Transistor Characteristics
In a transistor, only three terminals are available, viz., Emitter (E), Base (B) and Collector (C).
Therefore, in a circuit the input/output connections have to be such that one of these (E, B or C) is common to both the input and the output.
Accordingly, the transistor can be connected in either of the following three configurations:
Common Emitter (CE), Common Base (CB), Common Collector (CC)
`text(Common emitter transistor characteristics)`
When a transistor is used in CE configuration, the input is between the base and the emitter and the output is between the collector and the emitter.
The variation of the base current `I_B` with the base-emitter voltage `V_(BE)` is called the input characteristic.
The variation of the collector current `I_C` with the collector-emitter voltage `V_(CE)` is called the output characteristic.
The collector current changes with the base current.
`text(Input Characteristics :)`
The collector-emitter voltage `V_(CE)` is kept fixed while studying the dependence of `I_B` on `V_(BE)`. We are interested to obtain the input characteristic when the transistor is in active state. So the collector-emitter voltage `V_(CE)` is kept large enough to make the base collector junction reverse biased.
Since `V_(CE) = V_(CB) + V_(BE)` and for Si transistor `V_(BE)` is 0.6 to 0.7 V, `V_(CE)` must be sufficiently larger than 0.7 V.
Since the transistor is operated as an amplifier over large range of `V_(CE)`, the reverse bias across the base-collector junction is high most of the time.
Therefore, the input characteristics may be obtained for `V_(CE)` somewhere in the range of 3 V to 20 V.
The increase in `V_(CE)` appears as increase in `V_(CB)`, its effect on `I_B` is negligible.
It is enough to determine only one input characteristics. The input characteristics of a transistor is as shown in Fig.
`text(Output Characteristic :)`
The output characteristic is obtained by observing the variation of `I_C` as `V_(CE)` is varied keeping `I_B` constant.
If `V_(BE)` is increased by a small amount, both `I_B` and `I_C` will increase proportionately.
when `I_B` increases `I_C` also increases. The plot of `I_C` versus `V_(CE)` for different fixed values of `I_B` gives one output characteristic. So there will be different output characteristics corresponding to different values of `I_B` as shown in Fig.
The linear segments of both the input and output characteristics can be used to calculate some important ac parameters of transistors.
(i) `text(Input resistance)` (`r_i`) : This is defined as the ratio of change in base-emitter voltage (`ΔV_(BE)`) to the resulting change in base current (`ΔI_B`) at constant collector-emitter voltage (`V_(CE)`).
`r_i=((DeltaV_(BE))/(DeltaI_B))_(V_(CE))`
(ii) `text(Output resistance)` (`r_o`) : This is defined as the ratio of change in collector-emitter voltage (`ΔV_(CE)`) to the change in collector current (`ΔI_C`) at a constant base current `I_B`.
`r_o=((DeltaV_(CE))/(DeltaI_C))_(I_B)`
(iii) `text(Current amplification factor)` (`β`) : This is defined as the ratio of the change in collector current to the change in base current at a constant collector-emitter voltage (`V_(CE)`) when the transistor is in active state.
`beta_(ac)=((DeltaI_C)/(DeltaI_B))_(V_(C))`
This is also known as small signal current gain and its value is very large.
If we simply find the ratio of `I_C` and `I_B` we get what is called dc `β` of the transistor. Hence,
`beta_(dc)=(I_C)/(I_B)`
Since `I_C` increases with `I_B` almost linearly and `I_C = 0` when `I_B = 0`, the values of both `β_(dc)` and `β_(ac)` are nearly equal. So, for most calculations `β_(dc)` can be used. Both `β_(ac)` and `β_(dc)` vary with `V_(CE)` and `I_B` (or `I_C`) slightly.
Therefore, in a circuit the input/output connections have to be such that one of these (E, B or C) is common to both the input and the output.
Accordingly, the transistor can be connected in either of the following three configurations:
Common Emitter (CE), Common Base (CB), Common Collector (CC)
`text(Common emitter transistor characteristics)`
When a transistor is used in CE configuration, the input is between the base and the emitter and the output is between the collector and the emitter.
The variation of the base current `I_B` with the base-emitter voltage `V_(BE)` is called the input characteristic.
The variation of the collector current `I_C` with the collector-emitter voltage `V_(CE)` is called the output characteristic.
The collector current changes with the base current.
`text(Input Characteristics :)`
The collector-emitter voltage `V_(CE)` is kept fixed while studying the dependence of `I_B` on `V_(BE)`. We are interested to obtain the input characteristic when the transistor is in active state. So the collector-emitter voltage `V_(CE)` is kept large enough to make the base collector junction reverse biased.
Since `V_(CE) = V_(CB) + V_(BE)` and for Si transistor `V_(BE)` is 0.6 to 0.7 V, `V_(CE)` must be sufficiently larger than 0.7 V.
Since the transistor is operated as an amplifier over large range of `V_(CE)`, the reverse bias across the base-collector junction is high most of the time.
Therefore, the input characteristics may be obtained for `V_(CE)` somewhere in the range of 3 V to 20 V.
The increase in `V_(CE)` appears as increase in `V_(CB)`, its effect on `I_B` is negligible.
It is enough to determine only one input characteristics. The input characteristics of a transistor is as shown in Fig.
`text(Output Characteristic :)`
The output characteristic is obtained by observing the variation of `I_C` as `V_(CE)` is varied keeping `I_B` constant.
If `V_(BE)` is increased by a small amount, both `I_B` and `I_C` will increase proportionately.
when `I_B` increases `I_C` also increases. The plot of `I_C` versus `V_(CE)` for different fixed values of `I_B` gives one output characteristic. So there will be different output characteristics corresponding to different values of `I_B` as shown in Fig.
The linear segments of both the input and output characteristics can be used to calculate some important ac parameters of transistors.
(i) `text(Input resistance)` (`r_i`) : This is defined as the ratio of change in base-emitter voltage (`ΔV_(BE)`) to the resulting change in base current (`ΔI_B`) at constant collector-emitter voltage (`V_(CE)`).
`r_i=((DeltaV_(BE))/(DeltaI_B))_(V_(CE))`
(ii) `text(Output resistance)` (`r_o`) : This is defined as the ratio of change in collector-emitter voltage (`ΔV_(CE)`) to the change in collector current (`ΔI_C`) at a constant base current `I_B`.
`r_o=((DeltaV_(CE))/(DeltaI_C))_(I_B)`
(iii) `text(Current amplification factor)` (`β`) : This is defined as the ratio of the change in collector current to the change in base current at a constant collector-emitter voltage (`V_(CE)`) when the transistor is in active state.
`beta_(ac)=((DeltaI_C)/(DeltaI_B))_(V_(C))`
This is also known as small signal current gain and its value is very large.
If we simply find the ratio of `I_C` and `I_B` we get what is called dc `β` of the transistor. Hence,
`beta_(dc)=(I_C)/(I_B)`
Since `I_C` increases with `I_B` almost linearly and `I_C = 0` when `I_B = 0`, the values of both `β_(dc)` and `β_(ac)` are nearly equal. So, for most calculations `β_(dc)` can be used. Both `β_(ac)` and `β_(dc)` vary with `V_(CE)` and `I_B` (or `I_C`) slightly.