Common Emitter Amplifier The so called classic common emitter configuration uses a potential divider network to bias the transistors Base. Power supply Vcc and the biasing resistors set the transistor operating point to conduct in the forward active mode. With no signal current flow into the Base, no Collector current flows, (transistor in cut-off) and the voltage on the Collector is the same as the supply voltage, Vcc. A signal current into the Base causes a current to flow in the Collector resistor, Rc generating a voltage drop across it which causes the Collector voltage to drop.
Then the direction of change of the Collector voltage is opposite to the direction of change on the Base, in other words, the polarity is reversed. Thus the common emitter configuration produces a large voltage amplification and a well defined DC voltage level by taking the output voltage from across the collector as shown with resistor RLrepresenting the load across the output.
Please explain me the above paragraphs.
And also conclude that how is the input impedance of common emitter configuration high and how does this affect the high current gain and power gain of this type of configuration.
The so called classic common emitter configuration uses a potential divider network to bias the transistors Base. Power supply Vcc and the biasing resistors set the transistor operating point to conduct in the forward active mode. With no signal current flow into the Base, no Collector current flows, (transistor in cut-off) and the voltage on the Collector is the same as the supply voltage, Vcc. A signal current into the Base causes a current to flow in the Collector resistor, Rc generating a voltage drop across it which causes the Collector voltage to drop.
Then the direction of change of the Collector voltage is opposite to the direction of change on the Base, in other words, the polarity is reversed. Thus the common emitter configuration produces a large voltage amplification and a well defined DC voltage level by taking the output voltage from across the collector as shown with resistor RLrepresenting the load across the output.
Please explain me the above paragraphs.
And also conclude that how is the input impedance of common emitter configuration high and how does this affect the high current gain and power gain of this type of configuration.
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The input signal is applied between the base and emitter terminals while the output signal is taken between the collector and emitter terminals. Thus, the emitter terminal of a transistor is common for both input and output and hence it is named as common emitter configuration.
The supply voltage between base and emitter is denoted by VBE while the supply voltage between collector and emitter is denoted by VCE.
In common emitter (CE) configuration, input current or base current is denoted by IB and output current or collector current is denoted by IC.
The common emitter amplifier has medium input and output impedance levels. So the current gain and voltage gain of the common emitter amplifier is medium. However, the power gain is high.
To fully describe the behavior of a transistor with CE configuration, we need two set of characteristics – input characteristics and output characteristics.
Input characteristics
The input characteristics describe the relationship between input current or base current (IB) and input voltage or base-emitter voltage (VBE).
First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The input current or base current (IB) is taken along y-axis (vertical line) and the input voltage (VBE) is taken along x-axis (horizontal line).
To determine the input characteristics, the output voltage VCE is kept constant at zero volts and the input voltage VBE is increased from zero volts to different voltage levels. For each voltage level of input voltage (VBE), the corresponding input current (IB) is recorded. In common emitter (CE) configuration, the input current (IB) is very small as compared to the input current (IE) in common base (CB) configuration. The input current in CE configuration is measured in microamperes (μA) whereas the input current in CB configuration is measured in milliamperes (mA). In common emitter (CE) configuration, the input current (IB) is produced in the base region which is lightly doped and has small width. So the base region produces only a small input current (IB). On the other hand, in common base (CB) configuration, the input current (IE) is produced in the emitter region which is heavily doped and has large width. So the emitter region produces a large input current (IE). Therefore, the input current (IB) produced in the common emitter (CE) configuration is small as compared to the common base (CB) configuration. Due to forward bias, the emitter-base junction acts as a forward biased diode and due to reverse bias, the collector-base junction acts as a reverse biased diode. Therefore, the width of the depletion region at the emitter-base junction is very small whereas the width of the depletion region at the collector-base junction is very large. If the output voltage VCE applied to the collector-base junction is further increased, the depletion region width further increases. The base region is lightly doped as compared to the collector region. So the depletion region penetrates more into the base region and less into the collector region. As a result, the width of the base region decreases which in turn reduces the input current (IB) produced in the base region. From the above characteristics, we can see that for higher fixed values of output voltage VCE, the curve shifts to the right side. This is because for higher fixed values of output voltage, the cut in voltage is increased above 0.7 volts. Therefore, to overcome this cut in voltage, more input voltage VBE is needed than previous case. Output characteristics The output characteristics describe the relationship between output current (IC) and output voltage (VCE). First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The output current or collector current (IC) is taken along y-axis (vertical line) and the output voltage (VCE) is taken along x-axis (horizontal line). To determine the output characteristics, the input current or base current IB is kept constant at 0 μA and the output voltage VCE is increased from zero volts to different voltage levels. For each level of output voltage, the corresponding output current (IC) is recorded.
The input signal is applied between the base and emitter terminals while the output signal is taken between the collector and emitter terminals. Thus, the emitter terminal of a transistor is common for both input and output and hence it is named as common emitter configuration.
The supply voltage between base and emitter is denoted by VBE while the supply voltage between collector and emitter is denoted by VCE.
In common emitter (CE) configuration, input current or base current is denoted by IB and output current or collector current is denoted by IC.
The common emitter amplifier has medium input and output impedance levels. So the current gain and voltage gain of the common emitter amplifier is medium. However, the power gain is high.
To fully describe the behavior of a transistor with CE configuration, we need two set of characteristics – input characteristics and output characteristics.
Input characteristics
The input characteristics describe the relationship between input current or base current (IB) and input voltage or base-emitter voltage (VBE).
First, draw a vertical line and a horizontal line. The vertical line represents y-axis and horizontal line represents x-axis. The input current or base current (IB) is taken along y-axis (vertical line) and the input voltage (VBE) is taken along x-axis (horizontal line).
To determine the input characteristics, the output voltage VCE is kept constant at zero volts and the input voltage VBE is increased from zero volts to different voltage levels. For each voltage level of input voltage (VBE), the corresponding input current (IB) is recorded.
