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Op amp problems and solutions, Exercises of Analog Electronics

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Typology: Exercises

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EXERCISE IDEAL OP AMP ANALYSIS
Ideal Op Amp Exercise Rev. 1/6/2003 C. Sauriol Page 1
EXERCISE IDEAL OP AMP ANALYSIS
No.1 Assuming ideal op amps, determine Vo for each and every circuit shown below.
2k
10k
1k
30k
1k
2k
10k
2k 24k
3k
2K
2K
10K
10K
14k
16k
15k
15k
10k
3.9K
1,5k
3k
5,1k
2k
-2 V
Vo
1,8k
Vo
+0,1V
Vo
-0,5V
Vo
+1V
-1,5V
Vo
+15V
-15V
Vo
-2 V
+5V
+8V
A) B)
C) D)
E) F)
pf3
pf4
pf5
pf8

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EXERCISE IDEAL OP AMP ANALYSIS

No.1 Assuming ideal op amps, determine Vo for each and every circuit shown below.

2k

10k

1k

30k

1k

2k

10k 2k 24k

3k

2K

2K

10K

10K

14k

16k

15k

15k

10k

3.9K

1,5k

3k

5,1k

2k

-2V

Vo

1,8k

Vo

+0,1V

Vo

-0,5V

Vo

+1V

-1,5V

Vo

+15V

-15V

Vo

-2V

+5V

+8V

A ) B)

C) D)

E) F)

No.2 Assume typical op amp data for circuits A through E and worst case values for circuit F.

Op amp parameters for VSUP =±15V minimum typical maximum

O/P voltage swing ±12V ±13,5V -

I/P voltage range ±11V ±12,5V -

Short circuit current ±12 mA ±20 mA -

30K

4,7k

3,3k

1,8k

10k

V x

  • 6 V
  • 1 V
    • 8 V

V o

  • 1 5 V
  • 1 5 V
  • 2 V

  • 3 V

10k 20k

6,8k V^ o

  • 1 6 V
  • 1 6 V

Vi n

1K 2k

680 V o

  • 1 6 V
  • 1 6 V

2 0 0

  • 2 V

10K

6,8k (^) Vo

1 0 K

  • 1 2 V
  • 1 2 V
  • 5 V

10k 100k

10k (^) V o

  • 1 6 V
  • 1 6 V

V 1

V 2

R 3 1 0 0 K

R 3

A) Determine V x

B) Determine V (^) o for V (^) in =+6V and V (^) in =-6V.

C) Determine V (^) o.

D) Determine V (^) o.

E) Determine V (^) o.

F) Determine the maximum value of R 3 if we do not want to saturate the inputs of the op amp given that V and V2 range from 80V to100V.

SOLUTIONS

+ 10V -

2k

10k

1k

30k

1k

2k

10k 2k 24k

3k

2K

2K

10K

10K

14k

16k

15k

15k

10k

3.9K

1,5k

3k

5,1k

2k

- 2 V

Vo

1 , 8 k

Vo

+0 , 1 V

Vo

- 0 , 5 V

Vo

+1 V

- 1 , 5 V

Vo

+1 5 V

- 1 5 V

Vo

- 2 V

+5 V

+8 V

B)

C) D)

E) F)

0 A

1 mA

  • 1 mA 2 V

- 2 V

- 2 V

- 1 2 V

0 A

0 V

0 V

0 , 1 mA

0 , 1 mA 0 A 0 A

+ 3 V -

- 3 V

0 A

0 A

- 0 , 5 V

- 0 , 5 V

0 , 2 5 mA

mA

+ 2 , 5 V -

- 3 V

0 , 1 2 5 mA

0 , 3 7 5 mA

1 , 1 2 5 V

- 4 , 1 2 5 V

- 1 , 2 5 V

- 1 , 2 5 V

0 , 1 2 5 mA

0 , 1 2 5 mA

+ 2 , 2 5 V -

mA

mA

+ 1 1 , 2 5 V -

- 1 2 , 5 V

0 A

0 A

0 A

0 A

0 A

0 A

1 mA

1 mA

1 6 V

1 4 V

+1 V

+1 V

0 , 1 3 3 mA

mA

mA

0 , 1 3 3 mA

- 1 , 3 3 3 V +

+2 , 3 3 3 V

1 , 6 mA

+3 , 2 V

+3 , 2 V

+ 1 , 8 V -

- 5 , 2 V + - 0 , 6 8 V +

+3 , 8 8 V

1 , 2 mA 1 , 2 mA

mA (^) 0 , 1 3 3 3 mA

No.1 A )

4,7k

3,3k

1,8k

10k

Vx

- 3 .6 V

+6 V

- 1 V - 8 V

- 1 V

- 1 V

0 A

0 A

mA

0 , 7 8 9 mA

mA

- 2 .6 V +

+ 7 V -

No.2 A)

10k 20k

6,8k

Vo

+1 6 V

- 1 6 V

Vi n

0 A

+6 V 0 A

+6 V

- 1 4 , 5 V

mA

- 4 , 8 3 3 V

B)

10k 20k

6,8k

Vo

+1 6 V

- 1 6 V

Vi n

0 A

0 A

mA

- 6 V

- 6 V

+1 4 , 5 V

+4 , 8 3 3 V

Vo

+1 5 V

- 1 5 V

+2 V

+3 V

0 A

+3 V

- 1 3 .5 V

C)

No feedback, the output saturates with

a polarity determined by the sign of the

differential I/P voltage:

Vo = A d (V + -V - ) = (2-3) = -

Vo = -V sat = -13,5V

Positive feedback will make the output

saturate with a polarity determined by

the sign of the differential I/P voltage.

With +6V applied to the -ve I/P of the

op amp, the O/P should be of the opposite

polarity, therefore assume

Vo = -V sat = -14,5V and determine the

V+ and verify the sign of the differential

I/P voltage in order to validate your

assumption of V o = -V sat, that is:

Vo = A d (V + -V - ) = (-4,83-6) = -

therefore the assumption was valid.

Same procedure here, except now V -^ is

negative, therefore the O/P polarity is

expected to be positive:

Vo = +V sat = +14,5V

Verify assumption with sign of V d = (V + -V - )

Vo = A d (V + -V - ) = (+4,83-(-6)) = +

No.

If F

10 2 π RF C (^) F

=

10 2 π 100 k × 0,1 μ

= 159 Hz thenV (^) o ( PP ) = − (^) R^1

( ) E C F

Vin ( AC ) dt t 1

t 2

A)

V ( ave ) =^ V

  • (^) PW

( ) T +^ V^

− (^) SW

( ) T

Vin(DC) = +0,2V

V T

V area

V V dt

o PP

o PP

t

t

o PP in AC

1000 0. 8 0. 6

1000

1000

( )

( )

( ) ( )

2

1

∆ =− × ×

∆ =− ×

Integral does not apply for 50

Hz and 100 Hz.

0 , 6 T 0 , 4 T

-2V DC

Vin

+1V

-1V

Vout

∆V out = 0,48Vpp at 1 kHz

and 48 mVpp at 10 kHz

+0,2V

DC

AREA

AREA

B)

P PP

o PP

t

t

o PP in AC

V F

V

T

V area

V V dt

1 2

1000 1 2

1000

1000

1000

( )

( ) ( )

2

1

= × × = =

∆ =− ×

F = 500 Hz

F > 159 Hz integral OK.

2 V (^) pp DC level

T 2

AREA

AREA

I N P U T

1 V (^) pp DC level

O U T P U T

C)

F Hz

V F

T

V area

V V dt

PP

o PP

t

t

o PP in AC

375

1

375 4

3 1000 0 , 5

1000

1000

( )

( ) ( )

2

1

=

= = 

  

 × ×

∆ =− × =

F > 159 Hz integral OK.

2 V DC level pp 3 T 4

I N P U T

1 V (^) pp DC level

AREA A R E A O / P

+1V

-1V

V (^) ( ave ) = V

  • (^) PW

( ) T +^ V^

− (^) SW

( ) T Vin(DC)^ = +0,5V

No.3 D)

Vo ( PP ) = − 1000 Vin ( AC ) dt t 1

t (^) 2

Vo ( PP ) = − 1000 × area

= 1000 ×

100 μ × 5 2

 

 

Vo ( PP ) = 0,25 VPP

O/P is a parabolic wave, not a

sine wave.

DC level

DC level

10 VPP

0. VPP

100 μμμμ s I N P U T O U T P U T

No.4 A)

V (^) o = − RF CE

dVin dt

V (^) o = − 10 K × 0,1μ ±

10 V 2 ms

 

 

V (^) o = m 5 V (^) P^2 ms

10 V (^) pp

-5V p

+5V p

I

N

P

U

T

10 V (^) pp

O

P

+5V p

-5V p

B) On edges we have:

V o = − 10 K ×^ 0,1μ^ (±∞ )

V (^) o =^ m∞^ ⇒^ m^ V (^) sat

On flat portions we have:

V o = − 10 K × 0,1μ (± 0 )

V (^) o = 0

The O/P spikes settle down

in 5τ = 5RE C E = 50 μs

2 ms

  • 1 V p

+1Vp

0V

50 μs

50 μs

- V sat

+ V sat

I N P U T O U T P U T

C) To stabilise negative feedback in order to avoid self oscillations from the circuit.

No. 5 Phase shifter C o

X P ATAN 180 0 1 0 2 = 

  

 = Φ= ×

( )

( ) ( )

nF nF std FC TAN k TAN

P

P FCP

X TAN P

X ATAN P

X ATAN

o o

o C o C o C

  1. 026 9. 1 2 1000 100 10

1 2 10

1

2

1 20 2 10 10

1

1 1 1 1

= ⇒ × ×

= ×

=

 ⇒ = = 

  

  ⇒ = 

  

 Φ = = ×

π π

π

The +ve input sees 0 to 100k DC wise, average of 50k, and the –ve input sees R 1 IIR 1 = R 1 /2 = 50k

R 1 = 100K. This means that O/P DC offset will not be minimized for all P 1 settings.