common-emitter-amplifier: decreasing gain

[rudiri]

Hi,

I have a question to the simple common emitter amplifier with current source load as in the picture. I thought the voltage gain should be in this case "A=gm/go" which is around 2100, but its only 330. For small currents this formula seems to be correct, but it's getting more worse the higher the current. Can somebody explain me please the reason for this effect?

Thanks

 

[dick_freebird]

Your output impedance is roughly the Early voltage divided by
collector current (V/A) so more current brings down the Zout
and gain along with it. Leaving aside things like emitter L, R which
will also steal gain and may be outside the model scope, and
predriver impedances which are divided against Rb @ OP.

 

[rudiri]

Thanks for help,

but the transconductance (gm) increases in the same way as Zout decrease, so this should be independent from the current (A=-gm*Zout=-(Ic/Vt)*(Va/Ic)). The driver is in my case ideal and has an infinite output resistance.

I think the parasitic effects should not have that much influence, but maybe I'm wrong. Please tell me.

 

[FvM]

If gm and g0 are completely describing your transistor, the low frequency gain would around 2000. Apparently this prerequisite doesn't come true. Without further information about the involved transistor model and the evidence of the said g0 value, how can we know why?

 

[dedalus]

Hi rudiri,

Output resistance ro must be positive and thus go (1/ro) must be positive too, while in you screenshot it's negative. Are you sure that go is "output conductance"?

From the result of simulation (gain = 330) it follows that Early voltage is around 9 V (Va = gain*VT = 330*26mV). Does this value correspond to data from your Process Specification?

 

[rudiri]

Hi, thanks for help

from the process specification, the minimum early voltage is 40V

I added the model-parameters and the small-signal-parameters for this biasing as attachment.
I didn't find a description about the parameter names, so I had to guess about their meanings.
I agree with you, that "go" should be positive. I'm still a student and new in this field of engineering, so I don't know exactly what cadence does, when it is doing the small signal analysis.

Hope for further help in this topic

small-signal-parameters:
signal OP("/Q0" "??")

betaac 156.3
betadc 169.8
cbcp 3.823f
cbcx 1.163e-27
cbep 2.305f
cbex 0
cjc 710.6a
cje 22.31f
cmu 710.6a
cpi 22.31f
dib_dvbc 776.5p
dib_dvbe 108.5u
dic_dvbc -8.212u
dic_dvbe 16.97m
ft 93.09G
gm 16.96m
gmu 69.42a
go -8.211u
gpi 108.5u
ib 2.944u
ibc -2.607a
ibcp -17.99f
ibe 2.944u
ibep -48.53f
ibex 0
ic 500u
icc 500u
iccp 35.48y
igc 38.78p
ith 469.7u
pwr 469.7u
rbi 51.19
rbp 818.9m
rci 73.5
region 1
temp 29.35
type 0
vbc -71.07m
vbe 863.2m
vce 934.3m
vcs 934.3m

model-parameters:

signal MP("/Q0" "??")

afn 1
ajc -500m
aje -500m
ajs -500m
alarm 4
art 100m
avc1 4.6
avc2 13.41
bfn 1
bvbc 5
bvbe 2.5
bvce 2.5
bvsub 20
cbco 1f
cbeo 0
ccso 0
cjc 716.1a
cjcp 4.403f
cje 4.3f
cjep 2.354f
compatible 0
cth 0
dear 0
dskip 1
dtemp 0
dtmax 226.9
dtmp 0
ea 1.09
eaic 1.12
eaie 1.169
eais 1.12
eanc 1.12
eane 1.12
eans 1.12
eap 1.12
ebbe 0
fc 900m
gamm 13.39f
hrcf 1K
ibbe 1u
ibci 3.398a
ibcip 13.15f
ibcn 100y
ibcnp 1.511z
ibei 46.02z
ibeip 2.704a
iben 6.374f
ibenp 72.92f
ikf 6.31m
ikp 854.6u
ikr 2.218m
imax NaN
imax1 NaN
imelt NaN
is 16.52a
isp 284y
isrr 1
itf 350m
kfn 0
mc 261.1m
me 141.6m
minr NaN
ms 519.2m
mvt0 0
nbbe 1
nci 1.115
ncip 1.096
ncn 2.671
ncnp 980.2m
nei 1.041
nen 3.428
nf 1.058
nfp 952.2m
nkf 500m
nr 1.047
pc 670.3m
pe 1.178
ps 3
qbm 0
qco 33f
qtf 2
rbi 64
rbp 800m
rbx 67.2
rci 71.74
rcx 20.67
re 19.2
rev 0
rs 1m
rth 5K
selft 1
tavc -831.8u
td NaN
tf 300f
tnbbe 0
tnf 20.58u
tnom 27
tr 631f
trise 0
tvbbe1 0
tvbbe2 0
type 0
vbbe 0
vbcfwd 200m
vbefwd 200m
vef 45.27
ver 5.257
vers 1.15
version 1.15
vo 1.486
vrev 0
vrt 0
vsubfwd 200m
vtf 100
wbe 1
wsp 1
xii 992.5m
xikf 0
xin 3
xis 2.653
xisr 0
xrb 316m
xrbi 316m
xrbp 0
xrbx 0
xrc 3
xrci 3
xrcx 0
xre -1.07
xrs 0
xtf 208.9
xvo 1

 

[rudiri]

I forgot to tell the model name. It's VBIC(Rev.1.15).

Can someone tell me how to calculate the intrinsic gain from the small signal parameters in this operating point. I tried to do it as I told before, but it didn't mach with the simulation results. All the values should be given in my last post.

Thanks