This exercise is about a BJT transistor common emitter circuit, with a biasing resistor between the base and the collector. I am supposed to compute the signal amplification of the circuit \$V_o/V_s\$ (\$V_o\$ being at node1). I am given that the transistor has \$\beta = 50\$ and \$r_{\pi} = 1.1\$ for the \$\pi\$ equivalent model. enter image description here

  • 1
    \$\begingroup\$ What work have you done so far? Also, your value of \$r_{\pi}\$ seems low. Is that supposed to be in k\$\Omega\$? \$\endgroup\$
    – Null
    Jan 24 '15 at 16:55
  • \$\begingroup\$ Yes I'm sorry it is. I have drown the equivalent circuit with the pi model i just don't know where to put Rb, as part of the input or output? \$\endgroup\$
    – user3401
    Jan 24 '15 at 16:59
  • \$\begingroup\$ Do you have some knowledge about negative feedback? \$\endgroup\$
    – LvW
    Jan 24 '15 at 17:02
  • \$\begingroup\$ not really, i am a second year student, just getting started with electronic \$\endgroup\$
    – user3401
    Jan 24 '15 at 17:03
  • \$\begingroup\$ General: Pi model would need dragging out of memory and dusting off but this is general approach: Assume Z_AC source = 0 for DC. For model used, establish DC operating point. Establish Rbe internal from current.Solve. \$\endgroup\$
    – Russell McMahon
    Jan 24 '15 at 20:44

There are, in principle, two ways for solving the task. Please understand that I do not intent to present the solution for you. However, I will try to give you some hints to find the solution by yourself.

1.) Application of the superposition theorem

Supplementing the transistor Pi-model with the external resistors results in a circuit which contains two sources: A voltage source Vin and the controlled current source of the Pi-model. Thus, after applying the superposition principle you get two equations for finding the two unknown quantities: Vo/Vin and Vbe. (Please note that the transconductance of the Pi-model can be expressed by the two known values for beta and Rpi).

2.) Application of the general gain formula for negative feedback

The closed-loop gain is Vo/Vin=Acl=AoHf/(1-AoHr). (Note that Ao will be negative).

Gain without feedback Ao=Vout/V(base) (simple gain formula for common emitter);

Forward (damping)factor Hf=V(base)/Vin for Vout=0;

Return (feedback) factor Hr=V(base)/Vout for Vin=0.

Both factors are calculated using simple voltage divider rules.

  • \$\begingroup\$ This transistor circuit exploits the same idea (parallel negative feedback) as the op-amp inverting amplifier. \$\endgroup\$ Aug 18 '17 at 6:04

You can take the Rb and connect it directly between the base and VC and denote it as Rb+Rc. Then the circuit simplifies a great deal and then you can use the two variable method specified by LvW.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.