How do you calculate the input impedance and voltage gain of a common-base BJT amplifier?
1 Answer
To calculate the input impedance and voltage gain of a common-base BJT (Bipolar Junction Transistor) amplifier, you'll need to follow these steps:
Common-Base BJT Amplifier Configuration:
The common-base configuration has the base terminal as the input, the collector terminal as the output, and the emitter terminal common to both input and output. It provides high current gain but low voltage gain.
Small-Signal Model:
For small-signal analysis, you can use the small-signal model of the BJT. This model simplifies the transistor's behavior and is valid for small variations around the operating point (DC biasing point).
DC Biasing:
Before proceeding with small-signal analysis, you must establish a DC biasing circuit to set the transistor's Q-point (quiescent operating point). This involves selecting appropriate resistors to bias the transistor and establish the desired collector current (Ic) and base-emitter voltage (Vbe) at the operating point.
Small-Signal Parameters:
For a small-signal analysis of the common-base BJT amplifier, you need the following small-signal parameters:
rπ (pi) - The small-signal base-emitter resistance. It is approximately equal to 26 mV/Ib, where Ib is the base current.
gm - The transconductance of the transistor, representing the relationship between the base current and the small-signal change in collector current (Ic). It is given by gm = Ic/Vt, where Vt is the thermal voltage (approximately 26 mV at room temperature).
Input Impedance (Zin):
The input impedance of the common-base amplifier is the impedance looking into the base terminal. It is given by Zin ≈ rπ. The small-signal input impedance is relatively low and is influenced mainly by rπ.
Voltage Gain (Av):
The voltage gain of the common-base BJT amplifier is the ratio of the change in output voltage (ΔVout) to the change in input voltage (ΔVin) under small-signal conditions. The voltage gain (Av) is typically less than unity in the common-base configuration due to its current gain nature.
Av = -gm * (Rc || RL)
where:
gm is the transconductance of the transistor.
Rc is the collector load resistor.
RL is the load resistor connected to the output.
Keep in mind that the negative sign in the voltage gain formula indicates a 180-degree phase shift between input and output.
It's worth noting that the calculations above are based on the small-signal model and linear approximation of the BJT's behavior. For a more accurate analysis, simulation tools like SPICE or advanced models may be necessary.
Common-Base BJT Amplifier Configuration:
The common-base configuration has the base terminal as the input, the collector terminal as the output, and the emitter terminal common to both input and output. It provides high current gain but low voltage gain.
Small-Signal Model:
For small-signal analysis, you can use the small-signal model of the BJT. This model simplifies the transistor's behavior and is valid for small variations around the operating point (DC biasing point).
DC Biasing:
Before proceeding with small-signal analysis, you must establish a DC biasing circuit to set the transistor's Q-point (quiescent operating point). This involves selecting appropriate resistors to bias the transistor and establish the desired collector current (Ic) and base-emitter voltage (Vbe) at the operating point.
Small-Signal Parameters:
For a small-signal analysis of the common-base BJT amplifier, you need the following small-signal parameters:
rπ (pi) - The small-signal base-emitter resistance. It is approximately equal to 26 mV/Ib, where Ib is the base current.
gm - The transconductance of the transistor, representing the relationship between the base current and the small-signal change in collector current (Ic). It is given by gm = Ic/Vt, where Vt is the thermal voltage (approximately 26 mV at room temperature).
Input Impedance (Zin):
The input impedance of the common-base amplifier is the impedance looking into the base terminal. It is given by Zin ≈ rπ. The small-signal input impedance is relatively low and is influenced mainly by rπ.
Voltage Gain (Av):
The voltage gain of the common-base BJT amplifier is the ratio of the change in output voltage (ΔVout) to the change in input voltage (ΔVin) under small-signal conditions. The voltage gain (Av) is typically less than unity in the common-base configuration due to its current gain nature.
Av = -gm * (Rc || RL)
where:
gm is the transconductance of the transistor.
Rc is the collector load resistor.
RL is the load resistor connected to the output.
Keep in mind that the negative sign in the voltage gain formula indicates a 180-degree phase shift between input and output.
It's worth noting that the calculations above are based on the small-signal model and linear approximation of the BJT's behavior. For a more accurate analysis, simulation tools like SPICE or advanced models may be necessary.