Why the tail resistor can increase the amp's stability

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fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

31 Jan 2007 10:31 Why the tail resistor can increase the amp's stability

Hi,everyone :
I read a patent (US6407537) , it is said that the tail resistor can increase
the stability . May anyone explain it ?
Why the tail resistor can increase the amp's stability

Why the tail resistor can increase the amp's stability

jonashat

Joined: 07 Sep 2005
Posts: 75
Helped: 5

31 Jan 2007 12:10 Why the tail resistor can increase the amp's stability

My educated guess would be:

Increasing RTL will decrease the transconductance of the input pair and the UGF will go down which will increase your phase margin.

n1cm0c

Joined: 05 Sep 2006
Posts: 183
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Location: Santa Maria

31 Jan 2007 12:44 Re: Why the tail resistor can increase the amp's stability

Yes, i agree.

But what's the subject of the patent ? The current-splitting mirror in the second stage ? It works as a current multiplier?

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

31 Jan 2007 15:40 Re: Why the tail resistor can increase the amp's stability

What's the purpose of T7 and T8 ? They work as current source !

ender84567

Joined: 20 Jan 2007
Posts: 82
Helped: 7

31 Jan 2007 15:46 Why the tail resistor can increase the amp's stability

provides a feedback path, whats your level of engineering understanding, i'm not sure how technical i can get in explaining this, are you familiar with root locus or nyquist plots?

n1cm0c

Joined: 05 Sep 2006
Posts: 183
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Location: Santa Maria

31 Jan 2007 16:24 Re: Why the tail resistor can increase the amp's stability

T7 and T8 are supposed to work as a current source,to bias the input pair without losing input commond mode range due to voltage drop in Rtl.

I am curious about T13 T14 T11 T12.

In pair t13 and T14 one can do a 1:N mirror, and then have N times more current
in T11, where again one can do 1:K mirror and get N*K times more current,
but I don't see the reasoning there. One can have a different supply (higher voltage?) for the pair T11-T12, but I see a much reduced headroom for T13.

Anyone can say more about this ?

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

31 Jan 2007 17:11 Re: Why the tail resistor can increase the amp's stability

Hi , n1cm0c :

Why you said "T7 and T8 are supposed to work as a current source,to bias the input pair without losing input commond mode range due to voltage drop "
Can you explain it more ?

And what is the gain of the amp (T1,T2,T3,T4,T6,T7,T Cool

?
I still can 't understand why the RTL increase the stability .

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

31 Jan 2007 18:47 Re: Why the tail resistor can increase the amp's stability

Can anyone tell what is the gain of the amp composed of T1,T2,T3,T4,T6 ?

xjxmn

Joined: 31 Jan 2007
Posts: 27

31 Jan 2007 20:09 Why the tail resistor can increase the amp's stability

I think the feedback of the OA is positive, isn't it?

renwl

Joined: 26 Apr 2004
Posts: 455
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Location: shanghai,china

01 Feb 2007 2:57 Why the tail resistor can increase the amp's stability

the feedback of the OA is negative. there is no problem.

xjxmn

Joined: 31 Jan 2007
Posts: 27

01 Feb 2007 3:59 Re: Why the tail resistor can increase the amp's stability

renwl wrote:

the feedback of the OA is negative. there is no problem.

My analysis is as below.
I suppose VT2_G( voltage of the gate of T2) decrease, and VT8_D decrease, and the current from left to right flow through RTL will increase, and IT2D decrease, and IT3D increase, and IT4D increase, so VT5_G increase, IT5D decrease,IT13D and IT14D decrease, IT11D decrease, IT12D decrease. Finally, VT2_G decrease.
So it's positive feedback. If something is wrong in my analysis, please tell me, thanks.

renwl

Joined: 26 Apr 2004
Posts: 455
Helped: 23
Location: shanghai,china

01 Feb 2007 5:17 Why the tail resistor can increase the amp's stability

less IT5D means that IT13D decrease and IT14D increase.
so the voltage of the gate of T2 will increase.
it's negative feedback.

Btrend

Joined: 26 Dec 2003
Posts: 424
Helped: 55

01 Feb 2007 5:29 Re: Why the tail resistor can increase the amp's stability

The added R is called source degenarated resistor, it will reduce the gain of differential pair, and increase it's input range. Reduce gain will increase stability too.
But the circuit is still 2 pole system, some kind of compenstaion is needed except u make the gain of first stage very low (i.e. the non-domainant pole far away ur dominant pole).

xjxmn

Joined: 31 Jan 2007
Posts: 27

01 Feb 2007 5:32 Why the tail resistor can increase the amp's stability

I think IT13D should equal to IT14D, because they share the same gate and drain, and they are all in saturation region.

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

01 Feb 2007 7:12 Re: Why the tail resistor can increase the amp's stability

But if there is no RTL , the gain will be more samller for the high output R
of current source . So I think the RTL will increase the stability by decreasing
the gain is wrong !

Btrend

Joined: 26 Dec 2003
Posts: 424
Helped: 55

01 Feb 2007 8:44 Re: Why the tail resistor can increase the amp's stability

Had u run the simulation of that amp ?
What's the phase margin & DC gain ?
I don't understand why the gain will be larger without RTL?
Can u show us ur simulation of derivation ?

