to increase the Slew-Rate (as it is near the minimum value of 200M);
  
  
• Bw: 67.12kHz -> I think the Bandwidth is OK (above the minimum of 60kHz) and can be eventually reduced to increase the Small Signal Gain (Av);
  
  
• PM: 56.83 degrees -> I would need to increase the Phase Margin to a value near 60 degrees;
  
  
  
• GBW product: 234M -> This value is just to have an idea of the Gain-Bandwidth Product;
  
  
• Current Budget of 400uA is being respected, because the total current used on the circuit is around 320uA (can eventually be increased if needed);

QUESTIONS:

Should I use the same length for all the transistors?

Can you give me a clue regarding the parameters that I should modify in order to get better values for the specifications?

In this case, what is the theoretical expression for the slew-rate?

Thank you very much in advance!