Transistor-2

What are the different regions of operation of transistor?

In BJT due to the presence of two junctions, each junction can be operated in forward or reverse bias leading to three different regions of operation. Transistor when used as a switch in logic gates will be operated in extreme regions of input output characteristics in which both regions will be forward biased or reverse biased which are called saturation and cut off regions of operation simultaneously. Transistor when used as amplifier is operated in active region in which input junction will be forward biased and output junction will be reverse biased. Other region inverse active region is of less importance in practice.

What is Ebers model of transistor?

Ebers-moll model of transistor holds for all regions of operation of BJT. This model is based on assumption that base spreading resistance can be neglected.

Ebers Moll model of NPN transistor

Ebers Moll model of NPN transistor

Consider a diode with voltage v applied between its terminals. The current flowing through the junction in terms of applied voltage between its terminals is given by

                                                  I=Io (exp (V/Vt)-1)

Where Io is reverse saturation current of the transistor (Current which flows across junction when it is reverse biased will be Io as exp (V/Vt) tends to 0).From the diagram applying Kirchhoff’s current law at the collector node, we get

                                Ic = –αNIE+Ico (exp (VBC/Vt)-1)

Where αN is the current gain of common base transistor mentioned above, VBC is the base to collector voltage, Ico is the reverse saturation current of base collector junction. Similarly for emitter junction by applying Kirchhoff’s current law

                                   IE= –αIIC+IEo (exp (VBE/Vt)-1)

Where αI is the inverted current gain of common base transistor with roles of collector and emitter interchanged, VBE is the base to Emitter voltage, Ico is the reverse saturation current of base Emitter junction. The above equations are derived based on the assumption of low level minority carrier injection (the hole concentration injected into the base is very much less compared to the intrinsic electron concentration in base), in such a case emitter or collector current is mainly dominated by diffusion currents, drift current is negligible compared to drift currents.

How to obtain response of transistor to large signal and small signals?

A Bipolar Junction transistor acts as a nonlinear device if input-output voltage swing is large. In such a case large signal response of a transistor is obtained graphically. But for small signals transistor operates with reasonable linearity so that the small signal response of the transistor can be obtained analytically by using model of transistor. These small signal transistor models are small signal approximations of transistor in linear active region. The notion of small signal is valid for voltages of the order of milli volts around the bias point.

Small signal hybrid model of transistor?

The basic two port equations for transistor in common emitter configuration are

                                                   Vbe = hfe*ib + hre*Vce

                                           ic = hfe*ib + hre*Vce

Where Vbe, ib, Vce, ic are incremental currents and voltages (incremental means for example ib=IbIB where Ib is instantaneous base current, IB is quiescent base current)

Small signal hybrid model of transistor

Small signal hybrid model of transistor

Define Transconductance of a transistor?

The Transconductance of transistor is defined as the ratio of incremental collector current to incremental base to emitter voltage with collector to emitter kept constant. It is denoted by gm.

gm= |Ic|/VT

where VT = K*T/Q = 26 milli volts at room temperature of 27 Deg C,

gm= |Ic(mA)|/26.     

Small signal PI model of transistor?

Small signal PI model of transistor

Small signal PI model of transistor

Where gm = |Ic (mA)|/26, rΠ = β / gm, rO = VA / Ic where β is small signal current gain in common emitter configuration, VA is Early voltage, Ic is collector current, Vbe is incremental base to emitter voltage.

Small signal T model of transistor?

Small signal T model of transistor

Small signal T model of transistor

Where gm = |Ic (mA)|/26, re = α / gm, rO = VA / Ic

where α is small signal current gain in common base configuration α = β/(1+ β), β is small signal current gain in common emitter configuration, VA is Early voltage, Ic is collector current, Vbe is incremental base to emitter voltage. The relation between re and rΠ is rΠ = (1+ β)* re.

What is Q point?

Q-point is an acronym for quiescent point. Q-point is the operating point of the transistor (IC,VCE) at which it is biased. The concept of Q-point is used when transistor act as an amplifying device. Q-point is generally taken to be the intersection point of load line with the output characteristics of the transistor. There can be infinite number of intersection points but q-point is selected in such a way that irrespective of input swing transistor remain in active region. All the input AC signals variations happen around Q-point.

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