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Design >> Analog Design >> Output imepedance
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Message started by Rakesh on Aug 24^th, 2011, 11:39am

Title: Output imepedance
Post by Rakesh on Aug 24^th, 2011, 11:39am

Hi,
I was working on increasing the linearity of the transconductor.
I get a good linearity if i short the output and measure the current.
However if i allow a swing of say peak to peak 400mV on the output my linearity goes down to 60dB .
My output impedance is 400k. Can u suggest me techniques or ways of increasing the output impedance apart from cascoding.
Thanks
Rakesh

Title: Re: Output imepedance
Post by harpoon on Aug 24^th, 2011, 1:17pm

Make L bigger ... Or use feedback, I'll have to think about the circuit.

Title: Re: Output imepedance
Post by loose-electron on Aug 26^th, 2011, 10:25am

gmC filter transconductor?

Prior suggestions are good.
Also - Increase the static bias current of the device so the signal is a smaller part of the total current.

Also differential pair structures can be replaced with multi-tanh structures as gets used in LNA front ends.

Title: Re: Output imepedance
Post by Vladislav D on Aug 27^th, 2011, 4:49am

loose-electron wrote on Aug 26^th, 2011, 10:25am:

gmC filter transconductor?

Prior suggestions are good.
Also - Increase the static bias current of the device so the signal is a smaller part of the total current.

To increase current through the devices in order to increase output impedance? Sounds controversial.... :-/

Title: Re: Output imepedance
Post by loose-electron on Aug 28^th, 2011, 12:17pm

He is having problems with linearity vs. output impedance.
Thus the suggestion on how to improve linearity.

If we know the application a bit more may be able to make better suggestions, I am guessing a gmC filter (4 transconductors per biquad) , but it has not been stated.