Wide swing current mirror in folded Cascode Op Amp

Hi all,

I have designing folded cascode OTA as shown in the schematic. I can get reasonably good gain (74dB) and f-3dB=600kHz using 130nm (1.2V) process. What bothers me was the wide swing cascode current mirror of PM12 and NM16 (the circled transistor in the schematic) is in triode region. In the schematic I also display the operating point.

For PM12--> vdssat=98mV, vds=64mV
For NM16--> vdssat=80mV, vds=44mV.

Is this expected for a given current mirror topology? The transistor behaves like voltage controlled resistor in triode region, therefore small change of bias voltage will cause large variation in current. So clearly this is not what we want.

How to make those two transistors going more into saturation?

Thanks
sj

So all the circuitry on the left of your circuit is supposed to be there to provide the biasing for your amplifier but somehow you're using voltage sources at the cascode stage to bias those nodes so this looks kind of sketchy. For example, The gate of NM16 and NM17 should be connected to the gate of NM9 and NM10 to provide the biasing voltage of that node, but you're using an ideal voltage source to provide basically the same voltage (600 mV vs. 601.04 mV). What's happening with PM12, PM7 and PM8 is something similar. You're forcing their gate voltage to a constant value by using an ideal voltage source of 450 mV which biases PM7 and PM8 at the right place (saturation) but since the drain voltage of PM7 and PM8 is set by the bottom diode connected transistor (NM5) and the drain voltage of PM12 is set by the TOP diode connected transistor (PM11) that makes vds of PM12 to be different from the vds of PM7 and PM8 (and what you want is for them to be equal).

Now, the problem is that PM12 is not going to be able to bias PM7 and PM8 by itself so what you need to do is add another resistor in the PM11/PM12 branch to adjust the voltages properly and be able to get rid of the ideal voltage supplies.

Hope this helps,

diemilio

Hi diemilio,

Thank you for your suggestions. It helps. I attach the schematic and dc operating point after some modification. However it as you can see from the schematic, NM16 barely saturate. Is this ok? What is the safe margin for the transistor in saturation? vdssat+50mv?

Another question is the output voltage (drain NM10) is held at ~480mV instead of 600mV when 600mv common mode voltage is applied at the input. Does this cause any problem and should I adjust the transistor's ratio to get exactly 1/2VDD at the output?

sj

You do realise that PM9/PM13 and PM11/PM12 are doing exactly the same thing. You can save 50uA there straightaway.
Your biasing currents seem to be unusually wasteful, with 150uA wasted not doing anything.
If you want to increase the overdrive of NM16, just increase the value of R0. Of course, you must know why you want to increase or decrease this overdrive, and what result you want to achieve.

I think checkmate pretty much said everything I was going to add. Summarizing:

- The NM14/NM17/PM11/PM12 branch is not really needed, you can tap off the biasing voltages for PM3/PM4 right from PM9/PM10 and for PM7/PM8 from PM13/PM14, just make sure you have right ratios of W/L to get the current values you want.

- Vds of NM16 basically depends on the voltage drop across the resistor, the current you run through the transistor and its W/L value. What you need to do is size either the transistor's W/L or the resistor (or both) in such a way that you get the desired value of Vdsat for the current your running through (write down the equations for that branch and solve, it should be pretty straight forward).

- The margin between vds and vdsat depends on how critical the transistor is through out the signal path. Since this transistor (NM16) is just part of the biasing branch is not that critical because the voltages across its nodes are fairly constant (DC values). Now, you should also keep in mind that leaving vds close to vdsat makes the transistor more vulnerable to any other changes in the circuit, like supply variation, vth variation, etc...

- In practice it is going to be very very difficult to adjust the output value to exactly VDD/2 but usually it's a good idea to leave as close to it as possible to maximize swing. At the end, your DC common mode voltage at the output will be defined by the feedback you apply to the amplifier, but setting its open loop DC value close to VDD/2 is a good practice.

diemilio

if you want the Vds of NM16 higher, increase the resistance of R0.