PreLab VRE refers to the voltage across the emitter.
VB, VRE, IC, and VBE are calculated using the large scale model for a BJT
To calculate the rest of the table you will need the small signal model which has not been covered in class
The small scale model of a BJT is covered in section 7.2.2 of the text on page 478
To calculate gm and rπ you will need to use Equations 7.12 and 7.13 and VT on page 480 of the text.
The common emitter amplifier we are using is covered in section 7.4.1 of the text on page 491.
The circuit for the small scale model is Figure 7.13 (c) on page 492. You will need to use this to calculate AV and RIN. Ignore Ri
The effect of adding the emitter resistor is covered on page 495 of the text.
Lab: Remember to install the electrolytic capacitors with polarities as shown in the circuit diagrams
Once again you will need to come up with a method for measuring Rin for an AC signal
Notes on small signal (AC) analysis:
Figure 1. This is the circuit you are to analyze. The switch S1 is so we can analyze the circuit with and without capacitor CE.
Figure 2. This is the DC equivalent model for finding ICQ.
This part of the anlayisis is identical to what you have already done in the homework. Assume the diode is on, and use a
0.7 V drop across the diode.
Figure 3. This is the AC equivalent model. Note that the fuchsia-colored region is merely a more complete (rpi and ro account
for non-idealities of the transistor and diode) version of the DC forward active model for the transistor. You short the diode
because it will have a constant dc voltage – just like an independent voltage source.
This part of the anlaysis is covered in the sections noted above. Switch W1 connected when the capacitor is inserted. It
is off when the capacitor is not inserted.
Rin
Figure 4. Here, we have re-arranged the circuit to make it more clear how to find Vo.
Before you go any further, calculate the values for Rpi, and Ro based on ICQ, VA, and Vt. Note: Since Va =
infintiy, Ro = infinity. So, we can just replace it with an open circuit in our calculations.
When the emitter capacitor is inserted, RE gets shorted out, so Ve gets grounded. The output voltage then is
very easily found to be
Vo = -Vb*(gm*Vpi)*(Rc||RL); Vpi = Vb – Ve; Ve = 0; Vb = (R1||R2||Rpi)*VI/(RI + (R1||R2||Rpi));
Since Av = Vo/Vi, the gain is also found quite easily.
When the emitter capacitor is not inserted, you have to include RE in your calculations. This makes your
equations a bit more complicated, but the analysis is the same.
Vo = -Vb*(gm*Vpi)*(Rc||RL); Vpi = Vb – Ve; Ve = (Ib1 + gm*Vpi)/Re; Ib1 = Vpi/Rpi; (Vi - Vb)/Ri =
Vb/(R1||R2) + Ib1;
Again, once you get an expression for Vo in terms of Vi, the gain is easy to compute.