Discovering Electronics 4-6 Flashcards
What are the three types of switching devices discussed in Chapter 4?
- npn transistor
- n-channel MOSFET
- Voltage comparator
What is the MOSFET equation for drain current (I_D)?
I_D = g_m (V_GS - 3)
where g_m is transconductance and V_GS is gate-source voltage
What are the two basic rules for npn transistor operation in switching circuits?
- If V_IN < 0.7V: transistor is OFF, V_BE = V_IN, V_CE = supply voltage
- If V_IN ≥ 0.7V: transistor is ON, V_BE = 0.7V, V_CE = 0V
What is the current gain formula for a transistor?
h_FE = I_C / I_B
where I_C is collector current and I_B is base current
How does a light-activated switch using an LDR and transistor work?
In bright light: LDR resistance low → V_IN < 0.7V → transistor OFF → lamp off
In darkness: LDR resistance high → V_IN > 0.7V → transistor ON → lamp on
What is the purpose of a protection diode in transistor/MOSFET circuits?
To protect against high reverse voltages generated when inductive loads (motors/solenoids) switch off by limiting voltage to 0.7V
What are the key characteristics of an n-channel enhancement mode MOSFET?
- Normally OFF when V_GS = 0
- Turns ON when positive V_GS applied
- Very high input impedance (negligible gate current)
- Can handle large drain currents
How do you calculate the minimum V_GS needed for a MOSFET to operate a load?
V_GS = (I_D / g_m) + 3
where I_D is drain current and g_m is transconductance
What are the three regions of a transistor’s voltage transfer characteristic?
- Cut-off region (V_IN < 0.7V)
- Linear region (0.7V < V_IN < saturation)
- Saturation region (V_IN high enough that V_CE ≈ 0V)
What is the difference between TTL and CMOS logic ICs?
TTL (7400 series): Faster switching, higher power consumption
CMOS (4000 series): Lower power, slower, more static-sensitive (exam focuses on CMOS)
What is the truth table for a 2-input NAND gate?
B | A | Q |
| 0 | 0 | 1 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
How does a pull-up resistor work in a logic input circuit?
- When switch open: pulls input to VCC (logic 1)
- When switch closed: connects input to 0V (logic 0)
What is the Boolean expression for a NOR gate?
Q = A + B
(read as Q = NOT (A OR B))
How do you convert an AND gate to NAND gates only?
AND = NAND followed by inverter:
A──┐ │
NAND──INV──Q
B──┘
What is the purpose of a zener diode in voltage regulation?
Maintains constant output voltage in reverse breakdown region when current is above holding current
How does half-wave rectification work?
Single diode allows only positive half-cycles of AC to pass, blocking negative cycles
What is the voltage drop across a forward-biased silicon diode?
Approximately 0.7V
What happens when a zener diode’s load current exceeds its capacity?
Zener stops regulating, output voltage drops, and resistor dissipates more power
How do you design a transistor switching circuit for a given load?
- Determine required I_C from load specs
- Calculate I_B = I_C / h_FE
- Calculate V_RB = I_B × R_B
- Ensure V_IN = V_RB + 0.7V for saturation
What is the truth table for the logic system: A → NOT → AND ← B?
B | A | C | Q |
| 0 | 0 | 1 | 0 |
| 0 | 1 | 0 | 0 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 0 |
How do comparators convert slow sensor signals to digital outputs?
They amplify tiny differences between inputs, saturating to VCC or 0V for fast transitions
What is the Boolean identity for De Morgan’s first law?
A·B = A + B
How do you protect a MOSFET from static damage?
Handle with anti-static precautions (wrist straps, conductive foam) as high input impedance makes them sensitive
What is the current through a zener diode when load is disconnected?
All resistor current flows through zener (I_Z = I_R)
How do you calculate power dissipation in a resistor?
P = V × I
or P = V²/R
or P = I²×R
What are the pin connections for a BC337 transistor?
- Collector
- Base
- Emitter
What is the Boolean expression for a traffic light system where Green = NOT A AND B?
Green = A·B
How do you implement an OR gate using only NAND gates?
OR = 3 NAND gates:
A──┐ │
NAND──┐ │
B──┘ │ NAND──Q
NAND──┘
What is the voltage across a reverse-biased silicon diode?
Nearly all supply voltage (only leakage current flows)
How do you select a MOSFET for high-current applications?
- Check maximum drain current rating
- Verify transconductance (g_m) meets requirements
- Consider cost vs. performance
What is the purpose of the resistor in a zener regulator circuit?
Limits current to protect zener and provides voltage drop (V_R = V_in - V_z)
How does a temperature-activated switch using a thermistor work?
Low temp: thermistor R high → V_IN > 0.7V → transistor ON → lamp on
High temp: thermistor R low → V_IN < 0.7V → transistor OFF → lamp off
What is the truth table for a 3-input system where Q=1 when A=1 and B=C=0?
C | B | A | Q |
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 0 |
What are the characteristics of the LM358 op-amp as a comparator?
- Output current: 40mA max
- High output: ~1V below VCC
- Low output: ~10mV above 0V
How do you calculate series resistance for an LED?
R = (V_supply - V_LED) / I_LED
(typically limit I_LED to 20mA)
What is the Boolean expression for an XOR function?
Q = A·B + A·B
How do you identify redundant gates in NAND implementations?
Look for pairs of NAND inverters in series (double inversion cancels out)
What is the pinout for a 4081 CMOS AND gate IC?
Pins 1,2 → Gate 1 inputs
Pin 3 → Gate 1 output
Pins 4,5 → Gate 2 inputs
Pin 6 → Gate 2 output
Pin 7 → GND
Pins 8,9 → Gate 3 inputs
Pin 10 → Gate 3 output
Pins 11,12 → Gate 4 inputs
Pin 13 → Gate 4 output
Pin 14 → VDD
How does a voltage divider reference work in comparator circuits?
Sets switching threshold by dividing supply voltage (e.g., two equal resistors give V_ref = VCC/2)
What is the truth table for a NOR gate?
B | A | Q |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 0 |
How do you calculate resistor value for transistor base?
R_B = (V_IN - 0.7V) / I_B
where I_B = I_C / h_FE
What are the key parameters when selecting a transistor?
- Maximum collector current (I_C)
- Current gain (h_FE)
- Cost
How does a MOSFET differ from a BJT in switching applications?
MOSFET: Voltage-controlled, high input impedance, no gate current
BJT: Current-controlled, requires base current
What is the Boolean expression for a NAND gate?
Q = A·B
How do you implement a NOT gate using NAND gates?
NOT = Single NAND with both inputs tied together:
A──┐
NAND──Q
A──┘
What is the purpose of a flyback diode in relay circuits?
Dissipates inductive kickback when relay de-energizes to protect switching transistor
How do you calculate zener current in a regulator?
I_Z = I_R - I_LOAD
What is the truth table for a 2-input AND gate?
B | A | Q |
| 0 | 0 | 0 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
How does a comparator with hysteresis work?
Uses positive feedback to create two switching thresholds (reduces chatter)
What are the three terminals of a MOSFET?
Gate (G), Drain (D), Source (S)
How do you calculate power in a zener diode?
P_Z = V_Z × I_Z
What is De Morgan’s second law in Boolean algebra?
A + B = A·B
How do you design a light-sensing circuit with adjustable sensitivity?
Replace fixed resistor in voltage divider with a potentiometer (LDR + variable resistor)
What is the truth table for a traffic light controller where Red=NOT B
Yellow=A
How do you calculate load current in a MOSFET switching circuit?
I_D = P_load / V_load
or I_D = V_load / R_load
What is the Boolean expression for an AND gate?
Q = A·B
How does a thermistor-based temperature alarm work with a comparator?
Thermistor in voltage divider → comparator compares to reference → triggers when threshold crossed
What are the three leads of a BJT transistor?
Emitter (E), Base (B), Collector (C)
How do you determine if a transistor is saturated?
V_CE ≈ 0V and I_C no longer increases with I_B
What is the purpose of a current-limiting resistor in a transistor base circuit?
Prevents excessive base current that could damage the transistor
How do you implement a NOR gate using NAND gates?
NOR = 4 NAND gates:
A──┐ │
NAND──┐ │
B──┘ │ NAND──┐
NAND──┘ NAND──Q
What is the voltage across a conducting silicon diode in series with a lamp?
V_diode ≈ 0.7V, V_lamp = V_supply - 0.7V
How does a window comparator work?
Uses two comparators to detect when a signal is between two reference voltages
What is the truth table for an OR gate?
B | A | Q |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 1 |
How do you calculate transconductance (g_m) of a MOSFET?
g_m = I_D / (V_GS - 3)
What is the Boolean expression for an OR gate?
Q = A + B
How does a voltage regulator maintain constant output with varying load?
Zener diode maintains fixed voltage drop; resistor adjusts current flow to compensate for load changes
What are the key parameters when selecting a zener diode?
- Zener voltage (V_Z)
- Power rating (P_Z = V_Z × I_Z_max)
- Holding current requirement
How do you calculate base resistor value for saturation?
R_B = (V_IN - 0.7V) / (I_C / h_FE)
What is the truth table for a NOT gate (inverter)?
A | Q |
| 0 | 1 |
| 1 | 0 |
How does a NAND gate work as a universal gate?
Can create all other logic functions (AND, OR, NOT) through proper combinations of NAND gates
What is the purpose of the emitter being grounded in transistor switching circuits?
Provides common reference point and completes current path for both base and collector currents
How do you implement an AND gate using NOR gates?
AND = 3 NOR gates:
A──┐ │
NOR──┐ │
B──┘ │ NOR──Q
NOR──┘
What is the Boolean expression for output Q in the truth table where Q=1 when A≠B?
Q = A·B + A·B
How does a comparator interface with a MOSFET?
Comparator output drives MOSFET gate directly (no current limiting needed due to MOSFET’s high input impedance)
What are the three regions of MOSFET operation?
- Cut-off (V_GS < threshold)
- Linear/Ohmic (small V_DS)
- Saturation (V_DS > V_GS - V_th)
How do you calculate power dissipation in a transistor?
P = V_CE × I_C
(important to avoid overheating in linear region)
What is the truth table for a 3-input AND gate?
C | B | A | Q |
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 |
| 0 | 1 | 0 | 0 |
| 0 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 |
| 1 | 0 | 1 | 0 |
| 1 | 1 | 0 | 0 |
| 1 | 1 | 1 | 1 |
How does a pull-down resistor work in a logic input circuit?
- When switch open: pulls input to 0V (logic 0)
- When switch closed: connects input to VCC (logic 1)
What is the Boolean expression for output Q in the truth table where Q=1 only when A=1 and B=0?
Q = A·B
How do you calculate the minimum zener resistor value?
R_min = (V_in - V_z) / (I_z_min + I_load_max)
What are the characteristics of an ideal comparator?
- Infinite gain
- Instantaneous switching
- Output swings rail-to-rail
How does a transistor switch interface between logic and high-power loads?
Logic output provides small base current → transistor amplifies current to drive larger collector current through load
What is the truth table for the logic system: A → OR ← B → AND ← C?
C | B | A | D | Q |
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 0 |
| 0 | 1 | 1 | 1 | 0 |
| 1 | 0 | 0 | 0 | 0 |
| 1 | 0 | 1 | 1 | 1 |
| 1 | 1 | 0 | 1 | 1 |
| 1 | 1 | 1 | 1 | 1 |
How do you calculate the maximum load current for a zener regulator?
I_load_max = I_R - I_z_min
where I_R = (V_in - V_z)/R
What is the Boolean expression for a NOR gate?
Q = A + B
How does a voltage follower (buffer) work in comparator circuits?
Provides high input impedance and low output impedance to prevent loading of reference voltage divider
What are the key differences between TTL and CMOS logic families?
TTL: 5V operation, faster, more power
CMOS: 3-15V, slower, less power, more static-sensitive
How do you implement a NAND gate using NOR gates?
NAND = 4 NOR gates:
A──┐ │
NOR──┐ │
B──┘ │ NOR──┐
NOR──┘ NOR──Q
What is the truth table for a 2-input XOR gate?
B | A | Q |
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | 0 | 1 |
| 1 | 1 | 0 |
How does a transistor act as a current amplifier?
Small base current controls larger collector current (I_C = h_FE × I_B)
What is the Boolean expression for output Q in the truth table where Q=1 when A=B?
Q = A·B + A·B
How do you calculate the power supply current for a logic gate IC?
Sum of currents for all active gates in the package
What are the three basic Boolean operations?
