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In TI's doc Designing With Logic, p. 16, it shows a method to calculate the internal resistance (\$R_{o}\$): using the open-circuit output voltage (\$V_{o}\$) and short-circuit current (\$I_{os}\$).

$$ R_{o} = \frac{V_{o}}{I_{os}} $$

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I can understand the method to get \$R_{o}\$, but i can't understand how get the 'open-circuit output voltage', if it's output floating, ideally there will no current flow in Q1 and Q2, where the voltage come from?

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  • \$\begingroup\$ Perhaps the C_load (CL) is what contains voltage, and is assumed charged at the point of the open circuit? \$\endgroup\$
    – KyranF
    Commented Oct 20, 2014 at 9:27

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Notice how at \$Io \gtrapprox 0\$, Vo drops sharply from Vcc down to 3.5V. This is the output voltage with just enough load to bias Q1+Q2 so \$ V_o = V_{cc} - R_1 \cdot I(R_1) - V_{be}(Q_1) - V_{be}(Q_2)\$ with \$I(R_1) = I_b(Q_1) \approx 0\$. That's a bit less than 5v - 0.7v - 0.7v = 3.6v.

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  • \$\begingroup\$ Yes, so it can't be 'open circuit' entirely, from the graph at right, Ios is not start from zero. \$\endgroup\$
    – diverger
    Commented Oct 21, 2014 at 9:46
  • \$\begingroup\$ @diverger: the axis should be labeled 'Io', which does equal 0 with a disconnected output. 'Ios' is the value where the plot crosses 0V ("Io, short circuit"). The plot is missing the leftmost bit at Vo=Vcc, Io=0; the author must have left it out because it's not relevant to the dV/dI=Ro linearization. \$\endgroup\$
    – arielCo
    Commented Oct 21, 2014 at 16:04
  • \$\begingroup\$ You mean the the point (Io = 0, Vo = Vcc) does exist? If the Io = 0, the output will float entirely, where the Vo come from? \$\endgroup\$
    – diverger
    Commented Oct 22, 2014 at 10:16
  • \$\begingroup\$ Vo will come from the transistor that drives the base of Q1. Think of the diode curve: with zero load, the B-E junctions of Q1, Q2 are at 0v, but any significant current will increase Vbe and Vo will drop, which is what you see at the left of the plot. \$\endgroup\$
    – arielCo
    Commented Oct 22, 2014 at 14:16

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