- Transistor Definition: Transistors are semiconductor devices used to amplify or switch electronic signals and electrical power.
- Input Characteristics: These characteristics show how input current varies with input voltage for a constant output voltage, explaining the control behavior of transistors.
- Output Characteristics: This set of curves demonstrates the change in output current as output voltage varies, crucial for understanding transistor output response.
- Current Transfer Characteristics: These illustrate the relationship between input and output currents, highlighting the amplification capabilities of transistors.
- Transistor Configurations: Understanding different configurations like Common Base, Common Collector, and Common Emitter helps in selecting the right transistor type for specific applications, each with unique transistor characteristics.
Transistor Characteristics are the current and voltages curves of a transistor in a given connection. The device is treated as a two-port network. The plots fall into three groups:
- Input Characteristics: These describe the changes in input current with the variation in the values of input voltage keeping the output voltage constant.
- Output Characteristics: This is a plot of output current versus output voltage with constant input current.
- Current Transfer Characteristics: This characteristic curve shows the variation of output current in accordance with the input current, keeping output voltage constant.
Common Base (CB) Configuration of Transistor
In the common-base connection the base is shared by the input and the output (Figure 1). Input impedance is low, output impedance is high, resistance gain is high and voltage gain is high.
Input Characteristics for CB Configuration of Transistor
Input Characteristics for CB Configuration: Figure 2 illustrates how the emitter current, IE, varies with the Base-Emitter voltage, VBE, while keeping the Collector-Base voltage, VCB, constant.
This leads to the expression for the input resistance as
Output Characteristics for CB Configuration of Transistor
Output Characteristics for CB Configuration: Figure 3 displays the changes in collector current, IC, relative to VCB, maintaining a constant emitter current, IE. This graph also allows us to calculate the output resistance.
Current Transfer Characteristics for CB Configuration of Transistor
Current Transfer Characteristics for CB Configuration: Figure 4 presents how the collector current, IC, varies with the emitter current, IE, while keeping VCB constant. This results in a current gain of less than 1, expressed mathematically below.
Common Collector (CC) Configuration of Transistor
In the common-collector connection the collector is shared by the input and the output (Figure 5). It is also called an emitter follower. Input impedance is high, output impedance is low, voltage gain is less than one and current gain is large.
Input Characteristics for CC Configuration of Transistor
Input Characteristics for CC Configuration: Figure 6 outlines how the base current, IB, changes in relation to the Collector-Base voltage, VCB, with a constant Collector-Emitter voltage, VCE.
Output Characteristics for CC Configuration of Transistor
Figure 7 below shows the output characteristics for the CC configuration which exhibit the variations in IE against the changes in VCE for constant values of IB.
Current Transfer Characteristics for CC Configuration of Transistor
This characteristic of CC configuration (Figure 8) shows the variation of IE with IB keeping VCE as a constant.
Common Emitter (CE) Configuration of Transistor
In the common-emitter connection the emitter is shared by the input and the output (Figure 9). Input impedance, output impedance, current gain and voltage gain sit between the CB and CC extremes.
Input Characteristics for CE Configuration of Transistor
Figure 10 shows the input characteristics for the CE configuration of transistor which illustrates the variation in IB in accordance with VBE when VCE is kept constant.
From the graph shown in Figure 10 above, the input resistance of the transistor can be obtained as
Output Characteristics for CE Configuration of Transistor
The output characteristics of CE configuration (Figure 11) are also referred to as collector characteristics. This plot shows the variation in IC with the changes in VCE when IB is held constant. From the graph shown, the output resistance can be obtained as:
Current Transfer Characteristics for CE Configuration of Transistor
This characteristic of CE configuration shows the variation of IC with IB keeping VCE as a constant. This can be mathematically given by
This ratio is referred to as common-emitter current gain and is always greater than 1.
The curves above are drawn for BJTs. The same kind of plots can be drawn for FETs.
