1. When I was interviewing for a position at IBM during my Senior year, I was as
ID: 2080379 • Letter: 1
Question
1.When I was interviewing for a position at IBM during my Senior year, I was asked a barrage of rapid fire questions by one of the engineering managers. Here are a few of them that you can answer in a more leisurely manner than I was able to: “What is the name of the parameter that determines current gain in a BJT? How is it measured? What are reasonable ranges of this parameter? How does it vary with environmental or circuit conditions? How would you measure this parameter for a transistor wired in a circuit?” Extra credit (I was asked this one, too!): how might you measure the parameter for a series of transistors embedded in an integrated circuit?
2.You are trying to control a relay with an Arduino output pin. The relay requires about 50 mA to switch at which time it will have a voltage drop of about 3V, while the Arduino recommended output current is 20 mA, while switching between 0 and 5 V. Describe how an NPN transistor like the2N2222 or 2N3904 could be connected to the Arduino output, the relay, one or more resistors (specify the resistance value(s) and power dissipation), and a 5V power source to control the relay.
3.Every diode is designed to pass current in one direction only while blocking current flow in the other direction, a function known as rectification. Besides this basic operation, diodes can exhibit other characteristics that can be used for other purposes in electronic circuits. Identify at least two functions a diode can perform that we discussed in class and describe what physical characteristics of the diode allow this function. Describe more than two for extra credit
4.Describe the operation of a current mirror and illustrate how one might be used. Why might one want to use a current mirror as opposed to another method or achieving the same purpose? Are there any differences in operation that must be considered with a MOSFET or a BJT?
5.What is the condition in a FET that limits drain current, despite increases in the drain-to-source voltage? Explain the physical phenomenon that causes this condition to exist.
6.We discussed two common abnormal operating conditions in class that lead to failure in a bipolar junction transistor. Pick at least one and describe the conditions that lead to this failure, whether it is recoverable and how it can be prevented.
7.You are designing a common emitter amplifier using a silicon NPN transistor with a grounded emitter. The amplifier will be operated using four 1.5 volt batteries in series. Suggest a set of steady state voltages VE, VB, and VC that would ensure the amplifier would be operating in the active region for a low level signal input.
8.Contrast the mechanism of operation of a MOSFET with a BJT. What other type of device was presented in class that performs similar amplification and switching functions? Which type of transistor is it more similar to and how?
Explanation / Answer
1. What is the name of the parameter that determines current gain in a BJT? How is it measured? What are reasonable ranges of this parameter? How does it vary with environmental or circuit conditions? How would you measure this parameter for a transistor wired in a circuit?” Extra credit (I was asked this one, too!): how might you measure the parameter for a series of transistors embedded in an integrated circuit?
Sol. Current Gain of BJT is defined by beta( ).
It is meaured as = Ic/Ib. Ic and Ib are collector and base current.
It ranges from 60 to 120.
It increasse with increase in temperature.
In serier of transisitor , you can consider it as of darlington pair, where current gain munliflies and nearly equals to 2.
3.Every diode is designed to pass current in one direction only while blocking current flow in the other direction, a function known as rectification. Besides this basic operation, diodes can exhibit other characteristics that can be used for other purposes in electronic circuits. Identify at least two functions a diode can perform that we discussed in class and describe what physical characteristics of the diode allow this function. Describe more than two for extra credit
First Function
The voltage drop across a forward-biased diode varies only a little with the current, and is a function of temperature; this effect can be used as a temperature sensor or as a voltage reference.
Second Function
A semiconductor diode's current–voltage characteristic can be tailored by selecting the semiconductor materials and the doping impurities introduced into the materials during manufacture. These techniques are used to create special-purpose diodes that perform many different functions. For example, diodes are used to regulate voltage (Zener diodes), to protect circuits from high voltage surges (avalanche diodes), to electronically tune radio and TV receivers (varactor diodes).
4.Describe the operation of a current mirror and illustrate how one might be used. Why might one want to use a current mirror as opposed to another method or achieving the same purpose? Are there any differences in operation that must be considered with a MOSFET or a BJT?
Current Mirror is used to replicate the same amput of current in other branch of circuit.
You can operate MOS current mirrors in subthreshold in fact they're liable to be better, over a wider range of output
voltage, if you do - running a smaller device "hotter" will raise the "knee" voltage where your mirror becomes
accurate. BJTs have a wasted base current term, on the order of 2% at hFE=100 (1/100 the current into each of the bases). You can buffer the base current so long as you have 2*Vbe headroom on the reference side, or drive the rack's base with some amp and use one mirror output as feedback. This could be effective down to maybe 1V. The nice thing about BJTs is that your headroom is pretty fixed across a wide current range.
5.What is the condition in a FET that limits drain current, despite increases in the drain-to-source voltage? Explain the physical phenomenon that causes this condition to exist.
With increase in drain to source voltage scattering in the channel also increases which in turn decreases the mobility. Mobility is directly proportional to Drain Current, therefor Id reduces.
6.We discussed two common abnormal operating conditions in class that lead to failure in a bipolar junction transistor. Pick at least one and describe the conditions that lead to this failure, whether it is recoverable and how it can be prevented.
BJT Device will fail if collector base reverse biased voltage will be too large and leads to puch through in the the device i.e. depletion width in the base collector juction eatens away base.
It is non recoverable if device gets breakdown, before breakdown device may function again.
7.You are designing a common emitter amplifier using a silicon NPN transistor with a grounded emitter. The amplifier will be operated using four 1.5 volt batteries in series. Suggest a set of steady state voltages VE, VB, and VC that would ensure the amplifier would be operating in the active region for a low level signal input.
Let VB= 1.5 V such that emitter base juction be Forward biased(0.7V required).
and Let Vc be 1.5+1.5+1.5= 4.5 V so the collector base juction reverse biased. and let VE=0V(grounded).
8.Contrast the mechanism of operation of a MOSFET with a BJT. What other type of device was presented in class that performs similar amplification and switching functions? Which type of transistor is it more similar to and how?
BJT is current controlled device while MOSFET is VOltage controlled device.
MOSFET requires a Electric Field across the Gate Electron to create inversion in the channel for conduction, BJT requires a heavy injection of carriers into the base and collector to make the current flow.
MOSFET devices incurred with various capacitances the limits its operation at higher frequencies while BJT is have less number of parasitic capacitance that improves higher frequency response.
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