I have a specific question regarding this circuit: The circuit in question

I need to calculate the gain of this amplifier. However, when I do this, I get the value two times bigger than the simulation of this circuit, which I run in Multisim, suggests.

My line of thought is this: The gain should be equal to R1/((re1+R2)+(R4||(R3+re2)), where re1 and re2 are the thermal resistances of the corresponding transistors.

Calculaing gives me the value of approximately 6.2, when the simulation suggests that the gain is 3.3. I have double-checked and the simulation circuit has no errors in it.

Thanks for your help!

  • \$\begingroup\$ hFE only 20? try 200. did U choose f so Zc1=~0 \$\endgroup\$ Oct 17, 2016 at 22:54
  • \$\begingroup\$ @Tony Stewart. EE since '75 My source frequency is 1GHz, it's in the range where resistncae in negligible. I can't change the hFE values, as the task I'm given specifically states I should use 20. Even if I do, the change is just a small fraction of the result. \$\endgroup\$
    – Mu3
    Oct 17, 2016 at 23:06
  • \$\begingroup\$ ok I never expected 1GHz with 1uF due to ESL \$\endgroup\$ Oct 17, 2016 at 23:34
  • \$\begingroup\$ With R4=1K it works for me at low f with a gain of 4. But you never specified R4 \$\endgroup\$ Oct 18, 2016 at 0:19
  • \$\begingroup\$ R4 is 129 Ohms. Frankly, the frequency is not given either. I just assumed 1GHz to eliminate the capacitive resistances. \$\endgroup\$
    – Mu3
    Oct 18, 2016 at 13:49

1 Answer 1


Obviously, since the hFE of the transistor is 20, you cannot expect a large gain. You wrongly deduced that the gain is R1/((re1+R2)+(R4||(R3+re2)), which you could have formatted in a proper manner. But I am digressing. The gain, in fact, is R1/((re1+R4)+(R2||(R3+re2)). If you take a look at it from the right side of the base, it becomes painfully obvious.

The next point is your comment: ' I just assumed 1GHz to eliminate the capacitive resistances.'. You are an engineer - Think like one. Just 'assuming' and wildly guessing a number because you heard somewhere that high frequencies eliminate 'capacitive resistances' (which are capactive reactances) is no way to model a system. It is clear that you have no idea what is actually going on in the circuit, and little to none understanding of terminology.

Picking 517.3MHz will give you much better results. As for why - I will leave that to you to figure out. This brings us to the final conclusion - Implement the aforementioned changes and you will see a gain of 6.2.

And as a sidenote - Begging for homework answers is not a good way to become an engineer. I suggest you stop now, before you become dependent on others for as simple things as calculating circuit gains.

  • 6
    \$\begingroup\$ Answers on this site are expected to be generally useful to readers, not just the OP. While it may be fun for you to include snide remarks such as "painfully obvious", leave readers guessing with the value of 517.3 MHz, and criticize "begging for homework answers", they don't actually make this a helpful answer to the community at large. If you instead explained your rationale and derivation, this would be a much more useful answer. With that said, the question is also from 2016, so you're >4 years late on calling the OP out for their mistake and terminology. \$\endgroup\$
    – nanofarad
    Jan 5, 2021 at 20:05
  • 4
    \$\begingroup\$ The only thing worse than asking for homework solutions on this site is providing homework solutions. \$\endgroup\$ Jan 5, 2021 at 21:12

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