Active load Common Source Amp. design using Gm/id

common source amplifier active load

Hi All,

Any one one this design before using Gm/id. Please share your insights.

i got struck as my calculated vs simulated results are not matching.

Thanks,

common source amplifier with active load

There aren't any problems with using of Gm/Id respective to CS amp. You just need to select desired Gm/Id value and find Id and (W/L) from Gm/Id vs Id/(W/L) curve.
Difficulties come from trade-offs between circuit parameters: noise and mismatch fix W*L and region of desired Gm/Id (low Gm/Id for current processing, current mirrors; high Gm/Id for voltage processing, diff.pairs), gain requires high Gm/Id and long L, bandwidth - low Gm/Id and short L. Resonable head room (Vdsat, Vov=Vgs-Vth) also can limit the region of Gm/Id selection.

Your calculated and simulated results don't match? It's depend from data which you use for calculation and how it was ontained. Also simulation results can be obtained differently. May be the reason in simulation setup.
In any way nothing guaranties that this matching will be ideal.
Can you describe your problem more precise?

active load

what kinds of noise should be considered?

common source amplifier gain

Hi Denis,

Thanks,

here is my design spec.

Prcess :180nm
type : CS amp with active load
GBW= 50meg, Cload=1p, gain=~5, vdd=3.3

I have chosen (gm/id)nmos =10 corresponding id/w = 3.229 both values are for L=1u, Early voltage =71.3

(gm/id)pmos = 5 corresponding id/w=2.53 both values are for L=1u, early voltage=68

i calculated operating current frm this equation

GBW=(gm/id) x (id/2 x pi x Cload) and its value is 31u


but my simulated results are

gain=10, 3db bw=19meg, Id=48u.


as per the early voltage values my gain shld be around 340 (gm/id)nmos x vea_eff


please correct me.

othr question,
1. how to choose gm/ids for nmos & pmos connected in series, As they have different mobility but will have same current.


Please let me know if i m not clear


Thanks,

common source gm

I think GBW=gm/Cload

common source amplifier dc analysis

GBW=gm/(2pi*Cload) and you will get the value of gm. Pick a value for gm/id like
10, the id=gm/10 for strong inversion. Do the rest by simulation like Denis said.

active load circuit

Ok. The order proposed by yschuang is more correct. You have to choose Gm at the first from your GBW requirement. This gives your Gm=314uS and Id=31.4uA at Gm/Id=10(nmos). Than you are able to calculate pmos.
If your target gain is ~5 this will mean than after all you need to stabilize it with negative feedback or to use pmos or nmos in diode connection as load inctead active load [Gain=(Gm/Id)nmos/(Gm/Id)pmos].

Why simulations are different:
1) Do you use pmos current mirror circuit for active load? [Some guys just put gate-source voltage source what is incorect]
2) Do you stabilize DC operation point of your CS stage? Do you sure that devices don't fall in triode region? From this point the vcvs is useful element with gain of 1k. Connect it's inputs: plus to CS output, minus to VDD/2; outputs: plus to input of CS stage trough high time constant RC circuit before AC source, minus to ground. So you made negative feedback which will stabilize DC operation point of your CS stage.

And take a notice that equation for GBW doesn't take into account intrinsic capacitances of devices. And nothing guaranties that early voltages match well to real vds values.

cs amplifier with active load

There you go. Thanks for an excellent explanation by Denis. Follow steps like Denis did.