AEE Electrical Engineering Core · Chapter 14

Analog/Digital Electronics, Microprocessor, Power Electronics and Drives

What to remember

Op-amp gains: inverting = −Rf/Rin, non-inverting = 1 + Rf/Rin. NAND and NOR are universal gates. The 8085 is an 8-bit microprocessor with 16-bit address bus (64 KB memory).
An SCR conducts after a gate pulse and stays on until the current falls below the holding current. Controlled rectifier output is Vdc = (2Vm/π) cosα for a single-phase full converter.
Chopper (buck) Vo = D·Vs; boost Vo = Vs/(1−D). DC drives control speed by armature voltage below base speed and by field flux above base speed.

Analog electronics

Diode rectifiers (Vm = peak input voltage):

Type	Vdc	Ripple factor	Efficiency (max)
Half wave	Vm/π (0.318 Vm)	1.21	40.6%
Full wave	2Vm/π (0.637 Vm)	0.48	81.2%

Example: Vm = 100 V gives Vdc = 31.8 V (half wave) and 63.7 V (full wave). Peak inverse voltage (PIV) is Vm for a half-wave rectifier and for a bridge rectifier; for a centre-tap full-wave rectifier it is 2Vm. A capacitor filter reduces ripple. A Zener diode works in reverse breakdown and regulates voltage.

Transistor (BJT): IE = IB + IC. α = IC/IE; β = IC/IB; β = α/(1−α). If β = 99 then α = 0.99. A transistor is a switch in cut-off (off) and saturation (on), and an amplifier in the active region. In common emitter (CE) the amplifier has high voltage and current gain and 180° phase shift. Common collector (emitter follower) has gain near 1 and is used for impedance matching. FET/MOSFET is voltage controlled, with very high input resistance and unipolar conduction.

Operational amplifier (ideal): infinite gain, infinite input resistance, zero output resistance, infinite bandwidth. The virtual short means the two inputs are at the same potential.

Inverting amplifier: gain = −Rf/Rin. Rf = 100 kΩ, Rin = 10 kΩ gives −10.
Non-inverting amplifier: gain = 1 + Rf/Rin. Rf = 90 kΩ, Rin = 10 kΩ gives 10.
Voltage follower: gain 1. Summing amplifier adds weighted inputs. Integrator (capacitor in feedback) and differentiator are used in analog computing. Comparator gives a two-level output.

Feedback and oscillators: negative feedback gives stable gain, wider bandwidth and lower distortion. Oscillation needs the Barkhausen condition: loop gain = 1 and total phase shift 0° or 360°. RC phase-shift and Wien bridge oscillators work at audio frequency; LC (Colpitts, Hartley) and crystal oscillators at radio frequency. Crystal oscillators have the best frequency stability. 555 timer: astable (square wave generator) and monostable (single pulse) modes.

Amplifier classes: Class A conducts for 360° (low efficiency, low distortion); Class B for 180° (push-pull, higher efficiency); Class AB removes crossover distortion; Class C for under 180° (tuned RF amplifiers).

Digital electronics

Number systems: binary (base 2), octal, decimal, hexadecimal. Examples: 1011₂ = 11; 2F₁₆ = 47. Negative numbers are stored in 2's complement. Gray code changes only one bit between steps.

Gates: AND, OR, NOT, NAND, NOR, XOR, XNOR. NAND and NOR are universal: any logic can be built from either one. De Morgan: (A·B)′ = A′ + B′ and (A+B)′ = A′·B′. XOR gives 1 when inputs differ. Simplification uses Boolean algebra or the Karnaugh map (K-map, grouping in 1, 2, 4, 8 cells).

Combinational circuits:

Half adder: Sum = A⊕B, Carry = A·B. Full adder: Sum = A⊕B⊕Cin.
Multiplexer: 2ⁿ inputs need n select lines. Decoder: n inputs give 2ⁿ outputs. Encoder is the reverse.

Sequential circuits:

Flip-flops: SR, JK, D, T. JK with J = K = 1 toggles. T is a toggle (divide-by-2). D copies its input on the clock. A latch is level-triggered and a flip-flop is edge-triggered.
Counter: n flip-flops count up to 2ⁿ states (mod-2ⁿ). A mod-10 counter needs 4 flip-flops. Asynchronous (ripple) counter is simpler but slower; synchronous counter is faster.
Shift register: moves bits along; used for serial-to-parallel conversion.

Converters: DAC resolution (step) = full scale/2ⁿ. A 4-bit DAC with 16 V full scale has 1 V per step. ADC types: flash (fastest), successive approximation (medium), dual slope (slow, accurate, used in digital voltmeters). Sampling must be above twice the highest signal frequency (Nyquist).

Logic families: TTL (bipolar, fast, 5 V), CMOS (very low power, wide supply). The 74xx series is TTL.

Microprocessor and microcontroller

Item	8085	8086	8051
Type	8-bit µP	16-bit µP	8-bit µC
Address bus	16-bit (64 KB)	20-bit (1 MB)	16-bit
Supply	+5 V	+5 V	+5 V

8085: 40-pin chip. Lower address lines AD0–AD7 are multiplexed with data; ALE (address latch enable) separates them. Registers: accumulator A, general B, C, D, E, H, L (pairs BC, DE, HL), program counter (PC), stack pointer (SP). Five flags: Sign, Zero, Auxiliary carry, Parity, Carry. Interrupts: TRAP (non-maskable, highest priority), RST 7.5, 6.5, 5.5 and INTR. The instruction HLT stops execution. The program counter holds the address of the next instruction.
Memory and I/O: memory-mapped I/O uses memory addresses; I/O-mapped uses IN/OUT instructions. The 8255 gives parallel ports, 8253/8254 gives timers, 8251 serial communication.
8051 microcontroller: on-chip ROM, RAM, timers, serial port and four 8-bit ports. A microcontroller has memory and I/O on one chip; a microprocessor needs external memory and ports.

Power electronics

Power devices: diode, SCR (thyristor), TRIAC (AC switching), GTO (turned off by negative gate current), power BJT, power MOSFET (fast switching, low voltage), IGBT (medium to high power, common in drives).

SCR: four layers, three junctions, terminals anode, cathode, gate. It turns on by a gate pulse (also by excess forward voltage, high dv/dt or temperature, which are unwanted). Latching current is slightly above holding current. It turns off when current falls below holding current (commutation). Natural commutation occurs in AC lines; forced commutation is needed in DC circuits. Snubber circuits protect against high dv/dt; series inductance limits di/dt.

Controlled rectifiers (phase-controlled), firing angle α:

Single-phase full-controlled bridge: Vdc = (2Vm/π) cosα. Example: Vm = 100 V, α = 60° gives 31.8 V. For α > 90° the output is negative (inverter mode with a source on the DC side).
Single-phase half-controlled bridge: Vdc = (Vm/π)(1 + cosα). Same values give 47.7 V. It cannot operate in inverter mode.
Three-phase full converter: Vdc = (3Vml/π) cosα with Vml the peak line voltage. Output is zero at α = 90°.

Choppers (DC to DC):

Type	Output
Step-down (buck)	Vo = D·Vs
Step-up (boost)	Vo = Vs/(1−D)
Buck-boost	Vo = −D·Vs/(1−D)

D = duty ratio = Ton/(Ton + Toff). Example: D = 0.4, Vs = 200 V gives 80 V; boost with D = 0.5, Vs = 100 V gives 200 V.

Inverters (DC to AC): single-phase full-bridge with square wave output has rms output equal to Vs. Fundamental rms = 0.9 Vs. Three-phase bridge inverter runs with 180° or 120° conduction. PWM (pulse width modulation) controls output voltage and removes low-order harmonics. AC voltage controller (phase-controlled regulator) varies rms voltage at constant frequency. Cycloconverter changes AC to AC of lower frequency. UPS uses rectifier, battery and inverter.

