AC analysis: IAC or VAC

[analogpr]

Hello! I am new in analog design. I would like to know how can i choose between ac current source and ac voltage source when conducting ac analysis. why do we usually put 1A( or 1V) as an ac amplitude in ac analysis.
Thanks

 

[goldsmith]

Hello! I am new in analog design. I would like to know how can i choose between ac current source and ac voltage source when conducting ac analysis. why do we usually put 1A( or 1V) as an ac amplitude in ac analysis.
Thanks

Hi analogpr
So welcome to the analog circuit world which is pretty sweet !!!
Anyway , i think you mean the dead circuit isn't it ?
When we want draw thevenin or norton model , in circuits which have no independent current or voltage source and we just can see dependent sources and R or RLC elements , we can this circuit dead . so as you probably know every current source has infinite impedance and every voltage source has zero impedance . (DC sources in here ) but dependent current sources are not infinite Z and dependent voltage sources are not short circuit too , so we must be able to understand the real impedance of our circuit to mention it in norton or thevenin equivalent circuit , so we must investigate on behavior of circuit instead of real condition . so we can put real and independent sources in out put terminal of our circuits to understand the real impedance ( usually 1 volt or 1 ampere ) .
You said in AC circuit , so i think perhaps you're wrong with mentioning your question or perhaps i couldn't catch the right mean of your question anyway , it's because information of your question are so poor .
Any question ?

Best Wishes
Goldsmith

 

[crutschow]

Normally you are driving a relatively high impedance voltage input so you would use a voltage source (1V nominal for AC analysis). A current source will give an input voltage equal to the current times the input impedance of the circuit under test and, for typical input impedances, that can give a very high voltage input (perhaps in the thousands of volts).

 

[FvM]

Normally you are driving a relatively high impedance voltage input so you would use a voltage source (1V nominal for AC analysis). A current source will give an input voltage equal to the current times the input impedance of the circuit under test and, for typical input impedances, that can give a very high voltage input (perhaps in the thousands of volts).

In AC analysis, you don't worry about high voltage levels, the simulation is small signal by nature. You inject e.g. 1 A IAC and get a voltage corresponding to the impedance in ohms.

 

[LvW]

Hello! I am new in analog design. I would like to know how can i choose between ac current source and ac voltage source when conducting ac analysis. why do we usually put 1A( or 1V) as an ac amplitude in ac analysis.
Thanks

AC analyses are small-signal analyses which means: You are allowed to apply large voltage or current signals. Sounds contradictory?
No - it`s logical because all non-linear system characteristics are linearized and thus, large signals are treated as they were low signals (without distortion).
It is common to use 1V or 1A because in ths case, the simulaton results can be interpreted directly without building a ratio.
Example: Gain Vout/Vin. For Vin=1V the output voltage is identical to the wanted gain value.

 

[crutschow]

...........................................
It is common to use 1V or 1A because in ths case, the simulaton results can be interpreted directly without building a ratio.
Example: Gain Vout/Vin. For Vin=1V the output voltage is identical to the wanted gain value.

I just realized that in small-signal AC analysis all inputs are interpreted as volts even if you put in 1A. The suffix in this case is ignored.

 

[LvW]

I just realized that in small-signal AC analysis all inputs are interpreted as volts even if you put in 1A. The suffix in this case is ignored.

Currently, I am using PSpice and I am able to excite the circuits with Vac=1V or Iac=1A. Different ac sources are available.

 

[FvM]

I just realized that in small-signal AC analysis all inputs are interpreted as volts even if you put in 1A. The suffix in this case is ignored.

That's not specific to small-signal analysis. SPICE is evaluating scale suffixes (e.g. mega, k, m, u, n, f) and ignoring units because each quantity has an implicite unit.

 

[crutschow]

OK. I go back to my original statement. If you use a current source for the analysis you can get huge input voltages for typical amplifier input impedances. But as noted, that has no effect on the accuracy of AC (linear) analysis. It just may give a plot scale with impossible voltages for real circuits.

 

[LvW]

OK. I go back to my original statement. If you use a current source for the analysis you can get huge input voltages for typical amplifier input impedances. But as noted, that has no effect on the accuracy of AC (linear) analysis. It just may give a plot scale with impossible voltages for real circuits.

Yes - no doubt about this. However, for an ac current source of 1 A the result (the "impossible voltage") may be interpreted as a resistance with a realistic value.

 

[crutschow]

Yes - no doubt about this. However, for an ac current source of 1 A the result (the "impossible voltage") may be interpreted as a resistance with a realistic value.

That may be true for the input but not when an amp in the circuit gives an output with similar high voltages.

 

[LvW]

That may be true for the input but not when an amp in the circuit gives an output with similar high voltages.

Why not? In this case, the output-voltage-to-input-current ratio gives the transresistance of the circuit, correct?
This applies, for example, to all current conveyor circuits as well as current feedback amplifiers.