V-I characteristics of a transistor The behavior of a transistor is observed with the help of its V-I characteristic. There are two types of characteristic. 1. Input Characteristic 2. Output Characteristic Input Characteristic: Input characteristic of a transistor gives the relationship between input current and input voltage for a constant output voltage. Output Characteristic: Output characteristics of a transistor gives the relationship between output current and output voltage for constant input current.

V-I characteristics in cb configuration

Input characteristics The input characteristic is a curve plotted between IE and VEB. At constant base voltage VCB. Keeping VCB constant(e.g.2V) note down IEB for different VEB. This will give input characteristic.

V-I characteristics in cb configuration

Input characteristics Observations:-- (i)Emitter current is very small ,when emitter Base voltage VEB is below to the knee voltage. (ii)After knee voltage the emitter current increasers rapidly. Input Resistance(ri):--It is the ratio of change in emitter voltage ∆VEB to the resulting change in emitter current∆IE at constant collector base voltage VCB.

Output characteristics Output Charecteristic:-- It is the curve between IC and VCB at constant emitter current. Keeping emitter current IE fixed note the collector current IC for various values of collector base voltage VCB.

Output characteristics The output characteristics has 3 basic regions: Active region – defined by the biasing arrangements Cutoff region – region where the collector current is 0A Saturation region- region of characteristics to the left Of VCB = 0V

The curves (output characteristics) clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by IC ≈IE Once a transistor is in the ‘on’ state, the base-emitter voltage will be assumed to be VBE = 0.7V

OBSERVATION:-- 1. The collector current Ic varies withVCB only at very low voltage (VCB<0).The transistor is never operated in this region. 2. Collector current IC is almost constant when the voltage is increased beyond the cutin voltage.It means that now Icis independent of VCB and depends upon IE. OUTPUT RESISTANCE:--It is the ratio of change in collectorbase voltage (∆VCE) to change in collector current(∆IC)at constant emitter current.

V-I characteristics of CE configuration

V-I characteristics of CE configuration

The input characteristic is a curve plotted between IBand VBE The input characteristic is a curve plotted between IBand VBE. At constant Collecror Emitter voltage VCE. Keeping VCE constant note down IB for different VBE. This will give input characteristic.

Input characteristics IB is microamperes compared to miliamperes of IC. IB will flow when VBE > 0.7V for silicon and 0.3V for germanium Before this value IB is very small and no IB. Base-emitter junction is forward bias Increasing VCE will reduce IB for different values. Input characteristics for a common-emitter NPN transistor

Observation:--As compared to CB configuration,IB increases less rapidly with increase of VBE.Therefore input resistance of a CE configuration is higher than that of CB configuration. Input Resistance:- It is the ratio of change in base emitter voltage(∆VBE) to the change in base current(∆IB) at constant VCE

Output characteristics For small VCE (VCE < VCESAT), IC increase linearly with increasing of VCE VCE > VCESAT IC not totally depends on VCE  constant IC IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC IB=0 A  ICEO occur. Noticing the value when IC=0A. There is still some value of current flows.

Observation:- 1.The collector current IC varies with VCE between 0 and 1V only.After this collector current is almost constant,independent of VCE. 2. For any value of VCE above knee voltage, the collector current IC is approximately equal to βIB.. 3. The transistor are operated in the region above knee voltage. Output Resistance:- It is the ratio of change in collector emitter voltage(∆VCE) to change in collector current(IC) at constant IB.

Comparison of transistor configuration S.NO CHARECTERISTIC COMMON BASE COMMON EMITTER COMMON COLLECTOR 1 Input resistance Low (about 100 Ω) high (about 750 Ω) Very high (about 750 Ω) 2 Output resistance Very high (about 450 Ω) High (about 45k Ω) (about 50 Ω) 3 Voltage gain (about 150) (about 500) Very Low (Less than 1) 4 Leakage current Very small (5 μA for Ge) Very large (500μA for Ge) (500μA forGe) 5 Application For high frequency application For audio frequency application For impedence matching

Conclusion:-- Common emitter circuit is most efficient reason:-- 1.High current gain 2.High voltage and Power gain.