Hi,
I tried to run basic common source amplifier ...
# sky130a
ALOK PRATAP SINGH
03/28/2023, 1:48 PMHi,
I tried to run basic common source amplifier simulation in xscheme. when I run ac sim, i get gain around 33dB which is around 50mv. When I try to provide 2m p-p sinusoid, I dont get 100mv p-p (get around 80mv - i get same gain irrespective of input ), can you find what would be the possible reason for the same. I am attaching the schematic and simulation result along with schematic setup for trans.
l
Larry Harris
03/28/2023, 2:59 PMWhat Frequency is the frequency bandwidth of your amplifier? Is the 2mVp-p sine wave frequency above the frequency bandwidth of your amplifier?
Larry Harris
03/28/2023, 3:01 PMAlso check the dc bias of the input transistor, Is the dc input voltage of the input transistor very close to the same voltage of the Vth of the input transistor?
s
Stefan Schippers
03/28/2023, 3:56 PMI have done the simulation on your circuit. I get 24dB gain (~16) and transient sim shows vout with 32mV pp, so it seems consistent.
Stefan Schippers
03/28/2023, 3:57 PMtransient sim
Stefan Schippers
03/28/2023, 3:57 PMschematic.
Stefan Schippers
03/28/2023, 4:27 PMYou can get higher gain if you tune the operating point such that Vout is close to 0.9V instead of 1.44V.
Stefan Schippers
03/28/2023, 4:31 PMlike this . I0=0.095mA, vout=0.99, gain=36dB
s
srinivasan_muthukrishnan19
03/29/2023, 3:52 AMits little bit confusing because , ac sim in the post shows 35 and you get around 28, really strange.
a
ALOK PRATAP SINGH
03/29/2023, 9:08 AM@Larry Harris bandwidth is greater than 10 MHz. I used 1 KHz, frequency is not an issue. In ac analysis gain is 33dB but in tranisient it comes to only 24dB. Even though stefen has used same circuit to simulate. he has taken a bit more current than the reference which I used. But still something is strange, I guess the design is very current sensitive. For variation of 10uA we see loss of gain around 10dB.
ALOK PRATAP SINGH
03/29/2023, 9:57 AM@Stefan Schippers I made an error in transient analysis. I kept reference in ac sim as 100uA and in transient as 110uA.
ALOK PRATAP SINGH
03/29/2023, 10:07 AMBut for variation of 1uA of current, the output resistance of pmos load M2 transistor changes by 5 times is quite surprising
s
SUMANTO KAR
03/29/2023, 11:10 AM@ALOK PRATAP SINGH You may try using the following netlist
Copy code
* c:\users\vikash\esim-workspace\cmos_opamp\cmos_opamp.cir
.lib "sky130_fd_pr/models/sky130.lib.spice" tt
* Gate bias
Rg 1 2 680
X1 3 2 5 5 sky130_fd_pr__pfet_01v8
Rd 3 4 100
* DC source for current measure
Vid 4 0 DC 0V
Vgb 5 1 DC 0V
Vdd 5 0 DC 3.3V
.control
* Sweep Vds from 0 to 1.8V
dc Vdd 0 1.8 0.01 Vgb 0 1.2 0.01
let vgs = V(5) - V(1)
let vds = V(5) - V(3)
* Sweep Vgs from 0 to 1.2V
dc Vgb 0 1.2 0.01
# Find threshold
let ih=Vid#branch[98]
let il=Vid#branch[85]
let vh=V(5)[98] - V(2)[98]
let vl=V(5)[98] - V(2)[85]
let vth=((vl - vh) / (ih - il)) * ih + vh
echo threshold voltage
print vth
quit
.endc
.end2 Views