Zin of a circuit with independent sources

In summary, the conversation discusses how to calculate input impedance for a LNA circuit. One method suggested is to use the equation V1/Iin, but this still contains V1 and is not independent of it. Another method is to turn off all independent sources and use the equation s(Lg + Ls) + 1/(gg + sCgs) to calculate input impedance. The equation can be solved by replacing 's' with 2*pi*f*i and solving the resulting complex equation. The use of Kirchoff and the coherence of the two sources are also mentioned as potential methods for solving this problem.
  • #1
anhnha
181
1
Hi,
This is not a homework question. I need to calculate input impedance for a LNA and this is part of it.
How do you calculate input impedance, Zin, for the circuit below?
I tried by applying an input voltage, V1, and calculate Iin.
Input impedance will be V1/Iin. However, the expression still contains V1 and I can't figure out how to
make it independent on V1.

There is another method that I am not sure.
Input impedance will be equal to the impedance as all independent sources are turned off.
This means that Zin in the picture will be equal to s(Lg + Ls) + 1/(gg + sCgs)

gg is a conductance.

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  • #2
Your equation (s(Lg + Ls) + 1/(gg + sCgs)) is the impedance. Someone please confirm, but all you need to do is replace 's' with 2*pi*f*i where i is the imaginary number sqrt(-1) and f is the frequency in hertz. Then solve the resulting complex equation into real and imaginary parts.
 
  • #3
I don't know what the application could be but, assuming the two sources (V1 I1) are coherent, the input impedance (V12/i(in)) seen by V1 will depend upon the phase of the current I1. Have I read the circuit diagram right?

You could solve this using Kirchoff (as always) but there may be an easier way.
 

What is the Zin of a circuit with independent sources?

The Zin of a circuit with independent sources refers to the input impedance of the circuit, which is the total impedance that the sources see when connected to the circuit. It is a measure of how much the circuit will resist the flow of current from the sources.

How is Zin calculated in a circuit with independent sources?

Zin can be calculated by taking the ratio of the voltage across the input terminals to the current flowing into them. This can be done using Ohm's law, where Zin = Vin/Iin. It can also be calculated using Kirchhoff's laws and the circuit's transfer function.

Why is Zin important in circuit analysis?

Zin is important in circuit analysis because it helps us understand how the circuit will behave when connected to various sources. It also helps us determine the maximum power transfer and the stability of the circuit.

How do independent sources affect Zin?

Independent sources, such as voltage and current sources, greatly affect Zin as they are the driving forces in the circuit. The values of the sources will determine the overall impedance of the circuit and how it will behave.

What are the limitations of Zin in circuit analysis?

Zin is a useful tool for circuit analysis, but it has its limitations. It assumes that the circuit is linear, time-invariant, and has no mutual coupling between components. It also does not take into account the internal resistance of the sources, which can affect the overall impedance of the circuit.

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