Question

BJT in common-emitter configuration has

• A. High input, high output
• B. Low input, high output
• C. High input, low output
• D. Low input, low output

Hint:

The Common-Emitter amplifier serves as the most widely implemented configuration because it delivers both substantial voltage gain and current gain simultaneously, yielding the highest overall power gain among all three configurations.

Answer 1

The Correct Option is B

Concept: A Bipolar Junction Transistor (BJT) can be wired into three fundamental amplifier configurations: Common-Base (CB), Common-Emitter (CE), and Common-Collector (CC). Each profile possesses unique small-signal performance characteristics regarding input resistance (\(R_{\text{in}}\)) and output resistance (\(R_{\text{out}}\)). In a Common-Emitter (CE) configuration, the input signal is applied directly across the base-emitter junction, while the amplified output signal is extracted across the collector-emitter path. Let us analyze these terminals using small-signal hybrid-pi equivalent models:
• Input Impedance (\(R_{\text{in}}\)): Looking into the base, the input signal encounters the forward-biased base-emitter junction. The dynamic resistance of this junction is moderately small and is expressed as: \[ R_{\text{in}} \approx r_{\pi} = (1 + \beta)r_e \] where \(\beta\) is the common-emitter current gain and \(r_e\) is the internal emitter resistance. This value typically ranges between several hundred ohms to a few kilo-ohms (\(1\text{ k}\Omega - 5\text{ k}\Omega\)), which is classified as moderately low when compared to field-effect transistors or common-collector stages.
• Output Impedance (\(R_{\text{out}}\)): Looking back into the collector terminal, the collector-base junction is reverse-biased under active operational conditions. The internal output resistance is dictated by the transistor's Early effect parameter (\(r_o\)): \[ R_{\text{out}} \approx R_C \parallel r_o \] Since the internal resistance \(r_o\) is extremely large (often exceeding \(50\text{ k}\Omega\)), the parallel combination is dominated by the collector load resistor \(R_C\). This results in an output impedance that is classified as relatively high (typically several kilo-ohms to tens of kilo-ohms). Let's look at a comparative reference table for the three configurations: {|c|c|c|c|} Parameter & Common-Base (CB) & Common-Emitter (CE) & Common-Collector (CC)
Input Impedance & Very Low (\(\approx$ tens of $\Omega\)) & Low Moderate (\(\approx$ 1 k$\Omega\)) & Very High (\(\approx$ hundreds of k$\Omega\))
Output Impedance & Very High (\(\approx$ M$\Omega\)) & High (\(\approx$ tens of k$\Omega\)) & Low (\(\approx$ tens of $\Omega\))
This systematic comparison confirms that the common-emitter configuration is characterized by a low input impedance and high output impedance.