Electric drives

DC motor speed: N ∝ (V − IaRa)/φ. Below base speed vary armature voltage (constant torque); above base speed weaken the field (constant power). Example: V = 220 V, Ia = 10 A, Ra = 1 Ω, back emf = 210 V. Ward Leonard system uses a motor-generator set for smooth wide control. Modern drives use controlled rectifiers or choppers.
Four-quadrant operation: forward motoring, forward braking, reverse motoring, reverse braking. Braking types: dynamic (rheostatic), regenerative and plugging.
Induction motor drives: synchronous speed Ns = 120f/P (4 poles, 50 Hz: 1500 rpm). Methods: stator voltage control, V/f control (keeps flux constant; at 25 Hz a 400 V/50 Hz motor gets 200 V), rotor resistance control (slip ring motors), slip power recovery (Kramer and Scherbius drives). V/f control with a PWM inverter is the most common.
Other drives: BLDC motor with electronic commutation, stepper motor for position control, and traction drives.
Duty classes: continuous, short-time and intermittent.

Exam traps

Inverting gain is −Rf/Rin; non-inverting is 1 + Rf/Rin (never below 1).
Latching current is greater than holding current for an SCR.
A half-controlled bridge cannot invert; a full-controlled bridge can.
Buck lowers voltage; boost raises it.
A microprocessor needs external memory; a microcontroller has it on-chip.
TRAP is non-maskable; INTR is maskable.
8085 data bus is 8-bit and address bus 16-bit.
A latch is level triggered; a flip-flop is edge triggered.

One-liners

1. β = α/(1−α).
2. Half-wave rectifier ripple factor is 1.21.
3. NAND and NOR are universal gates.
4. A 4-bit counter has 16 states.
5. 8085 can address 64 KB.
6. The ALE signal separates address and data on the 8085.
7. Duty ratio D = Ton/(Ton + Toff).
8. Single-phase full converter Vdc = (2Vm/π) cosα.
9. TRIAC conducts in both directions.
10. IGBT combines MOSFET input and BJT output.
11. Ns = 120f/P.
12. Armature voltage control gives speed control below base speed.

Practice questions

The average output of a single-phase half-wave rectifier with peak input 100 V (ideal diode) is nearly
1. 63.7 V
2. 50 V
3. 70.7 V
4. 31.8 V
Answer
D. 31.8 V
Vdc = Vm/pi = 100/3.14 = 31.8 V.
The ripple factor of a full-wave rectifier without filter is about
1. 0.48
2. 1.21
3. 0.81
4. 0.21
Answer
A. 0.48
Full-wave ripple factor is 0.48; half-wave is 1.21.
The PIV of each diode in a centre-tap full-wave rectifier is
1. Vm/2
2. Vm
3. Vm/root 2
4. 2Vm
Answer
D. 2Vm
The full secondary voltage 2Vm appears across the non-conducting diode.
For a transistor with beta = 99, alpha is
1. 0.01
2. 0.99
3. 0.9
4. 1.01
Answer
B. 0.99
alpha = beta/(1+beta) = 99/100 = 0.99.
An inverting op-amp amplifier has Rf = 100 kilo-ohm and Rin = 10 kilo-ohm. The gain is
1. -11
2. -10
3. 10
4. 11
Answer
B. -10
Gain = -Rf/Rin = -10.
A non-inverting op-amp amplifier has Rf = 90 kilo-ohm and Rin = 10 kilo-ohm. The gain is
1. 10
2. -9
3. 9
4. -10
Answer
A. 10
Gain = 1 + Rf/Rin = 1 + 9 = 10.
The condition for sustained oscillations in an oscillator is that the loop gain is
1. infinite with 90 degree phase shift
2. zero
3. less than one
4. unity with zero phase shift
Answer
D. unity with zero phase shift
The Barkhausen criterion needs |A beta| = 1 and loop phase shift of 0 or 360 degrees.
Which oscillator gives the best frequency stability?
1. Wien bridge oscillator
2. Hartley oscillator
3. Crystal oscillator
4. RC phase-shift oscillator
Answer
C. Crystal oscillator
The quartz crystal has a very high Q and fixed resonant frequency.
A class B push-pull amplifier transistor conducts for
1. 270 degrees
2. 360 degrees
3. 90 degrees
4. 180 degrees
Answer
D. 180 degrees
Each transistor of a class B pair conducts for half the cycle.
The binary number 1011 equals decimal
1. 11
2. 13
3. 10
4. 9
Answer
A. 11
8 + 0 + 2 + 1 = 11.
The hexadecimal number 2F equals decimal
1. 37
2. 47
3. 31
4. 32
Answer
B. 47
2 x 16 + 15 = 47.
Which gate gives output 1 only when its two inputs are different?
1. XNOR
2. NOR
3. NAND
4. XOR
Answer
D. XOR
XOR output is high for unequal inputs.
Which pair of gates is universal?
1. XOR and XNOR
2. NAND and NOR
3. AND and NOT
4. AND and OR
Answer
B. NAND and NOR
Any Boolean function can be built using only NAND or only NOR.
A multiplexer with 16 data inputs needs how many select lines?
1. 2
2. 4
3. 8
4. 16
Answer
B. 4
2^n = 16 gives n = 4.
How many flip-flops are needed to build a mod-10 counter?
1. 3
2. 5
3. 4
4. 10
Answer
C. 4
2^3 = 8 is too few and 2^4 = 16 is enough.
A 4-bit DAC has a full-scale range of 16 V. The voltage step for one LSB is
1. 1 V
2. 0.5 V
3. 4 V
4. 1.6 V
Answer
A. 1 V
Step = 16 V/2^4 = 1 V.
Which ADC type is the fastest?
1. Dual slope
2. Counter ramp
3. Flash type
4. Successive approximation
Answer
C. Flash type
Flash uses parallel comparators and converts in one step.
The 8085 microprocessor can directly address a memory of
1. 64 KB
2. 16 KB
3. 256 bytes
4. 1 MB
Answer
A. 64 KB
A 16-bit address bus gives 2^16 = 65,536 locations.
In the 8085, the lower eight address lines are separated from the data lines by the signal
1. WR
2. RD
3. HOLD
4. ALE
Answer
D. ALE
Address latch enable latches AD0-AD7 as address.
Which interrupt of the 8085 is non-maskable?
1. RST 5.5
2. INTR
3. RST 6.5
4. TRAP
Answer
D. TRAP
TRAP has the highest priority and cannot be disabled.
The register in the 8085 that holds the address of the next instruction is the
1. stack pointer
2. flag register
3. program counter
4. accumulator
Answer
C. program counter
PC points to the next instruction byte to be fetched.
A single-phase fully controlled bridge rectifier has Vm = 100 V and firing angle 60 degrees (continuous current). The average output is nearly
1. 31.8 V
2. 47.7 V
3. 100 V
4. 63.7 V
Answer
A. 31.8 V
Vdc = (2Vm/pi) cos 60 = 63.7 x 0.5 = 31.8 V.
A single-phase half-controlled bridge has Vm = 100 V and alpha = 60 degrees. The average output is nearly
1. 95.4 V
2. 47.7 V
3. 31.8 V
4. 63.7 V
Answer
B. 47.7 V
Vdc = (Vm/pi)(1 + cos 60) = 31.83 x 1.5 = 47.7 V.
A step-down chopper has a source of 200 V and duty ratio 0.4. The average output is
1. 500 V
2. 120 V
3. 80 V
4. 200 V
Answer
C. 80 V
Vo = D x Vs = 0.4 x 200 = 80 V.
A boost chopper has a source of 100 V and duty ratio 0.5. The output voltage is
1. 50 V
2. 150 V
3. 100 V
4. 200 V
Answer
D. 200 V
Vo = Vs/(1 - D) = 100/0.5 = 200 V.
A single-phase full-bridge inverter with a 200 V DC supply and square-wave output has a fundamental rms voltage of nearly
1. 141 V
2. 180 V
3. 200 V
4. 254 V
Answer
B. 180 V
V1rms = 0.9 Vs = 180 V (the peak fundamental is 4Vs/pi = 255 V).
A four-pole induction motor is supplied at 50 Hz. Its synchronous speed is
1. 1500 rpm
2. 1000 rpm
3. 3000 rpm
4. 750 rpm
Answer
A. 1500 rpm
Ns = 120 x 50/4 = 1500 rpm.
A 400 V, 50 Hz induction motor is run at 25 Hz with constant V/f ratio. The supply voltage should be
1. 400 V
2. 100 V
3. 800 V
4. 200 V
Answer
D. 200 V
V/f = 8 V/Hz so V = 8 x 25 = 200 V.
A DC motor on 220 V has armature current 10 A and armature resistance 1 ohm. The back emf is
1. 210 V
2. 10 V
3. 220 V
4. 230 V
Answer
A. 210 V
Eb = V - Ia Ra = 220 - 10 = 210 V.
Speed control of a DC motor above base speed is obtained by
1. reducing armature voltage
2. reducing the field flux
3. adding armature resistance only
4. reversing the supply
Answer
B. reducing the field flux
N is inversely proportional to flux; field weakening gives constant-power operation.
Which device can be turned off by a negative gate current?
1. TRIAC
2. SCR
3. GTO
4. Diode
Answer
C. GTO
The gate turn-off thyristor has gate turn-off capability.
The minimum anode current that keeps an SCR on after the gate pulse is removed is the
1. breakover current
2. holding current
3. latching current
4. leakage current
Answer
C. latching current
Latching current is the value needed to stay on after the gate drive ends; holding current keeps it on once conducting.
Consider the statements on operational amplifiers. 1. The non-inverting gain can be less than one. 2. An ideal op-amp has infinite input resistance. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Gain 1 + Rf/Rin is always at least 1.
Consider the statements on controlled rectifiers. 1. A half-controlled bridge can work as an inverter. 2. A fully controlled bridge can work as an inverter. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
The half-controlled bridge has a freewheeling path and cannot give negative average voltage.
Consider the statements on the 8085. 1. It has an 8-bit data bus. 2. It has a 16-bit address bus. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
The 8085 is an 8-bit microprocessor with 64 KB address space.
Consider the statements on flip-flops. 1. A T flip-flop toggles when T = 1. 2. A JK flip-flop with J = K = 1 toggles. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both give a toggling output on each clock pulse.
Consider the statements on choppers. 1. A boost chopper output is always lower than the source. 2. A buck chopper output is always lower than the source. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Boost gives Vs/(1 - D), which is at least Vs.
Consider the statements on negative feedback in amplifiers. 1. It increases gain. 2. It reduces distortion. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Negative feedback lowers gain but improves stability, bandwidth and distortion.
Consider the statements on the microcontroller 8051. 1. It has no on-chip RAM. 2. It needs an external timer chip for its basic timing. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
It has on-chip timers, ports and serial port.
Consider the statements on induction motor drives. 1. V/f control keeps the air-gap flux nearly constant. 2. Rotor resistance control is possible in squirrel cage motors. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Rotor resistance can be varied only in slip-ring motors.
Consider the statements on SCR. 1. The holding current is greater than the latching current. 2. It conducts in one direction only. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Latching current exceeds holding current; the SCR conducts only anode to cathode.
Consider the statements on the three-phase fully controlled converter. 1. The average output voltage is proportional to cos alpha. 2. It is zero at alpha = 90 degrees (continuous current). Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Vdc = (3Vml/pi) cos alpha, which is zero at 90 degrees.
Consider the statements on the 8085. 1. TRAP is a maskable interrupt. 2. The HLT instruction increases the program counter by 16. Which is/are correct?
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
D. Neither 1 nor 2
TRAP cannot be masked; HLT stops execution.
Match the device with its use. P) TRIAC Q) 555 timer R) Zener diode S) IGBT
1. P-AC switching, Q-pulse generation, R-voltage regulation, S-motor drives
2. P-voltage regulation, Q-AC switching, R-motor drives, S-pulse generation
3. P-pulse generation, Q-motor drives, R-AC switching, S-voltage regulation
4. P-motor drives, Q-voltage regulation, R-pulse generation, S-AC switching
Answer
A. P-AC switching, Q-pulse generation, R-voltage regulation, S-motor drives
TRIAC is a bidirectional switch; 555 makes pulses; Zener regulates; IGBT is used in drives.
Match the converter with its function. P) Rectifier Q) Chopper R) Inverter S) Cycloconverter
1. P-DC to DC, Q-AC to DC, R-AC to AC, S-DC to AC
2. P-AC to AC, Q-DC to AC, R-DC to DC, S-AC to DC
3. P-DC to AC, Q-AC to AC, R-AC to DC, S-DC to DC
4. P-AC to DC, Q-DC to DC, R-DC to AC, S-AC to AC
Answer
D. P-AC to DC, Q-DC to DC, R-DC to AC, S-AC to AC
These are the standard conversions.

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