Symmetrical/3-current mirror OTA

[AMSA84]

Hi guys.

I am designing a Symmetrical OTA and I am facing a problem. The maximum gain that I am able to get is around 50dB with a current consumption of 700uA. The GBW is aprox. 130MHz.

The circuit that I am talking about is this one:

**broken link removed**

I would like to push the gain to > 60dB. One possible way from what I have seen is to put a cascode at the output. However I don't know how to implement the bias circuit.

Can someone give me a tip on how to do it?

Taking the advantage of this post, I'd like to ask another thing: If I want to increase the phase margin on this design, can I put a capacitor between node 4 and the gate of M6? Like the Miller 2-stage OTA amplifier. The difference is that this design doesn't have nothing to do with a 2-stage design!

Regards.

 

[Dominik Przyborowski]

If your process provides low threshold devices, use self cascode current mirrors. Make a stack of std Vth and low Vth devices and connect them gates together, like in figure 8.

If yout process doesn't provide low Vth fets, use high swing current mirrors maked as stack of two fets and resistor like on this scheme. The voltage drop on resistor is equal to vds of current source transistor and should be slightly higher than it vdssat.


//edit:
For the second option here You have an example: