Basic Electronics EC21101 Autumn 2009

Basic Electronics EC21101 Autumn 2009-2010Class test 2 Solutions1. An amplifier with voltage series feedback is having an overall voltage gain of 80. If the voltage gain of the internal amplifier is increased by a factor of 5, the increase in the overall voltage gain of the amplifier with feedback is observed to be only 5%. Find out the gain of the internal amplifier (without feedback) and the feedback factor.

Voltage gain with feedback, Av,f = Av / (1 + Av().

Thus

80 = Av / (1 + Av()

. . . . (1)

and

when the voltage gain Av is increased by a factor of 5, then

84 = 5Av / (1 + 5Av()

. . . . (2)

Solving equations (1) and (2), we get

Av = 1344 and ( = 0.01175595. (Ans)2. a) A 12 bit ADC has LSB of 1mV, find SNR in dB and rms quantization noise. b) A 24V DC motor needs digital voltage control to vary its speed with an accuracy of 0.1%. Estimate minimum number of bits required by DAC.

c) A 2 bit binary counter is realized with a ROM, Latch and clock input. The ROM has one mode control input M. When M=0 counter will count up and with M=1 it will count down. i. What should be the size of the ROM?

ii. Write the content of the ROM with Address and Data output for all states. a)

N=12

SNR = 6.02N + 1.76 dB = 6.02 x 12 + 1.76 = 74 dB

LSB = 1mV

RMS quantization noise = LSB / (12)0.5 = 1 / 3.46 = 0.288 mV

b)

Number of steps required = 24/(0.001x24) = 1000

Nearest binary number that is higher than 1000 is 210 = 1024.

Number of bits required by DAC = 10 bits.

c)

Two bit binary counter values for up count (when M=0) and down count (when M=1) are, M=A3 A2 A1

Out1Out2

000

01

001

10

010

11

011

00

100

10

110

01

101

00

100

11

LHS columns are addresses to the memory locations in the ROM and RHS are the outputs from the stored memory locations.Total memory size required is 23 x 2 = 8 x 2 bits = 16 bits

(for similar example please ref. to pages 468, 469 of Lessons in Electronics circuit vol. IV Digital , by Tony.R. Kuphaldt)

Out1

Out2

A1

A2

A3

M

Clock

ROM

Latch

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