I am trying to design a low power, fully Differential Input Single Ended Output amplifier with gain of atleast 50dB. I am using a VDD of 1.5 Volts using MOSFETS. I want to have maximum swing and my circuit along with different node voltages and MOSFET Parameters are shown in the attached picture. I want to have Vdsat for each and every MOSFET to be around 0.1~0.12 Volts. In the picture, the very first Voltage source that is connected to 2nd matched PMOS pair is vbias3 variable. The second Voltage source which is currently set to 1 V is vbias2 variable and the tail source biasing voltage source is vbias 3 variable. Later I will replace all of these voltage sources with a biasing generator that I will design later. The two matched pair NMOS (T-6,0,5,8) have channel width of 4uM and PMOS (T-1,4,2,3) transistors have Channel width of 8uM. The Tail current source NMOS T7 has channel width of 8uM. All of the transistors have channel length of 220 uM.

I am trying to have my output (the middle node between T1 and T0) to be at centre of the rail voltage (1.5V) so that I can maximize my swing. However I am stuck at this point and no matter what I do I can't bring that point down to ~750mV and have the gain of 50dB.

Need design help with following: - Bring down the Vdsat for top 4 PMOS to around 0.1~0.12 V - Bring the output node(currently at 1.037 V) down to 750 mV by keeping the gain as high as possible to maximize the swing. - Have all the MOSFETS operate in saturation

NOTE: Please don't worry the Tail current source NMOS as Vdsat seems to be greater than VDS in the current screenshot, however I can easily put my MOSFET in saturation by just bringing the biasing voltage down by 5mV.

Any help to achieve the above requirement would be greatly appreciated.


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1 Answer 1


The lower limit for the output swing is set by the NMOS transistors, i.e. the tail current source, the diff pair and the cascode.

Things to check:

  1. Saturation voltages of the NMOS transistors. Everything below, say, 120mV?
  2. Biasing of the cascode, it should be as low as possible and as high as necessary. It should be in the order of 3*Vdsat + 1*VT.
  3. Input common voltage. The output swing depends on the input common-mode mode voltage. For maximum swing, minimum input common mode voltage is required. The minimum would be around 2*Vds + 1*VT.

To bring the output to 750mV you need to apply feedback, so you should have a proper testbench.


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