Question

How do you determine the input impedance and current gain of a common-emitter BJT amplifier with emitter degeneration?

Answer 1

To determine the input impedance and current gain of a common-emitter Bipolar Junction Transistor (BJT) amplifier with emitter degeneration, you can follow these steps:

Circuit Description:
A common-emitter BJT amplifier with emitter degeneration typically consists of a BJT with the emitter resistor (Re) added in the emitter circuit. This resistor introduces negative feedback and stabilizes the gain, making it less dependent on the transistor's characteristics. The basic circuit schematic is as follows:
lua
Copy code
       Vcc
        +
        |
        Rb
        |
        +
  Vin --|>-- BJT -- RL
        |    |
        Re   Vout
        |    |
        +----+
        |
       GND

DC Biasing:
You need to set up the DC biasing for the amplifier. This involves choosing appropriate resistor values for Rb and Re, and calculating the quiescent operating point of the BJT. This point ensures the transistor is operating in its active region. To simplify the calculations, you can use the following formulas:
Choose a suitable value for Rb to bias the base-emitter junction at an appropriate DC voltage (usually around Vcc/2).
Calculate the emitter current (Ie) using Ohm's law: Ie = (Vcc - Vbe) / Re, where Vbe is the base-emitter voltage (approximately 0.7V for silicon BJTs).
AC Analysis:
To determine the input impedance (Zin) and current gain (Ai) with emitter degeneration, you need to analyze the circuit using small-signal models. The two main parameters to consider are the transconductance (gm) and the dynamic emitter resistance (re).

Transconductance (gm): It is the change in collector current with respect to the change in base-emitter voltage. For a BJT in the active region, gm ≈ Ie / Vt, where Vt is the thermal voltage (around 26 mV at room temperature).

Dynamic emitter resistance (re): It is the incremental resistance looking into the emitter terminal of the transistor. For a silicon BJT in the active region, re ≈ 1 / gm.

Input Impedance (Zin):
The input impedance of the common-emitter amplifier with emitter degeneration can be approximated as the sum of the base resistor (Rb) and the dynamic emitter resistance (re):

Zin ≈ Rb + re

Current Gain (Ai):
The current gain (Ai) of the amplifier is the ratio of the change in output current (ΔIc) to the change in input current (ΔIin). Since emitter degeneration stabilizes the current gain, it is approximately equal to the ratio of the load resistor (RL) to the dynamic emitter resistance (re):

Ai ≈ RL / re

Remember that these are approximate formulas, and actual circuit performance may vary due to factors such as early effect and other parasitic elements. Nonetheless, this analysis should provide a good estimation of the input impedance and current gain of the common-emitter BJT amplifier with emitter degeneration.