Chapter 8

Question 1

An op amp is a circuit that provides

Answer 1

A high voltage gain and an output proportional to the difference between the two inputs

Question 2

Op amp producing a moderate output swing requires

Answer 2

Only a small input difference due to its high voltage gain

Question 3

Inverting amplifier configuration

Answer 3

Inverting input - virtual ground

Question 4

Integrator applications

Answer 4

Analog filters, analog-to-digital converters

Question 5

Differentiators

Answer 5

High noise

Question 6

An inverting configuration using multiple input resistors tied to virtual ground node can serve as

Answer 6

A voltage adder

Question 7

Placing a diode around an op amp leads to a

Answer 7

Precision rectifier (can rectify very small input swings)

Question 8

Placing a bipolar device around an op amp provides

Answer 8

A logarithmic function

Question 9

Op amps suffer from various imperfections

Answer 9

Dc offsets

Input bias currents

Question 10

The speed of op amps are limited by

Answer 10

The bandwidth of op amps, finite slew rate for large signals

Question 11

Op-amp applications

Answer 11

Amplifiers, limiters, oscillators

Question 12

Ideal op-amp properties

Answer 12

Rin = infinity
V1=V2 (negative feedback)
Ao = infinity
Slew rate infinite
Rout = 0

Question 13

Unity gain buffer

Answer 13

If Ao increases, Vout = V1

Question 14

Non-inverting amplifier

Answer 14

Such a configuration makes it immune to manufacturing defects

Less affected by variations in resistance because gain is function of R1/R2

Question 15

Virtual ground

Answer 15

Can’t sink current (Vx)

Question 16

Open loop gain

Answer 16

Ao

Question 17

Closed loop gain

Answer 17

Av

Question 18

Integrator

Answer 18

Pole at origin

Acts as a low pass filter

Slope past pole is constant -20dB/dec

Question 19

The pole can be found by

Answer 19

Setting denominator to 0

Question 20

Differentiator (Ao = infinity)

Answer 20

Zero at origin

Slope of H(s) changed by +20dB/dec after a zero

High pass filter

Question 21

Non-linear function

Answer 21

Precision rectifier

Logarithmic amplifier

Square root amplifier

Question 22

Precision rectifier

Answer 22

Similar to unity gain buffer (w/o diode)

Normal diode based rectifiers need the input to be at least 0.7V to forward bias the diode

Vin=0, Vout=0.7V and Vout=Vin
-as Vin increases, Vout rises as well and Vout=Vin

Vout - drop across R1

Negative cycle of input - current should flow from ground to R to diode to op-amp
(Not possible due to the diode, Vout=0)

Question 23

Logarithmic amplifier

Answer 23

Vin negative - circuit will not operate (open loop circuit)

Question 24

Square root amplifier

Answer 24

If Vin increases, then Vout is more negative, Vgs more positive, and Vgs > Vt. MOSFET operates in saturation and I increases

Question 25

Input bias current

Answer 25

Op-amps are generally based on BJTs, which tend to draw small base current (which may cause inaccuracies)

Question 26

Offset voltage

Answer 26

Couple capacitors block DC, hence offset voltage

Question 27

Slew rate

Answer 27

V/us Slope of maximum allowable ramp Occurs when rate is too high and the fastest the op-amp can respond (causing delay and distortion) Input high - rate will be high as well so that may lead to output lagging input Generally 10V/us Happens because internal circuit acts like current source charging capacitors

Question 28

Op-amp bandwidth

Answer 28

Open loop gain not independent of frequency Finite BW makes it act like a capacitive circuit, gain decreases Non-infinite Ao pushes the pole further (BW increases, gain decreases) Higher the gain, the unity gain BW will be low

Question 29

Unity gain BW

Answer 29

Output = input Not useful is below 1 (not amplifying)