### DC Transfer function

The LTspice DC transfer function analysis calculates the low frequency gain and input impedance & output impedance of a circuit.

- Gain =output voltage/input voltage,
- Input impedance of a circuit=input voltage/input current,
- Output impedance of a circuit=output voltage/output current.

Let’s have a look at LTspice DC Transfer function analysis with the help of a simple voltage divider circuit. See the screen shot below.

The voltage divider circuit consists of a voltage source and two resistors like 1k and 2k. The two resistors are in series and they are in parallel with voltage source. Similarly, the voltage divider circuit divides the voltage between the two resistors depending on the value of resistors. Here the values are as follows.

- Input voltage source V1=10v
- Resistance R1= 1k
- Resistance R2=2k

in addition, the output voltage is taken at the node between resistance R1 and resistance R2 . The output voltage is marked as ‘out’ in the above figure.

So, let’s see how do we find the transfer function, input impedance and output impedance of an above circuit.

DC Transfer function = output voltage/input voltage.

Similarly, output voltage = input voltage*Resistance R2 / Total resistance.

therefore, v(out)=V1*R2/R1+R2

=10*2K/3 K

=6.66666V.

DC Transfer function = v(out)/Vin, 6.66666v/10v = 0.66666.

Input impedance of a circuit Z1 = V1/I1

V1 = 10v

I1 = V1/R1+R2, 10v/3k = 3.3ma .There fore Z1 = 1ov/3.3ma = 3k

Similarly, we can find output impedance Z2 = 666.667ohm.

In order to do this in LT spice, first click on the simulate button and then click on the edit simulation command. You will get a pop window. It is shown in the above figure. Now, click on the DC transfer and insert the values like output=V(out),source = V1. After entering the values, click on the run button. Finally, the obtained result is shown below.

The obtained result is same as theoretical result.

Finally, I hope this article helped you to learn LTSpice better. Let us know your likes and dislikes in the form of comments.

Happy reading…

Hi

such an knowledgeable and informative article thanks for sharing

keep it up for the good work

wonderful post. glad to read it. keep it rocking up !!

I disagree to your statement, “Output impedance of a circuit=output voltage/output current”. The simulated result of the output impedance does not tally with the statement.