The ratio of output to input impedance of a CE amplifier is _______

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Examoogle · Accepted Answer

Moderate

Explanation: The Common Emitter (CE) amplifier provides a moderate ratio of output impedance ($Z_o$) to input impedance ($Z_i$). While the input impedance is typically in the range of a few kilo-ohms and the output impedance is in the range of tens of kilo-ohms, their ratio falls into a moderate range compared to other configurations like Common Collector or Common Base.

Working Principle: In a CE configuration, the input signal is applied to the base-emitter junction (forward biased), providing moderate input resistance. The output is taken from the collector-emitter junction (reverse biased), which inherently exhibits higher resistance due to the depletion layer width. The ratio $\frac{Z_o}{Z_i}$ typically results in a value representing moderate impedance transformation capability.

Key Formulas: $Z_{in} \approx \beta r_e$ — Input impedance of CE amplifier; $Z_{out} \approx R_C$ — Output impedance of CE amplifier

Key Points: CE amplifiers are widely used due to their high voltage and current gain.; Input impedance is relatively low (1k$\Omega$ to 5k$\Omega$).; Output impedance is relatively high (10k$\Omega$ to 50k$\Omega$).; The moderate ratio makes it a balanced choice for general-purpose amplification.

Advantages: High voltage gain; High current gain; High power gain

Disadvantages: Phase inversion (180 degree shift); Limited bandwidth due to Miller effect

Applications: Audio frequency pre-amplifiers; General-purpose signal amplification

Additional Info: Common Collector (CC) has high input and low output impedance.. Common Base (CB) has very low input and very high output impedance.

Answer: D is correct — The CE amplifier exhibits a moderate impedance ratio as its output impedance is typically higher than its input impedance, but not by the extreme orders of magnitude found in other configurations.

Exam Tip: Remember: CE is for overall gain, CB is for impedance matching (low-to-high), and CC (Emitter Follower) is for impedance matching (high-to-low).

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