Help me design an opamp for the output of DAC

[Vabzter]

Hi all,
I have the following specs and I want to design an opamp. The application is that it should be connected at output of DAC and amplify from 1.2(full scale o/p of DAC) to 1.75 V. So will connect it in non inverting configuration..

Vcc=1.8V
Vout=1.75V
Cl = 35pF, Rl = 5Kohms
Idc <60uA
BW=200KHz.
SR is not fixed but can be a very slow Opamp.
I have designed a simple 2 stage Opamp but not getting the full output swing. Should I go for a cascode with class AB ouput?
Any sugestions/pointers with this design..
Thanks
Vabzter

 

[mists]

Opamp Design Help..

if the supply is 1.8v, i think cascode can't support 1.75v output.

 

[dreamteam]

Re: Opamp Design Help..

To get the maximum output swing, try to keep only Vds sat. 50mV will be hard to get....
Try a class AB, or an Rail to Rail output stage and choose W/L in order to have a low Vds sat.

A two stage opamp could match but you couldn't cascode.

 

[ieropsaltic]

Re: Opamp Design Help..

Cascode can't help you to achieve better output swing .Class AB can do the job I believe but will suffer from larger non-linearity .You can also try differential pair amplifiers .By differential amplifiers you can get double your original swing specially if you can get a differential input signal .If your input signal is single ended, I think that you still can get double the swing on the cost of half your gain .

 

[Vabzter]

Re: Opamp Design Help..

Hi
Thanks for all the replies..What what I undustood I can have a simple 2 stage with Class AB as an output stage. Is this correct?
Also can someone post some links/papers for class AB CMOS design..That would be very helpful tome.
Thanks
Vabzter

 

[crossbow_123]

Re: Opamp Design Help..

Hi
Thanks for all the replies..What what I undustood I can have a simple 2 stage with Class AB as an output stage. Is this correct?
Also can someone post some links/papers for class AB CMOS design..That would be very helpful tome.
Thanks
Vabzter

Hi,

I think the cascode structure is not suitable for you, You can take this paper for your information.

 

[isaacnewton]

Re: Opamp Design Help..

Hi all,
I have the following specs and I want to design an opamp. The application is that it should be connected at output of DAC and amplify from 1.2(full scale o/p of DAC) to 1.75 V. So will connect it in non inverting configuration..

You mean the output swing of the op amp is from 1.2V to 1.75 V?

 

[Vabzter]

Re: Opamp Design Help..

Hi all,
I have the following specs and I want to design an opamp. The application is that it should be connected at output of DAC and amplify from 1.2(full scale o/p of DAC) to 1.75 V. So will connect it in non inverting configuration..

You mean the output swing of the op amp is from 1.2V to 1.75 V?

Hi,
No what I meant was that the input to opamp is 0 to 1.2V and the output of opmap should be 0 to 1.75V. The opamp is directly connected after DAC whose full scale output is 1.2V.
Thanks
Vabzter

 

[tedd]

Re: Opamp Design Help..

You can also refer to IEEE papers ,such as
B. Sekerkiran, “A Compact Rail-to-Rail Output
Stage for CMOS Operational Amplifiers”, IEEE Journal
of Solid-State Circuits, vol. 34, pp. 107-110, 1999.

 

[maharshi_qis]

Re: Opamp Design Help..

I think for that low voltsges and to get full rail to rail swing its better to go for folded cascode NMS-PmOS differential amplifier

 

[spbhu]

Re: Opamp Design Help..

Definitely a class AB amplifier is required in your design!

Added after 2 minutes:

You can read philip allen's book and select one easy op-amp structure!