For the common-base configuration of Fig. 18, an ac signal of 10 mV is applied, resulting in an

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For the common-base configuration of Fig. 18, an ac signal...

For the common-base configuration of Fig. $18,$ an ac signal of $10 \mathrm{mV}$ is applied, resulting in an ac emitter current of $0.5 \mathrm{mA}$. If $\alpha=0.980$, determine: a. $Z_{i}$ b. $V_{o}$ if $R_{L}=1.2 \mathrm{k} \Omega$ c . $A_{v}=V_{o} / V_{i}$ d. $Z_{o}$ with $r_{o}=\infty \Omega$ e. $A_{i}=I_{o} / I_{i}$ f. $I_{b}$

For the common-base configuration of Fig. $18,$ an ac signal of $10 \mathrm{mV}$ is applied, resulting in an ac emitter current of $0.5 \mathrm{mA}$. If $\alpha=0.980$, determine:
a. $Z_{i}$
b. $V_{o}$ if $R_{L}=1.2 \mathrm{k} \Omega$
c . $A_{v}=V_{o} / V_{i}$
d. $Z_{o}$ with $r_{o}=\infty \Omega$
e. $A_{i}=I_{o} / I_{i}$
f. $I_{b}$

Key Concepts

Input Impedance

Input impedance in this context refers to the effective resistance faced by the incoming AC signal at the emitter terminal of a common-base amplifier. It is a critical parameter that determines how much of the input signal is dropped across the transistor versus being delivered to the amplifier, influencing the overall signal transfer and matching conditions with preceding stages.

Common-Base Amplifier

This concept refers to a transistor amplifier configuration where the base terminal is held at AC ground while the input signal is applied to the emitter and the output is taken from the collector. It is characterized by low input impedance, high output impedance, and a current gain defined by the relation between the emitter and collector currents, making it particularly useful in high-frequency applications.

Small-Signal Analysis

Small-signal analysis involves linearizing the behavior of non-linear devices, such as transistors, around a bias point to analyze their response to small input variations. This method simplifies the behavior of the transistor into equivalent circuits using parameters like input impedance, output impedance, and gain, enabling the calculation of AC signals without the complexity of the full non-linear model.

Voltage Gain

Voltage gain is the ratio of the AC output voltage to the AC input voltage. In a common-base amplifier, voltage gain is determined by the ratio of the load resistance at the collector to the effective input impedance at the emitter, with the transistor’s current amplification properties playing a significant role. This parameter indicates how effectively the amplifier increases the magnitude of the input signal.

Output Impedance

Output impedance is the resistance 'seen' by the load connected to the amplifier's output terminal, typically taken at the collector in a common-base configuration. It affects the voltage delivered to the load and is important in determining the amplifier’s ability to drive subsequent circuit stages or loads without significant voltage drop.

Current Gain

Current gain is the ratio of the output current to the input current. In the common-base configuration, this gain is closely related to the intrinsic properties of the transistor, particularly the parameter designated as alpha (?), which defines the proportion of the emitter current that appears at the collector. It is crucial for understanding the amplification of current signals and designing biasing networks.

Transistor Parameter ?

The parameter ? represents the common-base current gain and is defined as the ratio of the collector current to the emitter current. Typically close to unity, it indicates that nearly all the emitter current is transferred to the collector. This parameter is fundamental in analyzing transistor circuits and helps in determining relationships between various currents (emitter, base, and collector) and the overall performance of the amplifier.

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