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Bipolar Junction Transistor(BJT) Interview Questions and Answers
Question: Are There Any Reasons Why Junction-to-case (or Channel-to-case) Thermal Resistance Is Not Specified For Small-package Devices?
Neither junction-to-case thermal resistance, Rth (j-c), nor channel-to-case thermal resistance, Rth (ch-c), is specified because small-package devices do not dissipate much heat and because temperatures on the surface of small packages cannot be measured accurately. For thermal design, use either junction-to-ambient thermal resistance, Rth (j-a), or channel-to-ambient thermal resistance, Rth (ch-a), instead.
Question: Are Transistors Driven By Current Or Voltage?
Bipolar transistors are current-driven devices, and field-effect transistors (FETs) and insulated-gate bipolar transistors (IGBTs) are voltage-driven devices.
Question: Can A Transistor Be Obtained By Connecting Tow Semiconductor Diodes Back-to-back?
No. Because in case of two discrete back-to-back connected diodes there are four doped regions instead of three and there is nothing that resembles a thin base region between an emitter and a collector.
Question: Define Beta Of A Transistor?
The β factor transistor is the common emitter current gain of that transistor and is defined as the ratio of collector current to the base current:
Β = IC/IB
Question: Discuss The Need For Biasing The Transistor?
For normal operation, base-emitter junction should be forward biased and the collector-base junction reverse biased. The amount of bias required is significant for the establishment of the operating or the Q-point which is dictated by the mode of operation desired.
In case the transistor is not biased properly, it would:
- Work efficiently
- Produce distortion in the output signal
- With the change in transistor parameters or temperature rise, the operating point may shift and the amplifier output will be unstable.
Question: Explain How Bjt Can Be Used As An Amplifier?
A transistor operates as an amplifier by transfer of the current from low impedance loop to high impedance loop.
Question: Explain Why Iceo >> Icbo?
The collector cut-off current denoted by ICBO is much larger than ICBO. ICEO is given as:
ICEO = ICBO/ (1-α)
Because α is nearly equal to unity (slightly less than unity), ICEO >> ICBO
Question: How Can I Calculate The Junction-to-ambient Thermal Resistance, Rth (j-a), Of A Small-signal Transistor?
The junction-to-ambient thermal resistance, Rth (j-a), of a small-signal transistor can be calculated from its collector power dissipation as follows:
- Equation: Rth (j-a) ＝ (Tj Max – Ta） / PC MAX
- Rth (j-a): Junction-to-ambient thermal resistance.
- Tj Max: Maximum junction temperature.
- Ta: Ambient temperature (temperature condition of PC Max).
- PC Max: Maximum collector power dissipation.
Question: Is It Ok To Use A Transistor As A Diode?
Bipolar transistors are not intended to be used as diodes. Use diodes if you need the diode functionality. A bipolar transistor is modeled as equivalent to two diodes connected back-to-back with a common anode or cathode terminal. The common terminal, which serves as the base terminal of a bipolar transistor, is fabricated using a fine pattern process in order to achieve electrical characteristics necessary for a transistor; its structure is completely different from that of a diode.
Question: Technical Documents For Individual Power Transistors Contain A Safe Operating Area (soa) Graph. What Is It?
The safe operating area (SOA) is very important for the operation of a power transistor because the SOA graph specifies the voltage and current conditions in which a power transistor can be used safely. The borders of the safe operating area are broadly defined by a thermal dissipation limit and the limit given by secondary breakdown.
Question: What Are ‘emitter Injection Efficiency’ And ‘base Transport Factor’ And How Do They Influence The Transistor Operation?
The ratio of current of injected carriers at emitter junction to the total emitter current is called the emitter junction efficiency. The ratio of collector current to base current is known as transport factor
i.e. β* = IC/IB
The larger the value of emitter injection efficiency, the larger the injected carriers at emitter junction and this increases the collector current. The larger the β* value the larger the injected carriers across collector junction and hence collector current increases.
Question: What Are The Main Purposes For Which A Cc Amplifier May Be Used?
Because of its high input impedance and low output impedance, the common collector circuit finds wide application as a buffer amplifier between a high impedance source and low impedance load.
Question: What Do You Understand By Collector Reverse Saturation? In Which Configuration Does It Have A Greater Value?
When input current (IE in case of CB configuration and IB in case of CE configuration) is zero, collector current IC is not zero although it is very small. In fact this is the reverse leakage current or collector reverse saturation current (ICBO or simply ICO in CB configuration and ICEO in CE configuration). In case of CE configuration it is much more than that in case of CB configuration.
Question: What Is Meant By Operating Point?
Quiescent point is a point on the dc load line which represents VCE and IC in the absence of ac signal and variations in VCE and IC take place around this point when ac signal is applied.
Question: What Is The Difference Between Small-signal Transistors And Power Transistors?
There is no internationally uniform classification distinguishing between small-signal and power transistors. Toshiba refers to transistors with power dissipation (PC) of 1 W or less as “small-signal transistors,” and those with a PC higher than 1 W as “power transistors.”
Question: What Is The Function Of Bjts?
They change small signals to large signals. This is called amplification. The ratio of collector current IC and base current IB (IC/IB) is called DC current gain, denoted as hFE.
Question: What Is The Relation Between Base-emitter Current And Collector-emitter Current In A Bipolar Transistor?
When small current (IB) flows from base to emitter, current of IB x hFE flows from collector to emitter in NPN transistor.
Question: What Kinds Of Bipolar Transistors (bjts) Exist?
There are two types of bipolar transistors: NPN type and PNP type. The NPN-type lineup ranges from high- to low-voltage products, and the PNP-type lineup includes products under 400 V. (Products under 200 V are widely available.)
Question: What Kinds Of Transistors Exist?
Transistors are classified into three types: bipolar (BJTs), field effect (FETs) and insulated-gate bipolar (IGBTs).
Question: Which Configuration Among Ce, Cb, And Cc Gives Highest Input Impedance And No Voltage Gain?
Common collector configuration has the highest input impedance and has voltage gain less than unity.
Question: Which Of The Transistor Currents Is Always The Largest? Which Is Always The Smallest? Which Two Currents Are Relatively Close In Magnitude?
The emitter current IE is always the largest one. The base current IB is always the smallest. The collector current IC and emitter current IE are relatively close in magnitude.
Question: Why Cc Configuration Is Called A Voltage Buffer? What Is Other Name?
Because of its high input impedance and low output impedance, the common collector circuit finds wide application as a buffer amplifier between a high impedance source and low impedance load. It is called a voltage buffer. Its other name is emitter follower.
Question: Why Ce Configuration Is Most Popular In Amplifier Circuits?
CE configuration is mainly used because its current, voltage and power gains are quite high and the ratio of output impedance and input impedance are quite moderate.
Question: Why Collector Is Always Reverse-biased W.r.t Base?
Collector is always reverse-biased w.r.t baseso as to remove the charge carriers from the base-collector junction.
Question: Why Collector Is Made Larger Than Emitter And Base?
Collector is made physically larger than emitter and base because collector is to dissipate much power.
Question: Why Emitter Is Always Forward Biased?
Emitter is always forward biased w.r.t base so as to supply majority charge carriers to the base.
Question: Why Is There A Maximum Limit Of Collector Supply Voltage For A Transistor?
Although collector current is practically independent of collector supply voltage over the transistor operating range, but if VCB is increase beyond a certain vale collector current IC is eventually increases rapidly and possibly destroys the device.
Question: Why Silicon Type Transistors Are More Often Used Than Germanium Type?
Because silicon transistor has smaller cut-off current ICBO, small variations in ICBO due to variations in temperature and high operating temperature as compared to those in case of germanium type.
Question: Why The Width Of The Base Region Of A Transistor Is Kept Very Small Compared To Other Regions?
Base region of a transistor is kept very small and very lightly doped so as to pass most of the injected charge carriers to the collector.