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n-Type on the current flowing to the collector $C$ can be written as: $I_c = I_e = I_{e0} ext{exp}(V_{be}/V_T)$. The output current is zero for $V_b < V_{ON}$, where $V_{ON} hickapprox 0.7V$. For $V_b hickapprox V_{ON}$, the device is at the edge of the active region, and as $V_b$ is increased further, the transistor enters the active region where the current is exponentially dependent on $V_b$. Eventually, $V_b$ becomes high enough that the device enters the saturation region, where $V_{cb} hickapprox 0V$. The collector-emitter voltage $V_{ce} = V_c - V_e$ and for $V_e = 0V$, we have $V_{ce} = V_c = V_{cc} - R_c I_c$. The collector current at the edge of saturation is $I_{csat} = rac{V_{cc} - V_{cesat}}{R_c}$. So for $I_{csat} hickapprox rac{V_{cc}}{R_c}$, $V_b hickapprox V_{ON} + V_T ext{ln}(V_{cc}/R_cI_{e0})$. For a typical choice of $V_{cc} = 5V$, $R_c = 1k ext{ extOmega}$, and $I_{e0} = 10^{-15}A$, we have $V_{ON} hickapprox 0.7V$ and $V_{b,sat} hickapprox 0.82V$. In this range, the transconductance is $g_m = rac{dI_c}{dV_b} = rac{I_c}{V_T}$. This transconductance is very high, allowing the BJT to serve as a very good switch. However, there's a problem when the transistor is in saturation. As $V_{cb}$ goes from positive to negative, the collector-base junction becomes forward-biased and the collector starts emitting electrons into the base. These electrons then get collected by the emitter, leading to a large amount of excess charge (electrons) in the base. This excess charge must be removed (or recombined) before the transistor can be turned off, which can take quite a while (storage time). This delay makes the standard BJT switch relatively slow. This led to the development of better BJT-based switches. 1. **Explain the basic operation of a BJT as a switch.** (Include the cutoff, active, and saturation regions). 2. **Describe the main limitation of using a standard BJT as a switch in high-speed applications.** (Mention the storage time due to excess charge in the base during saturation). 3. **Identify and explain an improved BJT-based switch designed to overcome this limitation.** (Focus on the Schottky-clamped BJT). 4. **Illustrate your explanation with a schematic diagram of the improved switch.** (Description or ASCII representation of a Schottky-clamped transistor). 5. **Briefly describe the mechanism of the improved switch.** (How the Schottky diode prevents the transistor from entering deep saturation). 6. **Give an example of a logic family that utilizes this technology.** (TTL or specifically Schottky TTL like 74S or 74LS series). 7. **List one disadvantage of the improved switch compared to the standard BJT.** (Slightly higher $V_{CE(sat)}$). 8. **Conclude with a summary of the trade-off.** (Speed vs. saturation voltage).### BJT as a Switch: Limitations and Improvements In digital electronics, the Bipolar Junction Transistor (BJT) is often used as a switch, operating primarily in two states: **Cutoff** (OFF) and **Saturation** (ON). #### 1. Basic Operation * **Cutoff Region ($V_{BE} < V_{ON}$):** The base-emitter junction is not forward-biased. No current flows ($I_C hickapprox 0$), and the output voltage $V_{CE}$ is approximately $V_{CC}$. The switch is