For an assignment I was given a problem which consists of two BJT transistors, a PNP and an NPN, in series with their bases tied together.


simulate this circuit – Schematic created using CircuitLab

The question is to find the voltage at each node. Additionally we are given the fact that \$\beta\$ of Q1 is 70 and \$\beta\$ of Q2 is 120. Q1 has an early voltage of 80, and Q2 has an early voltage of 100. Both transistors have a \$V_{eb}\$ or \$V_{be}\$ drop of 0.7 V, respectively. I tried solving using the simple models of PNP and NPN transistors but I keep getting current-controlled current sources in series. I don't understand what the current through each collector would be given that the bases share the same current. Is there a reasonable way to solve this that I am missing?

  • \$\begingroup\$ Obviously, the collector currents have to be the same, the base currents have to be the same, and the emitter currents are going to be the same. Therefore, one of the transistors is in saturation, which means that its effective gain is less than its "natural" value. \$\endgroup\$ – Dave Tweed Dec 15 '18 at 23:15
  • 1
    \$\begingroup\$ given that R1 is only 5 times bigger than R2 both transistors will be in saturation. \$\endgroup\$ – Jasen Dec 15 '18 at 23:28

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