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Design >> Analog Design >> Designing fully differential Telescopic Opamp
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Message started by aditya_12p on Mar 20^th, 2014, 1:35pm

Title: Designing fully differential Telescopic Opamp
Post by aditya_12p on Mar 20^th, 2014, 1:35pm

Design fully differential telescopic OPAMP(single stage only)
a) Analysis of all equations of your design, with a
systematic derivative of all transistors W/L ratios
and spectre simulation of circuit for the following
specifications.
i) Open loop gain(DC gain) ≥ 80 dB
ii) UGB ≥ 300 MHz
iii) Differential Output voltage swing ≥ 3V
technology 0.18um

Can somebody tell me procedure to bias the circuit and schematic of it. I am trying to bias the circuit but unable to understand the concept behind the calculation of W/L ratio.Can you explain?It will be appreciated if somebody does analyse and calculate W/L values.
Is it necessary to use CMFB to bias the circuit.

Title: Re: Designing fully differential Telescopic Opamp
Post by aaron_do on Mar 20^th, 2014, 6:20pm

Hi,

since it is fully differential, you will need CMFB.

For the W/L ratios, note that it depends on the load. i.e. what resistance/capacitance are you trying to drive?

As a start, you can take out your circuit, and write down what you want for all of the biasing voltages. For example, suppose you have a 1.8 V supply, then you might want the output common-mode voltage to be 1.0 V, and the bias voltage of the cascode node to be 1.4 V. You also might want your gate overdrive for the PMOS current sources to be equal to 0.2 V. So you can easily write down all the node biasing voltages that you want. After that it is a simple matter of using your equations to calculate W/L. However, since you haven't defined the power consumption or the load, its all going to be relative...

One more thing, in order to get 80 dB gain, some of your lengths will probably need to be more than minimum.

regards,
Aaron

Title: Re: Designing fully differential Telescopic Opamp
Post by aditya_12p on Mar 21^st, 2014, 10:01pm

Hi,
I implemented the fully differential telescopic opamp with cmfb circuit . I got a gain of 85dB voltage swing more than 3V but UGB is 80Mhz . Any ideas on how to increase the UGB of the opamp?
Power consumption should be less than 3mW and Vdd is 3.3V

Title: Re: Designing fully differential Telescopic Opamp
Post by aaron_do on Mar 23^rd, 2014, 5:53pm

Hi,

its likely to be a matter of sizing/biasing point. Is your opamp driving any load?

Aaron

Title: Re: Designing fully differential Telescopic Opamp
Post by aditya_12p on Mar 24^th, 2014, 10:27am

schematic of the circuit that i used.how to decide the bias point ?

Title: Re: Designing fully differential Telescopic Opamp
Post by AnalogDE on Mar 24^th, 2014, 11:27am

Try increasing the input pair gm to push out the dominant pole.
You may need to burn more power.

Title: Re: Designing fully differential Telescopic Opamp
Post by aaron_do on Mar 24^th, 2014, 5:53pm

Quote:

Is your opamp driving any load?

You seem to be avoiding the most important question...are C0 and C1 the load?

Title: Re: Designing fully differential Telescopic Opamp
Post by loose-electron on Mar 26^th, 2014, 11:20am

He needs some basic books and classes in the subject first.

Title: Re: Designing fully differential Telescopic Opamp
Post by RobG on Mar 30^th, 2014, 10:02am

loose-electron wrote on Mar 26^th, 2014, 11:20am:

He needs some basic books and classes in the subject first.

Hell, this place would implode if everyone did that. ;)

I don't know why books "calculate" W/L. Mostly what you need to know is that gm =~ 2I/(Vgs-Vt) in strong inversion. In weak inversion gm becomes largely independent of Vgs-Vt so maximum gm will be ~I/75mV.

This is an approximation, but now you have gm in terms of I and Vgs-Vt, which are the only physical design variables that you can change. I is limited by power, and Vgs-Vt is limited to ~150mV for the diff pair and by headroom requirements for the loads and cascodes.

UNITY GAIN Bandwidth is =gm/Cload/2pi =~ so maximum bandwidth is (I/75mV)/Cload/2pi.

From this you can determine how much current you need to achieve your desired UNITY GAIN bandwidth for a given capacitive load. If you are close you can see if you can get enough gm in weak inversion (realizing your input devices are going to get very wide and increase the input capacitance), but if you are off by a factor of two you either have to relax your power requirements and increase I, or decrease your load capacitance. You don't have any other options.

I emphasized UNITY GAIN because if you are configured for a higher gain your bandwidth will be scaled by your feedback factor.

I'm sure there is a paper or book out there that covers this way of designing but I'm not aware of it.

rg

Title: Re: Designing fully differential Telescopic Opamp
Post by loose-electron on Apr 1^st, 2014, 12:45pm

Implosion might be a good thing at times...