Important Short Questions and Answers: Biasing of Discrete BJT and MOSFET

BIASING OF DISCRETE BJT AND MOSFET

1. Define operating point.

     The zero signal values of Ic & Vce are known as operating point. It is also called so because the variations of Ic and Vce take place about this point, when the signal is applied.   

2. Why the operating point is selected at the centre of the active region?

     The operating point of a transistor is kept fixed usually at the center of the active region in order that the input signal is well amplified. If the point is fixed in the saturation region or the cut off region the positive and negative half cycle gets clipped off respectively.

3. What is DC load line?       

     It is the line on the output characteristics of a transistor circuit which gives the values of Ic & Vce corresponding to zero signal (or) DC Conditions.       

4. What is the need for biasing in transistor amplifier?

     The proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of signal is known as transistor biasing.

     When a transistor is biased properly, it works efficiently and produces no distortion in the output signal and thus operating point can be maintained stable.   

5. What are the factors to be considered to design a biasing circuit?

9 It should ensure proper zero signal collector current.

9 The emitter base junction must be forward biased and collector base junction must be reversing biased.

9 The transistor should be operated in the middle of the active region or operation point should be fixed at the centre of the active region.

9 The operating point should be made independent of the transistor parameters (such as β).

9 It should ensure that VCE does not fall below 0.5 V for Ge transistors and 1 V for

9 silicon transistors at any instant.

5.   List out different type of biasing.

•      Voltage divider bias

•      Fixed bias

•      Emitter feedback bias

•      Collector feedback bias

6.   Define stability factor of an amplifier. What is ideal value?

The rate of change of collector current IC w.r.t. the collector leakage current *ICO at constant β and IB is called stability factor i.e.

Stability factor, S = dI_C / dIco dI at constant I_B and β

8.  What is thermal run away in a transistor?

The collector current, being equal increases with increase in temperature. This leads to increased power dissipation with further increase in temperature. .Being accumulative process it can lead to thermal runaway resulting in burn out of transistor. Self destruction of an un-stabilized transistor is called thermal runaway.

9.  Why thermal runaway is not there in FETs?

The FET has a positive temperature coefficient of resistivity. In FET, as temperature increases its drain resistance also increases, reducing the drain current. Thus, unlike BJT, thermal runaway does not occur with FET.

10.       What are the advantages and disadvantages of fixed bias circuits? Merits:

•      It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (R_B).

•      A very small number of components are required.

Demerits:

•      The collector current does not remain constant with variation in temperature or power supply voltage. Therefore the operating point is unstable.

•      Changes in V_be will change I_B and thus cause R_E to change. This in turn will alter the gain of the stage.

•      When the transistor is replaced with another one, considerable change in the value of β can be expected. Due to this change the operating point will shift.

•      For small-signal transistors (e.g., not power transistors) with relatively high values of β (i.e., between 100 and 200), this configuration will be prone to thermal runaway. In particular, the stability factor, which is a measure of the change in collector current with changes in reverse saturation current, is approximately β+1. To ensure absolute stability of the amplifier, a stability factor of less than 25 is preferred, and so small-signal transistors have large stability factors.

11.  How self-bias circuit is used as constant current source?

In the self bias circuit if Ic tends to increase because of ICO has increasing as a result of temperature, the current in RE increases. As consequences of the increase in voltage drop across RE that provides negative feedback, the base current is decreased. Hence constant IC value is maintained in the self bias circuit.

12. How FET is known as Voltage variable resistor?

In the region before pinch off, where VDS is small, the drain to source resistance rd can be controlled by the bias voltage VGS. Therefore FET is useful as voltage variable resistor (VVR) or Voltage dependent Resistor (VDR)