Download The Common Emitter Configuration-Amplifiers in Basic Electronics-Lecture Slides and more Slides Basic Electronics in PDF only on Docsity!
Common Emitter Configuration
The most frequently encountered configuration for pnp and npn transistors is common emitter configuration. The emitter is common between the input and output sides. The input for common- emitter configuration is base terminal and output for common-emitter configuration is collector terminal.
Input and Output Parameters
Input Terminal: Base terminal
Input Current: IB (Base Current)
Input Voltage: VBE (Voltage difference between base and emitter)
Output Terminal: Collector Terminal
Output Current: IC (Collector Current)
Output Voltage: VCE (Voltage difference between collector and emitter)
Input and Output Characteristics
The behaviour of a transistor can be described using characteristics of the input parameters and output parameters
Input Characteristics
For common emitter configuration the input characteristics is the relation between input current (IB) and the input voltage (VBE) at different values of output voltage (VCE)
Increasing VCE will have little effect on IB and IB will slightly decrease with the increase in VCE. This trend is evident by the graph and it happens because the width of base region decreases with the increase in VCE hence the base current also decreases. The magnitude of IB is in microamperes whereas the output current IC is in milliamperes.
Output Characteristics
The output characteristics of the common-emitter configuration are a plot of output current (IC) verses the output voltage (VCE) for a range of input currents (IB).
Beta (β)
In dc mode Beta is the ratio of collector current to the base current.
C dc B
I
I
IC and IB are measured at particular operating point on the characteristics curve. β generally lies between 50 to 400. Both α and β reveals the relative magnitude of one current to the other i.e. for a device with β = 200 the collector current will be 200 times that of the base current. β can also be mentioned as the common-emitter forward current gain. In transistor data sheets β is mentioned as hFE where h stands for hybrid parameter, F for forward current gain and E stands for common emitter configuration. For ac signals βac is defined as
VCE =constant
C ac B
I
I
Calculation of β
The value of beta can be calculated by using the output characteristic curves. Operating point of the transistor is defined by VCE = 7.5V and IB = 25μA as indicated on the figure. βdc is the value of beta on the operating point. βac is the value of beta in the neighbouring region defined by IB1 and IB2.
Relationship between α and β
Alpha and Beta relates the collector, emitter and base currents are interrelated and their relation can be found using the following relations
By putting the values IC and IB in the KCL equation
1 2
1 2
C C C
ac B B B
I I I
I I I
3.2 2.2 1
30 20 1
C
ac B
I mA mA mA
I A A A
(^)
ac 100
108 25
C
dc B
I mA
I A
C B
I I
C
E
I I
I (^) E I (^) C IB
