Design the OPamp with rail to rail and low output impedance

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hebu

Joined: 15 Nov 2004
Posts: 195

27 Jun 2005 6:25 Design the OPamp with rail to rail and low output impedance

I can realize the input stage designed in r2r style, but how to implement
an opamp with low output impedance and r2r? It's diffcult to utilize source
follower to do this??

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gnarejo

Joined: 27 Jun 2005
Posts: 2

27 Jun 2005 9:04 Re: Design the OPamp with rail to rail and low output impeda

You can connect the output of the OPAMP to next OPAMP and take the out put from the cascaded one .

surianova

Joined: 01 Sep 2004
Posts: 410
Helped: 21
Location: ASIA

28 Jun 2005 4:28 Re: Design the OPamp with rail to rail and low output impeda

hi

To get a low output impedence (need when drive resistive load), you can use opamp as a buffer, prefer use rail to rail opamp because r2r output can go to
very low voltage.

If your output driving capacitive load, high output impedence has no problem
to drive, for example gate of the CMOS.

Suria

sunking

Joined: 25 May 2004
Posts: 914
Helped: 46

28 Jun 2005 10:42 Design the OPamp with rail to rail and low output impedance

source flower is not fit for r2r.
you can search "rail to rail" in this board for more

hebu

Joined: 15 Nov 2004
Posts: 195

28 Jun 2005 14:45 Re: Design the OPamp with rail to rail and low output impeda

ya, I can design rail to rail but noly drive cap load. Now I need a
low output impedance so as to drive low impedance. But I found
source follower can't meet my requirement.
Anybody has the idea?

surianova

Joined: 01 Sep 2004
Posts: 410
Helped: 21
Location: ASIA

30 Jun 2005 3:30 Re: Design the OPamp with rail to rail and low output impeda

hebu wrote:

ya, I can design rail to rail but noly drive cap load. Now I need a
low output impedance so as to drive low impedance. But I found
source follower can't meet my requirement.
Anybody has the idea?

If you can design a rail to rail opamp with high gain and connected in negative feeback, the output impedence will become very low according to formula as below

ro= Ro/(1+A)

Ro= output impedence without feeback
ro= output impedence with feeback

So, the output impedence is low, just like using source follower...

Surianova

hebu

Joined: 15 Nov 2004
Posts: 195

30 Jun 2005 5:26 Re: Design the OPamp with rail to rail and low output impeda

But if the output stage is a current source output, the Ro can be as high as
several hundred ohms or more. So, if the Ro=500K ohms, the Gain=1000,
the equivalent output impedance is still larger than 500ohm.
It may not be used in some case.

surianova

Joined: 01 Sep 2004
Posts: 410
Helped: 21
Location: ASIA

30 Jun 2005 13:17 Re: Design the OPamp with rail to rail and low output impeda

hebu wrote:

if your output driving capacitive load, then it is not a concern. What is the problem
occur when you are using source follower ? nmos source follower or pmos source
follower?

Surianova

tyanata

Joined: 27 Nov 2003
Posts: 122
Helped: 3
Location: Sofia

01 Jul 2005 7:55 2 stupid qestions

ro= Ro/(1+A)

Ro= output impedence without feeback
ro= output impedence with feeback

1. In this equation A is opened loop gain or closed loop?

2. Is this formula suitable for amplifier which is connected as follower( The signal is connected to INP, OUT is connectede to INN?