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I am doing an assignment which has asked me to find the effect of varying \$V_{EB}\$ on paramaters such as voltage gain , current gain and input resistance for the following circuit : enter image description here

I attempted this question by hooking up a voltage source like this : enter image description here

But when i run the circuit , it shows "Failed to find DC Operating Point for AC analysis" .

Why it's showing such an error and how do i get to plot \$V_{EB}\$ along the x-axis ?

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  • \$\begingroup\$ Do you have to use this specific circuit? You may change Vbe @ DC operating point by changing any of the resistors or V2, but this would also change Ib, Ic and Vce, so you would not be evaluating the change in Vbe exclusively. \$\endgroup\$
    – devnull
    Commented Mar 7, 2021 at 14:40
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    \$\begingroup\$ @Starboy: What would happen in reality if you connected a battery to the base and emitter as you have V3 connected? \$\endgroup\$
    – JRE
    Commented Mar 7, 2021 at 15:03
  • \$\begingroup\$ @JRE It would burn the circuit due to a high value of current without a resistance in place ? \$\endgroup\$
    – Starboy
    Commented Mar 7, 2021 at 15:15
  • \$\begingroup\$ Right. What ever you are trying to do, destroying transistors won't get you there. \$\endgroup\$
    – JRE
    Commented Mar 7, 2021 at 15:34
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    \$\begingroup\$ @Starboy It's called sensitivity analysis, I think. Once you understand the idea, it's really easy to apply. You don't need Spice for this. It's pure math. \$\endgroup\$
    – jonk
    Commented Mar 7, 2021 at 16:29

1 Answer 1

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For the x-axis, select the waveform window, hover the mouse over the X-axis until the cursor changes into a ruler, then RClick on it and change time to V(Ve,Vb), or V(Ve)-V(Vb) (same thing). It may not work since you seem to have assigned two labels to the same node. LTspice will take precedence to the last assigned name (except for the ground).

For changing the BE voltage, you'll have to either change the parameters of the transistor .model -- I doubt this is what's needed -- or change the resistor network such that the DC operating point changes. Since this is an assignment, I'll leave that up to you. Is C1 meant to be tied to ground, or was there supposed to be a load afterwards? I ask because your source is modelled as a sine.

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  • \$\begingroup\$ to vary the DC Bias point , I have varied R1 from 1k to 1000k in steps of 1k. Then did the AC analysis at 1kHZ. However , the option to change "R1" is greyed out . Yeah , C1 is meant to be tied to the ground. \$\endgroup\$
    – Starboy
    Commented Mar 7, 2021 at 15:31
  • \$\begingroup\$ Yes , I have removed V3 , and the circuit is same as the first image except that I have varied R1 and removed "Iin" \$\endgroup\$
    – Starboy
    Commented Mar 7, 2021 at 15:34
  • \$\begingroup\$ Do you need to find out the DC characteristics, or a frequency analysis? Think well, because for the 1st you don't need any SINE() input or capacitors and an .OP simulation is what you're looking for, and for the second you'd need a load after C1, yet you say it needs to be tied to ground (i.e. it shorts out RE at high frequencies) I don;t understand what you mean by "R1 is greyed out". \$\endgroup\$ Commented Mar 7, 2021 at 15:44

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