I've been doing some circuit analysis questions as practice but came across one that seemed very odd:


As far as I can tell, this question is missing information and therefore unsolvable? I was able to find Iload since I had all the information needed there. However, I am unable to find RB simply because I'm missing V (voltage for the base). As there are two unknowns there, the problem cannot be solved. Am I correct in this judgement, or am I not seeing something?


  • \$\begingroup\$ You are given Vbe. \$\endgroup\$
    – JRE
    Commented Oct 16, 2018 at 6:19
  • 1
    \$\begingroup\$ It is possible that the problem allows you to choose both \$V\$ and \$R_\text{B}\$? (I think their value for \$V_\text{BE}\$ is really the value for \$V_{\text{BE}_\text{SAT}}\$. So you know the base voltage (relative to ground), by definition.) \$\beta_\text{SAT}\$ isn't specified. But you may be allowed a reasonable guess for that, as well. But who knows? \$\endgroup\$
    – jonk
    Commented Oct 16, 2018 at 6:19
  • \$\begingroup\$ i don't think there is enough information. and \$V_{BE} = 0.95\$ V seems awful high. \$\endgroup\$ Commented Oct 16, 2018 at 6:45
  • \$\begingroup\$ The voltage measurements indicate the Collector voltage is well below the Base voltage, thus the transistor is operating in Saturation. You know the Base voltage, so derive a formula that depends on the unknown Vin. \$\endgroup\$ Commented Oct 17, 2018 at 6:31

1 Answer 1


You can’t come up with a numerical answer, however you can come up with an expression for Rb using the unknown voltage V and the forced beta (often Ic/Ib = 10), since you can easily calculate the collector current.


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