Explain the operation of a common-emitter transistor amplifier and its voltage gain.
1 Answer
A common-emitter transistor amplifier is a popular configuration used to amplify weak signals. It is based on a bipolar junction transistor (BJT) with its emitter as the common terminal, meaning the input signal is applied between the base and emitter, and the output is taken between the collector and emitter.
Here's how a common-emitter transistor amplifier works:
Transistor Biasing: The transistor must be properly biased to ensure it operates in the active region, where small changes in the base current can result in significant changes in the collector current. This is typically achieved using a voltage divider network of resistors connected to the base-emitter junction.
Input Signal (AC Coupling): The input signal is typically an AC (alternating current) signal superimposed on a DC (direct current) bias. To remove the DC component and allow only the AC part of the signal to reach the base-emitter junction, a coupling capacitor is used. The capacitor blocks the DC voltage but allows the AC signal to pass through.
Transistor Amplification: The AC input signal modulates the base current, causing the collector current to vary accordingly. Due to the transistor's characteristics, small variations in base current result in much larger variations in collector current. This amplification process forms the core of the common-emitter amplifier.
Load Resistor: The collector of the transistor is connected to a load resistor, which provides a voltage drop across it proportional to the collector current. This resistor develops the output voltage of the amplifier.
Output Signal (AC Coupling): Since the amplifier's output contains both AC and DC components, a coupling capacitor is used at the output to block the DC component and obtain only the amplified AC signal at the output terminal.
Voltage Gain of a Common-Emitter Transistor Amplifier:
The voltage gain (Av) of a common-emitter transistor amplifier is the ratio of the output AC voltage (Vout) to the input AC voltage (Vin). It represents how much the input signal is amplified by the circuit. The voltage gain can be approximated by the following formula:
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Av â - β * Rc / re
where:
β is the current gain of the transistor (commonly known as hFE or βdc), representing the ratio of collector current (Ic) to base current (Ib) when the transistor is in the active region.
Rc is the collector resistor. It is the load resistor connected to the collector of the transistor.
re is the dynamic resistance of the transistor's base-emitter junction. It is given by re â 25 mV / Ib, where Ib is the base current.
Note: The negative sign in the formula indicates that the output signal is inverted compared to the input signal in a common-emitter configuration.
The voltage gain can also be expressed in decibels (dB):
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Av_dB â 20 * log10(|Av|)
In practice, the actual voltage gain may vary slightly due to factors like biasing arrangements, transistor parameters, and external circuit components. The choice of appropriate resistors and capacitors in the circuit design helps achieve the desired voltage gain and optimize the amplifier's performance.
Here's how a common-emitter transistor amplifier works:
Transistor Biasing: The transistor must be properly biased to ensure it operates in the active region, where small changes in the base current can result in significant changes in the collector current. This is typically achieved using a voltage divider network of resistors connected to the base-emitter junction.
Input Signal (AC Coupling): The input signal is typically an AC (alternating current) signal superimposed on a DC (direct current) bias. To remove the DC component and allow only the AC part of the signal to reach the base-emitter junction, a coupling capacitor is used. The capacitor blocks the DC voltage but allows the AC signal to pass through.
Transistor Amplification: The AC input signal modulates the base current, causing the collector current to vary accordingly. Due to the transistor's characteristics, small variations in base current result in much larger variations in collector current. This amplification process forms the core of the common-emitter amplifier.
Load Resistor: The collector of the transistor is connected to a load resistor, which provides a voltage drop across it proportional to the collector current. This resistor develops the output voltage of the amplifier.
Output Signal (AC Coupling): Since the amplifier's output contains both AC and DC components, a coupling capacitor is used at the output to block the DC component and obtain only the amplified AC signal at the output terminal.
Voltage Gain of a Common-Emitter Transistor Amplifier:
The voltage gain (Av) of a common-emitter transistor amplifier is the ratio of the output AC voltage (Vout) to the input AC voltage (Vin). It represents how much the input signal is amplified by the circuit. The voltage gain can be approximated by the following formula:
Copy code
Av â - β * Rc / re
where:
β is the current gain of the transistor (commonly known as hFE or βdc), representing the ratio of collector current (Ic) to base current (Ib) when the transistor is in the active region.
Rc is the collector resistor. It is the load resistor connected to the collector of the transistor.
re is the dynamic resistance of the transistor's base-emitter junction. It is given by re â 25 mV / Ib, where Ib is the base current.
Note: The negative sign in the formula indicates that the output signal is inverted compared to the input signal in a common-emitter configuration.
The voltage gain can also be expressed in decibels (dB):
scss
Copy code
Av_dB â 20 * log10(|Av|)
In practice, the actual voltage gain may vary slightly due to factors like biasing arrangements, transistor parameters, and external circuit components. The choice of appropriate resistors and capacitors in the circuit design helps achieve the desired voltage gain and optimize the amplifier's performance.