So I'm currently doing an assignment where I'm meant to design my own common emitter. I'm supposed to choose the Q-point of my collector current and choose my own signal amplification (Av). During this design, I'm assuming (I_C=I_E, if I_B << I_C) and (I_R1=I_R2, if I_B << I_R1)
I'm using the hybrid-pi model.
I got the following value:
- Vcc = 22V
- Voltage over R_C: 11V
- Collector resistance: 3.7k ohm
- I_CQ (Q-point of collector): 3mA
- Small signal amp: Av = 6 times.
- Emitter resistance: R_E = -(R_C/|Av|) = 615 ohm
- Voltage across emitter resistance V_Re: 1.85V
- Voltage at base V_B = Vbe + V_Re = 0.7 + 1.85 = 2.55V
- I_R1 = I_R2 = I_C/10 = 3mA/10 = 0.3mA
- V_R2 = 2.55V (Same as V_B)
- R_2 = 8.5k ohm
- V_R1 = 19.45V
- R_1 = 65k ohm
- My voltage (V_CE) across the transistor is 9.15 V
When I'm going over to small-signal analysis, to calculate R_in and R_out. I need to know the base current I_b or current amplification beta/hfe, however, I know neither with makes me get stuck.
- r_pi = beta/gm
Circuit diagram in LTSpice: