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Voltage Sources (one or two source is needed to power up the circuit)?

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ehsanaiman

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Hello,

I want to ask about voltage source to power up two-stage amplifier. Currently im simulating the two stage amplifier using 90nm technology in Hspice and i found different gains produced by the same circuit if I change the number of voltage source from one into two, from single source 1.2V connected to VDD & ground into two source +/- 0.6V connected to VDD and VSS).

The circuit i try to simulate is like in the picture acttached.
feedbackaccommon.PNG

Can someone explain the theory behind single voltage source or two voltage source? do I need to use 1.2V single source or i need to use two voltage source +/- 0.6V to power up the circuit?
 

Could you be more clear? How are you connecting the 1.2V supply
 

I connect the single 1.2V source just like in picture attached.
feedbackaccommon.PNG
and i get different gain from configuration two voltage source.
 

The differential amplifier input must be biased within the suitable common mode range. Your AC sources don't include a DC offset, unless the amplifier common mode range is including negative rail, your single supply setup can't work.

You should know the common mode range and design the test circuit respectively.


B.t.w. why are you applying same magnitude and phase to the differential inputs? Is this a common mode rejection measurement?
 

Hello FvM, thank you. So , it means that if some amplifier in paper said that the voltage source used is 1.2V, they used +/0.6V right? not just one 1.2V Vsource right? I ve been meaning to ask this question because the amplifier schematic doesnt put VDD and VSS in the circuit which VDD connected to +0.6V and VSS connected to -0.6V right?

Yeah, supposedly that is CMRR measurement. By the way, if i am applying same magnitude and different phase to differential inputs, does it counted as DC gain measurement?
 

Hi,

It's just a question where the voltages (in paper as well as in real circuit) are referenced to.
A usual 5 pin OPAMP has no dedicated GND.
Thus you are free to use +0.6V / -0.6V or +1.2V / 0V or any other combination where (VDD - VSS ) = 1.2V. --> The OPAMP will never know.

Within an ASIC for sure you have limitations in supply voltage, but the result is the same (within these limits).

When the paper talks about "single power supply" then it will have specified the Common_mode_input_voltage_range in the same manner.
When the paper talks about "dual power supply" then it will have specified the Common_mode_input_voltage_range in the same manner.

--> Read the documentation carefully.

Klaus
 

Hi Klaus, thank you for very complete answers to my questions, I get it, single or dual supply considered same in opamp.
 

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