Question
For the circuit in Figure P14.33, (a) calculate the current that flows to ground through an input when a single input is at a logic low state. (b) Calculate the base current into Q2 when the output is in a logic low state.
Step 1
Look for resistors, transistors (Q1, Q2), and the power supply voltage. Note the values of the resistors and the configuration of the transistors. Show more…
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The three-terminal device circled in Fig. $\mathbf{P 2 6 . 8 1}$ is an NPN transistor. It works using three simple rules: I. The net current into the device is the same as the net current out of the device, so $I_{\mathrm{C}}+I_{\mathrm{B}}=I_{\mathrm{E}} .$ II. The potential at point $e$ is always $0.60 \mathrm{~V}$ less than the potential at point $b$. Thus, $V_{\text {in }}-V_{e}=0.60 \mathrm{~V}$. III. The current into point $c$ is always a fixed multiple of the current into point $b$, so $I_{\mathrm{C}}=\beta I_{\mathrm{B}},$ where $\beta \gg 1$ is a parameter characteristic of the transistor. Use these rules to answer the questions. (a) What is the potential $V_{\text {out }}$ in terms of $V_{\mathrm{in}}$, in the limit $\beta \rightarrow \infty$ ? (Hint: Determine $I_{E}$ in terms of $V_{\text {in }}$,and use this to determine $I_{\mathrm{C}}$ in terms of $V_{\mathrm{in}}$ and $\beta .$ The potential difference across the $1.0 \mathrm{k} \Omega$ resistor can then be used to determine $V_{\text {out }}$. Take the limit $\beta \rightarrow \infty$ in this result.) (b) What value of $V_{\text {in }}$ is needed so that $V_{\text {out }}=7.5 \mathrm{~V} ?(\mathrm{c})$ If the input potential includes a small time-dependent "signal," so that $V_{\text {in }}=15.0 \mathrm{~V}+v_{\text {in }}(t),$ then the output potential is $V_{\text {out }}=7.5$ $\mathrm{V}+G v_{\mathrm{in}}(t),$ where $G$ is a "gain" factor. What is $G$ for this circuit?
Given $I_{o}=2 \mathrm{mA}$ in the circuit in Fig. P2.104, find $I_{A}$.
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