I'm trying to solve a question that says to find \$gm\$ and \$R_c\$, considering a gain of 50 using a BC547b transistor. But I can't find a way to calculate these parameters with the information I have. Could someone help me?

The BC547 model:


  • BR=10 NC=2 ISC=47P IKR=12M VAR=10
  • RB=280 RE=1 RC=40 TR=.3U
  • CJE=12P VJE=.48 MJE=0.5 CJC=6P VJC=.7 MJC=.33 TF=.5N)

So, because of that I'm considering Va = 50V, Is = 20e-15 and Beta = 530. Right?

The circuit of this question:



Consider worst case hFE not max do it might be hFE=200 for loading effects on base bias.

  • Rc~50*Re
  • Rin~hFe(min)*Re
  • Ic=10V/(50+1)* Re. *50%
  • Vb~0.7V+ Ve(=10V/51=0.2V)=0.9V
  • choose base bias with Rin loading effects for 0.9V from 10V
  • \$\begingroup\$ Tony Stewart: with Rc=50Re=5k and Ic=10V/50Re we have 10V=Vcc across Rc. Instead, we should select Ic=1mA instead of Ic=2mA, don`t you think so? \$\endgroup\$ – LvW Oct 18 '20 at 9:29
  • \$\begingroup\$ Yes sorry Only 5V across Rc to allow AC swing , but 4.5V is better to avoid Vce(sat), and that increases to 2V partial saturationfor high current when Rc=100 and Re=2. Due to variation in hFE effects on THD \$\endgroup\$ – Tony Stewart EE75 Oct 18 '20 at 9:50
  • \$\begingroup\$ Better design for symmetrical swing is to use Av =>100 then use Rcb/Rsb(series to base) and achieve gain of 50~ with high hFE and full swing9.9V. (1% less due to Av). Unlike OA’s hFE is not high enuf to make gain= Rcb/Rs. Include 10R in source impedance. Negative feedback is always better than open loop with just Emit. Dej. \$\endgroup\$ – Tony Stewart EE75 Oct 18 '20 at 10:22

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