+ Post New Thread
Results 1 to 6 of 6

24th March 2007, 00:16 #1
 Join Date
 Jul 2006
 Posts
 150
 Helped
 13 / 13
 Points
 3,097
 Level
 13
Common source with Ideal current source loadshort channel
Hi everybody,
Here is a question for you. suppose you Have an amplifier with common source configuration with ideal current source load.
The gain of this amplifier is therefore the intrinsic gain = gm*ro.
(assume this is a device with short channel lenght e.g., 0.18um).
Since the output resistance ,ro, is a function of Vds of the transistor (Channel length modulation, DIBL, hot electrons)...then the intrinsic gain is a function of Vds!
i.e., gain=gm*ro=f(Vds)
Assuming that you want gain of 20[V/V] this means that the output swing needs to be confined from one min voltage to output voltage.
The question: If the gain is changing with respect to the output swing, doesnt it means that the amplifier is "very very nonlinear"?
(assuming very small input signal which dont excite the nonlinearity due to the input).
Attached file is a figure from EE240 taught last year.
Regards,

Advertisement

24th March 2007, 02:37 #2
Re: Common source with Ideal current source load
yes, the gain is a function of output voltage, and this is called gainnonlinearity. this effect will cause distortion. one solution method is to reduce the gainnonlinearity, using longlength transistor and working the signal in linear region; the other method is to improve the dc gain of circuit to reduce this effect.
hope it's helpful

Advertisement

24th March 2007, 08:55 #3
 Join Date
 Jul 2006
 Posts
 150
 Helped
 13 / 13
 Points
 3,097
 Level
 13
Re: Common source with Ideal current source load
But usually when you deal with the nonlinearity of the common source it depends on the input signal:
Ids=k'(W/L)(Vgs,dc+vgs,acVt)^2
when you expand this relation you will get the coeffiecient for the nonlinearity.
But this doesnt depends on the output voltage or on the output resistance.
I've never seen calculation of nonlinearity of an amplifier where the nonlinearity due to output voltage is taken into accound due to the output resistance change.

Advertisement

24th March 2007, 10:13 #4
Re: Common source with Ideal current source load
surely, when you consider the current it's be affected in firstorder approximately, but the output voltage vout=i*ro is influenced.
in a closedloop amplifier, vout=vin*A(s)/(1+f*A(s)), where A(s)=A(vout)/(1+s/w) is amplifier openloop transfer function, the output voltage is gaindependent.

24th March 2007, 10:32 #5
 Join Date
 Jul 2006
 Posts
 150
 Helped
 13 / 13
 Points
 3,097
 Level
 13
Re: Common source with Ideal current source loadshort chann
Then why In calculation of CS nonlinearity you dont see the output resistance taken into account?
You can design theoretically high IIP3 amplifier with hand calculation but when you simulate it, the results will be a disaster because of the output resistance change vs. Vds (and not because of the VI nonlinear relationship!!)
Therefore, If this is true, the design procedure for linearity in amplifiers is very complicated (and the theory and literature is missleading) and the 1st order approximation (assuming constant rout) is irrelevant.
I'm attaching a paper by Razavi where he mentiones that the output resistance contributes to the nonlinearitys:
Razavi paper:
============
"Another troublesome effect is the output resistance of shortchannel
MOS transistors, and in particular its variation with
the drainsource voltage even in the saturation region. Shown
in Fig. 3, this phenomenon causes the intrinsic gain gM*ro to
depend on the output potential, thereby creating nonlinearity
in amplifiers."

Advertisement

24th March 2007, 11:40 #6
Re: Common source with Ideal current source loadshort chann
the gain is a function of output voltage A(vout), and when output voltage change, the output resistance change, so the output resistance influences the gain. in design, we set the output transistors' length be long in order to reduce the channel length modulation, and we also hold the output transistors in deep saturation to make the gain constant as possible. in general, make the Vds of output transistors twice the saturation voltage Vds,sat.
it's impossible to cancel the nonlinearity completely incured by output resistance, so how much cost we should pay to reduce the effect depends on the application.
+ Post New Thread
Please login