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

01 Feb 2007 9:35 Re: Why the tail resistor can increase the amp's stability

If there is no the RTL , the transconductance will be :
Gm = gm /1 + gmro
where gm is the transconductance of the input device, ro is the output
resistor of the current source .
For ro is very big , t
Gm = 1/ro
But if there is RTL ,
then
Gm = gm/[1 + gm(RTL/2] = 2/RTL
So if there is no the RTL , the gain should be small .

Btrend

Joined: 26 Dec 2003
Posts: 424
Helped: 55

01 Feb 2007 11:09 Re: Why the tail resistor can increase the amp's stability

fanrong wrote:

Why the Gm of differential pair without RTL is 1/ro ?
if it was true , then u will always get output gain as Gm*ro=1 ?
The Gm should be equal to gm , such that without RLT ur first stage should
have gain gm*ro !

dingjingfeng

Joined: 12 Aug 2005
Posts: 196
Helped: 9

01 Feb 2007 14:14 Re: Why the tail resistor can increase the amp's stability

without RTl means you should replace this resistor with a short line.
and it is a virtual ground at this point for diffrecial circuit

then you should recalculate you Gm:D

fanrong wrote:

n1cm0c

Joined: 05 Sep 2006
Posts: 183
Helped: 25
Location: Santa Maria

01 Feb 2007 15:32 Re: Why the tail resistor can increase the amp's stability

T1 T2 T3 T4 are standard differential input with current mirror load.

if RTL is zero, then T7 and T8 are two current sinks in parallel. If RTL is non-zero then it's also a standard configuration, and the fact that the bias currents do not go through it keep the input common mode range unchanged.

T5 is a standard second-stage, common source. The load of T5 is not directly
TCL, which would be the standard, but T13 and T14. The way they are connected
the input resistance looking from the drain of T5 into T13 is 1/gm + (N+1)*Ro,
where Ro is the output resistance of TCL. The N is the multiplication factor of the mirror. There's no net increase by doing this, since the current in TCL is increased also by (N+1) , so its Ro was divided by (N+1). Adding T13 and T14 has then another function.

In small signal, it copies current variations in T5 to T11, and T11 copies it to the output with T12. One could then multiply currents by N and then by K in these two mirrors, and use a different supply (higher voltage?) at the output.

The problem is, as already pointed out, that the feedback back to T2 seems to be positive. If T12 sources current into the resistive divider the voltage there will rise, but if if does not then the node will fall to some voltage between the external ground (where ZL is) and VSS. consider TCL a short to ground , and notice that the drain of T5 rises and falls together with the output, to see that the connection
back to T2 makes the feedback positive.

What is the purpose of the circuit, in the patent?

fanrong

Joined: 20 Oct 2006
Posts: 126
Helped: 1

02 Feb 2007 1:33 Re: Why the tail resistor can increase the amp's stability

Firstly , without RTL means open .
If without RTL , the two differential input transistor and their tial current source
compose of voltage follower . If the current source's output resisotr are very
big , the drain 's current will not change when input changes . But for definity
Ro of the current source , the drain current will change as V/ro .
So ,the Gm is 1/ro .

But my question is why the writer of the patent said that the RTL will increase
the stability !

Btrend

Joined: 26 Dec 2003
Posts: 424
Helped: 55

02 Feb 2007 2:56 Re: Why the tail resistor can increase the amp's stability

fanrong wrote:

I'm sorry why "the two differential input transistor and their tial current source
compose of voltage follower " ? Can u explain it ?
It's a general differential input stage composed of 2 NMOS differential & 2 PMOS active load, so why this input differential pair is a source follower ?

n1cm0c

Joined: 05 Sep 2006
Posts: 183
Helped: 25
Location: Santa Maria

02 Feb 2007 10:28 Re: Why the tail resistor can increase the amp's stability

If RTL has a zero value, then it's a short circuit (zero resistance). Open would be infinite RTL.

Look up an explanation of a differential pair, bipolar or MOS, to see why the
the differential transconductance is gm, and how RTL reduces it.

xjxmn

Joined: 31 Jan 2007
Posts: 27

02 Feb 2007 14:30 Why the tail resistor can increase the amp's stability

I agree with n1cm0c, RTL range from zero to infinite, gm will decrease, and the main pole almost not changed with RTL， so the phase margin will increase, the system will be more stable.
But why not use miller compensation to stablize the OA. What's the advantage of this structure?

dingjingfeng

Joined: 12 Aug 2005
Posts: 196
Helped: 9

02 Feb 2007 16:11 Why the tail resistor can increase the amp's stability

I don't agree without RTL means open
usually, Compared with the impendence of the current source, the RTL should be much smaller and will generate a ground at the center of itself.
then half of the resistor works as a source degenarated resistor.

and a source follower is usually used as voltage level, and have no gain.
I strongly agree Btrend's idea

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Jim cage

Joined: 02 Jul 2006
Posts: 153
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03 Feb 2007 22:16 Re: Why the tail resistor can increase the amp's stability

my opinion is that this is an OTA with current multiplication to increase the Gm of the amplifier. The RTL resistor maintains a constant gm for the first stage (1/RTL) to withstand process and temperature variations.

jinnose

Joined: 24 Feb 2007
Posts: 20
Helped: 1

27 Feb 2007 9:14 Why the tail resistor can increase the amp's stability

To me, it looks like LDO. So T12 is the power mosfet. But, the configuration is wrong as everyone mentioned (positive feedback). T14 needs to be cascode configuration to support higher Vi than Vdd. Otherwise, T14 will be stressed.