This paper presents an analysis of the common-emitter, common-collector, and common-base amplifier circuits under DC conditions. The common-emitter amplifier is one of the most fundamental configurations in analog electronics, often introduced in textbooks. While static operating points, AC and DC gain are typically analyzed, real-world design requires careful selection of resistor values like R1 and R2.
The NPN transistor 2N2219 is a silicon device with a typical base-emitter voltage (Ube) of 0.7V during normal operation. When choosing R1, it's important to consider that the maximum base current (Ib) for this transistor is around 800mA, but in practice, Ib is usually in the range of several milliamps or microamps. For example, if we choose R1 = 10kΩ, the resulting Ib would be approximately 0.43mA. Assuming a current gain (β) of 100, the collector current (Ic) would be about 43mA.
To ensure the transistor operates in the active region, the collector-emitter voltage (Uce) must be greater than Ube, ideally above 1V to avoid saturation distortion due to AC signals. Using this, R2 can be calculated as (12V - 1V)/43mA ≈ 256Ω. Therefore, R2 should be less than 256Ω, and a practical value might be 200Ω or lower.
Simulations were conducted with different R2 values:
- **Figure 2 (R2 = 50Ω)**: Shows a higher Ic.
- **Figure 3 (R2 = 200Ω)**: Indicates a more balanced operating point.
- **Figure 4 (R2 = 350Ω)**: Demonstrates the transistor entering the saturation region.
If R1 is reduced to 1kΩ, the base current increases to 4.3mA, leading to a much higher Ic of 430mA, which significantly limits the allowable R2 value, making it too small for practical use.
Another observation is that when R2 is 50Ω or 200Ω, the collector current (Ic) changes even when the base current (Ib) remains constant. This behavior aligns with the transistor output characteristics, where Ic increases with Uce, especially in the active region.
Understanding the various states of the transistor is crucial. The output characteristic curves show how Ic behaves with varying Uce and Ib. In the saturation region, increasing Uce does not lead to a significant increase in Ic, even if Ib rises.
For the **common-collector** amplifier, R1 and R2 are chosen to ensure proper biasing. Since R2 is connected to the emitter, the collector-emitter voltage (Uce) must be sufficiently high to maintain reverse bias on the collector junction. Typically, R1 and R2 can range from hundreds of ohms to several kilohms, depending on the desired current levels.
In the **common-base** configuration, the input is applied to the emitter, and the output is taken from the collector. Here, the emitter current (Ie) is usually in the tens of milliamps. For instance, if Ie = 43mA, R1 might be around 100Ω, and R2 can be selected between tens of ohms and 200Ω to maintain proper Uce and ensure reverse bias.
These considerations highlight the importance of selecting appropriate resistor values to ensure stable and efficient operation of transistor amplifiers in different configurations.
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