# small-signal analysis

#### zdzislavv

Joined Jun 5, 2009
22
Hello!
Here there is what I want to solve: http://images38.fotosik.pl/135/35e73fda57d25875.jpg
I found some informations on Q-point stabilization: http://images43.fotosik.pl/139/09ebd31e0c12d268.jpg
And finally I tried to draw scheme with substituting transistors to models (but some of wires are connected to both transistors - and I don't know what to do with them, e.g. Rc and Rb): http://images49.fotosik.pl/139/c2964a1dbbc28db2.jpg
I want to solve the whole exercise but the most important part for me is small-signal analysis. May you help me with drawing the proper circuit and give some advices so that I can try to finish this exercise?
Thank you very much in advance
Greetings!

#### Jony130

Joined Feb 17, 2009
4,927
If I am not-mistaken the correct small-signal schematics should look like this.
And good luck with the Q-point calculation, And remember that current flow through RB must be equal I=0.6V/Rb

I nie za trudne te zadanie dla ciebie?

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#### zdzislavv

Joined Jun 5, 2009
22
Thanks very much, Jony130 .
I see that capacitors are shorted - that's right. But shouldn't Rg be connected directly to G (gate) of JFET, not through Rb? In Your schematics it looks like there is connection with no resistors between D (drain) of JFET and emitter of BJT. Shouldn't it be rather connection between D and collector through Rc? And what about diode, Rz and Re?
Greetings!

#### Jony130

Joined Feb 17, 2009
4,927
Rg is directly connected to gate. Look at Uge voltage arrow.
For AC signal, Drain, Rc are shorted by rz=0Ω (nie mylić z "rz" ) of Zener diode, and emitter is short by Ce.

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#### zdzislavv

Joined Jun 5, 2009
22
http://images39.fotosik.pl/147/266ced692a1e4832.jpg

Thanks! I've got two questions. First one - why is this Rc placed here? In your schematic it is between C and E, ok. And there is just wire between D and E (they are the same point). In other words D-C is the same as E-C so Rc is placed in proper way between drain and collector. So I know why it is placed in this way - but what for? Wouldn't it be more natural to draw it in this way: http://images44.fotosik.pl/151/de2e3e90b40662a9.jpg?

The other question is connected with rather basic knowledge about circuit theory. Why cannot I replace diode with this voltage source as shown in my schematic (second photo)?

Greetings!

#### Jony130

Joined Feb 17, 2009
4,927
Wouldn't it be more natural to draw it in this way: http://images44.fotosik.pl/151/de2e3e90b40662a9.jpg?
Well maybe, but Zener diode has rz=0Ω and that means that Drain and upper side of Rc is short to the GND.

The other question is connected with rather basic knowledge about circuit theory. Why cannot I replace diode with this voltage source as shown in my schematic (second photo)?
The same question you can ask about the BJT and base to emitter junction and Vbe=0.7V.
http://www.edw.com.pl/pdf/k01/35_05.pdf
http://www.edw.com.pl/pdf/k01/37_02.pdf
http://www.edw.com.pl/pdf/k01/18_12.pdf