Author Topic: Small signal analysis when base is grounded ?  (Read 743 times)

0 Members and 1 Guest are viewing this topic.

Offline lordvader88Topic starter

  • Frequent Contributor
  • **
  • !
  • Posts: 935
  • Country: ca
Small signal analysis when base is grounded ?
« on: April 29, 2019, 07:17:10 pm »
I finally learned the basics of the hybrid T-model for BJT AC signals, so that's great. Say u have a BJT voltage reg. circuit, where the AC ripple from rectifier/filtering, is on the source voltages of everything. And have cap'a on the bases of BJT's. So ignoring the actual reactance of the cap, if u do small signal analysis with the model, u short the base. So there is no ac base current? Even tho I can see the voltage ripple. So it's common-mode,

Also in for instance a Colpitt's oscillator, where let's say a parallel tank circuit has C1 between B-E and C2 between E-gnd. Also the DC bias across the tank is usually always way way more than uV and mV considered in small signal analysis. So am I missing something or is this model is not for this case at all ?


Also I forget how I once did the loop gain of this type of oscillator, and I can't find the video, so I have to learn that I know. But in that analysis I was using the actual reactances, and I guess just the large scale BJT model ?


I have made a model in LTspice of my broken Tektronix 422 PSU, and it works quite well.

What of the small signal analysis can be applied to this ? Any where a BJT does't have a cap on it's base ?

Or should I be able to work out all relevant stuff from doing a massive mesh/node analysis with large scale BJT model, with common-mode AC ignored, is that right ?
« Last Edit: April 29, 2019, 07:26:44 pm by lordvader88 »
 

Offline lordvader88Topic starter

  • Frequent Contributor
  • **
  • !
  • Posts: 935
  • Country: ca
Re: Small signal analysis when base is grounded ?
« Reply #1 on: April 29, 2019, 07:40:33 pm »
https://www.seas.upenn.edu/~ese319/Lecture_Notes/Lec_19_Colpitts_Osc_09.pdf

Time for another go this then

Ok and I'm realizing I never used node/mesh analysis with complex numbers and impedance that way. So now I realize I should use that on big and small BJT models. So I'll try that and see if it matches with LTspice.

I'm forgetting what I know and don't know
« Last Edit: April 29, 2019, 11:49:03 pm by lordvader88 »
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf