• posted

I'm looking at problem 7, chapter 9 of T. Lee "The Design of CMOS Radio-frequency integrated circuits" regarding the input impedance of a capacitively-loaded source follower. Cgd and rg are assumed zero. DC biasing of the source is a constant-current sink so doesn't feature in the AC analysis. There is only Cgs and load capacitance CL. It asks over what range of load capacitance the real part of the input impedance is negative? Is this a trick question? I think it is for all CL:

Vg = Vgs + Vs

Vg = Ig (Ig + gm.Vgs) ------ + ------------- jw.Cgs jw.CL

Vg = Ig 1 Ig ------ + ----- (Ig + gm.------) jw.Cgs jw.CL jw.Cgs

Vg jw.CL + jw.Cgs + gm

-- = ------------------- Ig jw.CL . jw.Cgs

Vg -gm - jw(Cgs+CL)

-- = ---------------- Ig w^2 . Cgs . CL

View in a fixed pitch font.

• posted

Can you post the model they use?

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

They don't specify any model for the problem; but earlier in chapter 9 they use a simple model comprising only rg, Cgs, Cgd and current generator gm.Vgs which is what I'm using here.

• posted

Took 4 pages of scribbling (whew!), but I get...

Real(Zin) negative when

w > sq-rt((rg*D)/(gm*B))

Where

B = Cgs*CL

D = (w^4)*(A*Cgd+B)^2 + (w^2)*gm*Cgd

A = Cgs + CL

I like to do these exercises just to keep my loop/node analysis skills sharpened ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

Hello Andrew,

No it isn't a trick question. For a wide range of source capacitance, the input impedance shows a negative real part, which may cause parasitic oscillation. Normally spoken there are real parts in the transistor (that limits it's gain), these real parts limits the range of capacitance where the input impedance has negative real part.

The negative input impedance is widely used in common collector and common drain oscillators (where an inductance is between base or gate and the ground (collector or drain).

The common collector oscillator is my favorite when I quickly need some HF signal without high stability.

Best regards, Wim PA3DJS

when you remove abc, the address is correct.

• posted

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

Thanks Jim. Substituting the simplifying assumption rg=0, as stipulated by the text book problem, into your inequality yields w>0, which convinces me it was a trick question - in the nicest possible sense. This is typical of the style of humour which generally makes Mr Lee's book very readable; but did leave me slightly unsure in this instance.

• posted

Thanks Wim. This particular text book problem stipulates two simplifying assumptions which make real(Zin) negative for *all* load capacitances, so I think it is a trick question; but only with the intention of reinforcing the lesson in the memory of the student.

• posted

Yep, It was a lot of work to get to "reductio ad absurdum" ;-)

But it does teach plainly the need for resistors in series with power MOSFET gates.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
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We should ship Weasly Clark to Vietnam and let them keep him in a POW camp for five years to see how he likes it. Make sure the rice has bugs in it, and the water has plenty of crap in every glass. He would break in no time, and confess to every crime has has done and those he just thought about doing while he was in office.

```--
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If you have broadband, your ISP may have a NNTP news server included in```
• posted

It's been rumored for a long time, long before Obama-lackey Weasel Clark got into politics, that he was _asked_ to retire. I'll poke around and see what I can find.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

DC

he

er what

ve?

=A0 =A0 ...Jim Thompson

=A0 | =A0 =A0mens =A0 =A0 |

=A0 | =A0 =A0 et =A0 =A0 =A0|

=A0|

=A0 =A0 =A0 |

Well now I'm impressed. I figured you were useless without your computer. McCain should be so lucky!

-- Joe

• posted

I almost always doodle on paper _before_ entering the idea into simulation. I date way back to the times when simulators would produce nonsensical and non-physical results.

McCain is one mean-ass guy who has been run through "charm school"(*) for this contest. _If_ he attains the Presidency he's going to kick some ass and shake Washington up like it's never been shook before.

(*) Note the feigned laugh and smile before he answers any question? That's to regain composure so he doesn't bite off someone's head in public ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

What about Jackson? (You're old enough to remember Jackson, right? ;-) ) They say he had a mean temper at times.

Tim

```--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms```
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Some heads SHOULD be bitten off in public, spit down their throats, then kick the head around like a deflated soccer ball. (They do play Soccer down under, don't they?) In that case we need someone who isn't afraid to bare his teeth at idiots, liars and thieves. :)

```--
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If you have broadband, your ISP may have a NNTP news server included in```
• posted

Yes, He did ;-)

But I'm not quite that old... I date back only to the second Roosevelt.

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
• posted

Wim is exactly right. This is not a "trick", it happens with any C-loaded follower (bipolar or FET), though it is small enough that it isn't always a problem.

For one example, take a look at the Tektronix 465 input source-follower: why do you think that C18 and R18 are there?

-f

• posted

Frank, the text book problem stipulates "assume rg=0" and asks "over what range of load capacitance the real part of the input impedance is negative?" The point is it's negative for all load capacitances if rg=0. There is no range, unless you count C>0 as a range. The sign of the impedance is independent of CL.

• posted

what

McCain dates to times before paper. ;-)

```--
Paul Hovnanian	paul@hovnanian.com
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• posted

[snip]

Ho! Ho! Ho!

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
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