- AND (·)
- OR (+)
- NOT (¯)
How does a zener diode maintain voltage regulation?
In reverse breakdown, voltage remains nearly constant despite current variations
What is the truth table for the logic system: A → NAND ← B → NOR ← C?
C | B | A | D | Q |
| 0 | 0 | 0 | 1 | 0 |
| 0 | 0 | 1 | 1 | 0 |
| 0 | 1 | 0 | 1 | 0 |
| 0 | 1 | 1 | 0 | 1 |
| 1 | 0 | 0 | 1 | 0 |
| 1 | 0 | 1 | 1 | 0 |
| 1 | 1 | 0 | 1 | 0 |
| 1 | 1 | 1 | 0 | 1 |
How do you calculate the maximum zener current?
I_z_max = P_z / V_z
What is the Boolean expression for output Q in the truth table where Q=1 when majority of inputs are 1?
For 3 inputs: Q = A·B + A·C + B·C
How does a transistor switch avoid operation in the linear region?
Ensure V_IN is either <0.7V (OFF) or sufficiently high for saturation (V_CE ≈ 0V)
What are the key specifications for the IRF510 MOSFET?
- Drain current: 5.6A max
- g_m: 1S typical
- Power: 40W
- Package: TO-220AB
How do you implement a NOR gate using NAND gates?
NOR = 4 NAND gates:
A──┐ │
NAND──┐ │
B──┘ │ NAND──┐
NAND──┘ NAND──Q
What is the truth table for a 3-input OR gate?
C | B | A | Q |
| 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 1 |
| 0 | 1 | 0 | 1 |
| 0 | 1 | 1 | 1 |
| 1 | 0 | 0 | 1 |
| 1 | 0 | 1 | 1 |
| 1 | 1 | 0 | 1 |
| 1 | 1 | 1 | 1 |
How does a MOSFET handle higher currents than a BJT?
Lower ON resistance (R_DS(on)) allows more current with less power dissipation
What is the Boolean expression for output Q in the truth table where Q=1 when odd number of inputs are 1?
For 3 inputs: Q = A⊕B⊕C
How do you calculate the power dissipated in a switching transistor?
P = V_CE(sat) × I_C
(very low when saturated)
What are the characteristics of the BD437 transistor?
- Collector current: 4A max
- h_FE: 40 @ 2A
- Package: TO-126
- Cost: 30p
How does a comparator with open-collector output work?
Requires pull-up resistor, can drive higher voltages than supply rail
What is the truth table for the logic system: A → AND ← B → OR ← C?
C | B | A | D | Q |
| 0 | 0 | 0 | 0 | 0 |
| 0 | 0 | 1 | 0 | 0 |
| 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 1 | 1 | 1 |
| 1 | 0 | 0 | 0 | 1 |
| 1 | 0 | 1 | 0 | 1 |
| 1 | 1 | 0 | 0 | 1 |
| 1 | 1 | 1 | 1 | 1 |
How do you calculate the minimum input voltage for a MOSFET switch?
V_GS_min = (I_D / g_m) + 3
What is the Boolean expression for output Q in the truth table where Q=1 when all inputs are equal?
For 3 inputs: Q = A·B·C + A·B·C
How does a zener diode protect against overvoltage?
Conducts heavily when voltage exceeds breakdown, clamping the voltage
What are the key specifications for the BC337 transistor?
- Collector current: 800mA max
- h_FE: 100-250
- Package: TO-92
- Cost: 2p
How do you implement an XOR gate using NAND gates?
XOR = 4 NAND gates:
A──┐ │ │
NAND──┐ │
B──┘ │ NAND──┐
NAND──┘ NAND──Q
What is the truth table for a 2-input XNOR gate?
B | A | Q |
| 0 | 0 | 1 |
| 0 | 1 | 0 |
| 1 | 0 | 0 |
| 1 | 1 | 1 |
How does a transistor switch provide isolation between control and load circuits?
Separate power supplies can be used for base and collector circuits
What is the Boolean expression for output Q in the truth table where Q=1 when exactly two inputs are 1?
For 3 inputs: Q = A·B·C + A·B·C + A·B·C
How do you calculate the power dissipated in a zener diode?
P_Z = V_Z × I_Z
