BEE Semiconductor Devices and Circuits 5 — Questions and Answers
Question 1: In a differential amplifier with a tail current source ITAIL, increasing ITAIL primarily affects which parameter?
- Common-mode gain ACMR
- Transconductance gm and thus differential gain (Correct answer)
- Input offset voltage
- Output resistance of the tail source
Correct answer: Transconductance gm and thus differential gain
Each transistor's bias current is ITAIL/2, so increasing ITAIL raises gm = IC/VT and therefore increases the differential voltage gain.
Question 2: The Miller effect in an inverting amplifier stage causes the input capacitance to appear as:
- Cgd/(1 + |Av|)
- Cgd × (1 + |Av|) (Correct answer)
- Cgd + Cgs
- Cgd − Cgs
Correct answer: Cgd × (1 + |Av|)
By Miller's theorem, a feedback capacitance Cgd appears at the input multiplied by (1 + |Av|), significantly increasing effective input capacitance.
Question 3: What type of semiconductor material results when silicon is doped with phosphorus?
- p-type, with holes as majority carriers
- Intrinsic semiconductor with no net charge
- n-type, with electrons as majority carriers (Correct answer)
- Degenerate insulator
Correct answer: n-type, with electrons as majority carriers
Phosphorus has five valence electrons; donating one to the lattice creates a free electron, making the silicon n-type.
Question 4: In a cascode amplifier topology, placing a common-gate (or common-base) stage above a common-source (or common-emitter) stage primarily provides:
- Lower voltage gain but higher input impedance
- Increased output resistance and reduced Miller effect for wider bandwidth (Correct answer)
- Lower transconductance to reduce power consumption
- Reduced DC bias stability
Correct answer: Increased output resistance and reduced Miller effect for wider bandwidth
The cascode's common-gate stage shields the input transistor from drain voltage swings, suppressing the Miller effect and raising output resistance, extending bandwidth.
Question 5: A tunnel diode exhibits negative differential resistance (NDR) because of which quantum mechanical phenomenon?
- Impact ionization across the depletion region
- Quantum tunneling of carriers through a very thin junction barrier (Correct answer)
- Recombination of electron-hole pairs at defect sites
- Phonon-assisted carrier scattering in the bulk semiconductor
Correct answer: Quantum tunneling of carriers through a very thin junction barrier
In heavily doped tunnel diodes, electrons quantum-mechanically tunnel through the thin junction, producing a current peak followed by an NDR region.
Question 6: For a PMOS transistor to operate in the saturation region, which condition must be satisfied?
- VSD ≥ VSG − |VTP| (Correct answer)
- VDS ≥ VGS − VTP
- VGS ≥ VTP (VTP negative)
- VSG ≤ 0
Correct answer: VSD ≥ VSG − |VTP|
For a PMOS in saturation, VSD ≥ VSG − |VTP|, which ensures the channel is pinched off at the drain end.
Question 7: The transit frequency fT of a BJT is defined as the frequency at which the common-emitter current gain β drops to:
- 0.707 of its low-frequency value
- 10% of its low-frequency value
- Unity (1) (Correct answer)
- Zero
Correct answer: Unity (1)
The transit frequency fT is defined as the frequency where the magnitude of the common-emitter short-circuit current gain equals 1.
BEE Semiconductor Devices and Circuits 5
In a differential amplifier with a tail current source ITAIL, increasing ITAIL primarily affects which parameter?