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For my homework I need to find the small signal gains of the circuit below.

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For the first part of the circuit the small signal gain is calculated with

$$A_{V1} = \frac{u_{a1}}{u_e}$$

and for all of the circuit it is calculated with:

$$A_{V2} = \frac{u_{a2}}{u_e}$$

I simulated the circuit in LTspice like this:

But when I measure the voltage at the \$u_{a1}\$ point, LTSpice, naturally, gives me a sine voltage like this:

I don't know much about this topic. Do I need to calculate the average value of this sine voltage(s) and divide it/them to \$u_e\$ to find the small signal gain(s)?

EDIT: The circuit that I simulated in LTSpice does not involve the \$R_L\$ resistor, I'm aware of it. That part of the experiment just does not involve that resistor. So there is no problem about that.

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  • \$\begingroup\$ Why are you showing the arrow directions downwards (unconventionally) towards 0 volts? \$\endgroup\$
    – Andy aka
    Commented Dec 2, 2022 at 12:00
  • \$\begingroup\$ @Andyaka if you are talking about the first picture (the schematic), that is from the homework. So my teacher drew it. Is it wrong? I always assume it is true. \$\endgroup\$
    – Efe
    Commented Dec 2, 2022 at 12:08
  • \$\begingroup\$ Does LTspice accept commas as decimal points? It doesn't on mine, but maybe it depends on language settings. \$\endgroup\$
    – GodJihyo
    Commented Dec 2, 2022 at 12:13
  • \$\begingroup\$ You might try setting the stop time to something much more than 200 us, to allow the capacitors and DC levels to stabilize. \$\endgroup\$
    – PStechPaul
    Commented Dec 2, 2022 at 22:03

1 Answer 1

Reset to default
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You can use an AC analysis.

Edit your signal voltage source, under 'small signal analysis' set AC Amplitude to 1.

Edit your simulation command. Select AC Analysis, Octave, 200 points per octave, start frequency 1 Hz, stop frequency 100 kHz. You can change the frequency range to suit your needs.

Run the simulation and plot the output of the circuit.

The plot will show gain in dB vs. frequency. To get the voltage you can right click the scale on the left side of the chart and change it to linear or logarithmic. Your gain will then be the plotted voltage divided by the input voltage which in this case is the 1 V that you set the AC Amplitude to.

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  • \$\begingroup\$ Thanks for your answer but I don't think that's what I need. Because in our lectures we always find a "V/V" value for small signal, not dB. Is that a different thing? \$\endgroup\$
    – Efe
    Commented Dec 2, 2022 at 12:22
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    \$\begingroup\$ The words "small signal" imply an .AC analysis. Sure, you could try to do it in .TRAN but, chances are you're looking for .AC. Just because the output will be in dB by default, nobody stops you from changing that into V/V. Also, I don't know how the locales influence your LTspice but, you could try 1.2k instead of 1,2k and see if that works. Otherwise LTspice will consider it as 1. \$\endgroup\$
    – a concerned citizen
    Commented Dec 2, 2022 at 14:12
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    \$\begingroup\$ @Efe It's just a different way of expressing it. See my update to the answer for how to get the voltage ratio. Also, the plot will show phase which can be hard to tell apart from the magnitude plot, you can right click the scale on the left side of the plot and click the 'don't plot phase' button in the pop-up dialog to hide it. \$\endgroup\$
    – GodJihyo
    Commented Dec 2, 2022 at 14:12
  • \$\begingroup\$ Thanks for your answer, again. I have one more question though. What if I try to calculate with the formula that I wrote in the question? I mean, right now LTspice is calculating the small signal gain, but what if I calculate? I guess there will be more than one \$A_V\$ value (as LTspice shows) since there is no DC voltage but sine AC voltage. Am I right? \$\endgroup\$
    – Efe
    Commented Dec 2, 2022 at 15:10
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    \$\begingroup\$ @Efe The overall gain will be the gain of of the first stage times the gain of the second stage. In this circuit the second stage voltage gain should be be close to 1 as it's an emitter follower. And yes, for small signal analysis you're looking at AC gain, you would look at DC when finding the operating point. \$\endgroup\$
    – GodJihyo
    Commented Dec 2, 2022 at 15:34

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