Problem with Ibias in MOS Op-Amp circuit

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haloo

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Current bias (urgent)

hi guys........

if i was using two stage MOS op-amp circuit....... and there was a part Ibias. what should i need to do with it? just add current source? i try to simulate it if using current course, but it doesnt give me the desired waveform.

can anyone help me? or show me the Ibias circuit?
 

Re: Current bias (urgent)

for simplicity, you can use a current source, but the circuit isn't work, well, can you upload the opamp?
or you can use a simple current mirror biased by a ideal current source
 

Re: Current bias (urgent)

here is the file.......... if i'm using the left side circuit, can you pls teach and show me the Ibias circuit? i'm new to this field and actually i'm not really understand what is I bias.
 

Current bias (urgent)

yes, there should be a current source. however, in order to make the circuit work properly, you need to figure out what's the correct value for that current.
 

Re: Current bias (urgent)

firstly, check the first stage, ensure MOSs is work properly
then, set (w/l)7/(w/l)5=2*(w/l)6/(w/l)4 to make output near midsupply
good luck
 

Re: Current bias (urgent)

that i bias circuit you can use either internal current or external current. for internal current you need to design the current sink circuit. for external current, you can supply the current externally with very precise resistor. however you need to select what value of current to drive the circuit. it is all depend on your characteristic of opamp that you desired. if you want really high gain, you can use high current because gain=gmro, gm is the transconductance of circuit while ro is the output impedance.

one more thing, you need to assure all the transistor operate in active region. i suspect your dc biasing might be wrong.
 

Current bias (urgent)

Just like "gingerjiang" said, i think your device size may not be correct.
But it's highly likely that you define wrong iBIAS definition in simulation. Even for a bad design, it may still working "sth" in transient simulation. If you really having nothing in waveform, then maybe your iBIAS definition is not correct.

ibias M8_drain VSS 10u (left diagram)
ibias VDD M8_drain 10u (right diagram)

it depends on which circuit you're talking about
 

Current bias (urgent)

lets say to the left diagram...... if i was replace the I bias with one MOS transistor and voltage to generate the currect bias, is there sth wrong with my circuit?
 

Re: Current bias (urgent)

haloo said:
lets say to the left diagram...... if i was replace the I bias with one MOS transistor and voltage to generate the currect bias, is there sth wrong with my circuit?
Click to expand...

i think you better use current sink to generate the current. you can use simple current mirror topology or you can use cascode structure for better design.
 

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