Transistor Input Resistance Analysis

Understanding the input resistance ($R_{in}$) characteristics of different bipolar junction transistor (BJT) configurations is crucial for amplifier design. The input resistance determines how much input voltage is required to drive a certain input current into the transistor.

Input Resistance Comparison

Let's compare the typical input resistance for the three basic BJT configurations:

• Common Base (CB): The input signal is applied to the emitter, and the output is taken from the collector. The base is common. The input current is the emitter current ($I_E$). The input resistance is approximately equal to the dynamic emitter resistance, $r_e$. $R_{in(CB)} \approx r_e = \frac{V_T}{I_E}$ where $V_T$ is the thermal voltage (approx. 26mV at room temperature) and $I_E$ is the emitter current. Since $I_E$ is usually larger than the base current ($I_B$), $r_e$ is typically very low, often in the range of tens of ohms.
• Common Emitter (CE): The input signal is applied to the base, and the output is taken from the collector. The emitter is common. The input current is the base current ($I_B$). The input resistance is significantly higher than in CB because it is multiplied by the current gain, $\beta$. $R_{in(CE)} \approx \beta r_e$ This results in input resistances typically in the kilohms (k$\Omega$) range.
• Common Collector (CC): Also known as an emitter follower, the input signal is applied to the base, and the output is taken from the emitter. The collector is common. This configuration provides high voltage gain (close to unity) and high input resistance. $R_{in(CC)} \approx \beta R_L$ where $R_L$ is the load resistance connected to the emitter. This configuration offers the highest input resistance, often in the hundreds of kilohms or megaohms (M$\Omega$) range.

Lowest Input Resistance Configuration

Based on the analysis:

• CB: Lowest input resistance (tens of $\Omega$).
• CE: Medium input resistance (k$\Omega$).
• CC: Highest input resistance (hundreds of k$\Omega$ to M$\Omega$).

Therefore, the Common Base (CB) configuration has the lowest input resistance among the three basic BJT amplifier configurations.