What is the main limitation of fixed‑bias transistor circuits?
Voltage gain and DC bias cannot be set independently.
Why does β variation cause problems in fixed‑bias circuits?
Changes in β shift the operating point, causing distortion.
What is the purpose of voltage‑divider bias?
To make the emitter current IE (≈ IC) nearly independent of β.
What sets the base voltage in a voltage‑divider bias circuit?
The resistor divider R1 and R2.
What are the three key transistor operating limits?
Maximum collector current, maximum VCE, maximum power dissipation.
What is the formula for transistor power dissipation?
P = VCE × IC.
When does maximum power dissipation occur?
When VCE = VCC/2 in the linear region.
Typical maximum collector current for small NPN transistors?
Around 100–200 mA.
What is VCE(sat) typically?
0.1–0.3 V.
What is the collector cutoff current?
Small leakage current when the transistor is off.
What are the two switching states of a transistor?
Cutoff and saturation.
What happens in cutoff?
IB = 0, IC ≈ 0, VCE ≈ VCC.
What happens in saturation?
IB large, IC = IC(sat), VCE ≈ 0.1–0.3 V.
Formula for IC(sat)?
IC(sat) = VCC / RC.
Design rule for saturation base current?
IB ≈ 2 × IC(sat) / β.
Formula for base resistor RB in switching?
RB = (Vi − VBE) / IB.
Why do inductive loads cause voltage spikes?
Collapsing magnetic field generates large reverse voltage (VL = L dI/dt).
What protects a transistor from inductive kickback?
A diode placed across the inductor (flyback diode).
What does the flyback diode do?
Provides a safe path for current when the transistor switches off.
What is the effect of propagation delay in switching circuits?
Slows response and can distort timing.
Why are transistors used in logic gates?
They act as electronic switches in cutoff and saturation.
What logic function does two transistors in series implement?
AND gate.
What logic function does two transistors in parallel implement?
OR gate.
What is the purpose of RB in a switching circuit?
Limits base current to a safe value.
