Is it possible to get full swing in simple cascode OPamp with single ended output

[shrikant_joshi7]

Hi m designing a telescopic opamp with single ended output and the swing is only 600mV with a supply of +/- 500mV.The ac analysis shows a d.c gain of 52 db i.e.498.
I given an input of 10mV sinusoidal wave so the output should be 4.98V(as it is showing in ac analysis) i.e .the output should saturate to +/- 0.5 V and should show a square wave in the output.But the output is only -170mV to 400mV.
Why the positive and negative swings are different?
Plz help.

 

[BradtheRad]

Can't say I know how to get around it, but your results resemble the same characteristic of conventional op amps. Output will not go below 2V when the device is powered from a positive supply.

And when you hook up the device to a bipolar supply, output will not get any closer to the bottom level than 2 V above it.

It may have to do with the bottom resistor in the current mirror. It has a potential across it. This voltage is added to the output.

I didn't spot a resistor in your schematic, but perhaps you have a mosfet performing the same function as a resistor?

 

[lamoun]

Hello shrikant,

No you can't have a large swing with a telescopic opamp.
You can either use another architecture (this might add a couple of hundreds mV), or if you really need a full output swing, you should use a rail-to-rail output stage.

When you have a negative swing the 3 lower transistors are the restricting factor (should be in saturation ~3Vdsat) in contrast to the upper two (~Vdsat) when you have a positive swing.