Test your understanding of transistor biasing and small-signal amplifiers, focusing on BJT and MOSFET circuits. This quiz covers concepts such as biasing methods, amplifier configurations, and key operating principles for electronics students and enthusiasts.
In a bipolar junction transistor (BJT) amplifier, which region should the transistor operate in to function as a small-signal amplifier?
Explanation: The active region allows the BJT to amplify small signals linearly, making it the proper mode for amplifiers. The cutoff region is for 'off' operation and does not permit amplification. The saturation region is used for switching and also does not provide linear amplification. The breakdown region is unsafe and destroys proper amplifier function.
Why is biasing necessary in MOSFET amplifier circuits?
Explanation: Proper biasing of the MOSFET ensures the device operates at a stable quiescent point (Q-point) for reliable, linear amplification. Preventing gate current flow is inherent in MOSFETs due to the insulated gate, not related to biasing. Increasing source resistance and reducing gate area are not primary reasons for biasing.
Which biasing method uses two resistors connected to the base of a BJT to set the base voltage?
Explanation: Voltage divider bias uses two resistors to form a potential divider, setting a stable base voltage. Collector feedback bias uses a resistor from collector to base instead. Fixed bias uses a single resistor for the base, and emitter feedback relies on a resistor in the emitter leg for stabilization.
What is the primary function of a capacitor connected in parallel with the emitter resistor in a BJT common emitter amplifier?
Explanation: Bypassing the emitter resistor with a capacitor allows AC signals to avoid the resistance, increasing amplifier gain for AC while maintaining DC stability. Filtering DC supply noise and coupling input signals are not performed by this specific capacitor. It is not used as a voltage divider in this context.
In the small-signal model of a BJT, what does the parameter 'r_pi' represent?
Explanation: The r_pi parameter represents the small-signal input resistance seen at the base terminal of the transistor. Output resistance at the collector is a different parameter, current gain is beta, and transconductance is denoted as gm, not r_pi.
Which biasing technique offers the best temperature stability for a BJT amplifier circuit?
Explanation: Voltage divider bias provides better thermal stability and less sensitivity to parameter changes compared to the other options. Fixed bias and base bias only are more prone to changes in temperature and transistor parameters. Collector feedback only offers some improvement but is less effective than voltage divider bias.
A MOSFET is said to be in cutoff when which condition is met?
Explanation: When the gate-source voltage (V_GS) is less than the threshold, the MOSFET remains off and no channel forms. Drain current being maximum indicates strong conduction, not cutoff. The source potential and V_DS being zero are unrelated to the cutoff state.
If a common source MOSFET amplifier has an output of 2 V peak for each 0.1 V peak input, what is the voltage gain?
Explanation: Voltage gain is output voltage divided by input voltage: 2/0.1 equals 20. Values 0.2 and 2 are incorrect as they do not reflect this ratio. 10 is half the correct value, likely due to calculation error.
What problem can occur if the base bias resistor in a BJT fixed bias circuit is chosen too large?
Explanation: A large base bias resistor limits base current, potentially turning the transistor off (cutoff). It does not cause excessive collector voltage drop, which is related to collector current. Saturation would happen with too much base current, not too little, and oscillation is unrelated to bias resistor size.
In a class A small-signal BJT amplifier, what is the typical position of the Q-point?
Explanation: The Q-point is set midway to allow maximum undistorted swing in class A amplifiers. Placing it at cutoff or saturation limits output swing and increases distortion. Operation near breakdown is unsafe for the device.
What is the approximate value of gate current in an ideal enhancement-type MOSFET?
Explanation: The insulated gate draws negligible current in an ideal enhancement-type MOSFET. Unlike BJTs, where base current flows, MOSFET gates are nearly zero current. Drain and source currents are much larger, and equating gate current to BJT base current is incorrect.
What is the role of the collector resistor (R_C) in a BJT common emitter amplifier?
Explanation: R_C produces a voltage drop proportional to collector current changes, thus converting current variations into the amplified output voltage. Input bias stabilization is handled by the bias network. Limiting emitter current is the job of the emitter resistor, and R_C's main function is not thermal runaway prevention.
Why is a coupling capacitor used at the input of small-signal transistor amplifiers?
Explanation: The coupling capacitor blocks DC components while allowing desired AC signals into the amplifier. It does not increase DC gain and certainly does not function as a heat sink. Providing base bias is done with resistors, not with capacitors.
What effect does adding a resistor in the source lead (source degeneration) of a MOSFET amplifier have?
Explanation: Source degeneration with a resistor stabilizes the operating point but typically lowers the voltage gain. It does not increase voltage gain and doesn't affect gate current, as the latter remains nearly zero. Transconductance is not disabled; instead, its impact is moderated by the source resistance.
Which component in a small-signal amplifier has the greatest impact on the lower cutoff frequency?
Explanation: The coupling capacitor and associated resistances determine the amplifier's lower cutoff frequency by high-pass filtering low-frequency signals. The collector resistor primarily affects gain and bias, not frequency limits. The emitter itself is not a component, and the bias resistor has limited influence on the lower cutoff.
In a common emitter BJT amplifier, what is the phase relationship between the input and output voltage signals?
Explanation: A common emitter amplifier inverts the signal, so the output is 180 degrees out of phase with the input, meaning it is flipped. In-phase (0 degree) and 90-degree shifts are not characteristic of this configuration. Claiming there is no fixed phase relationship is incorrect for this classic circuit.