1. Applications of operational Amplifiers
UNIT 3
Applications of operational Amplifiers

2. Oscillators Oscillators are circuits that generate periodic signals
An oscillator converts DC power from the power supply into AC signal power spontaneously - without the need for an AC input source
Figure Repetitive ramp waveform.

3. Linear Oscillators
Figure A linear oscillator is formed by connecting an amplifier and
a feedback network in a loop.

4. Barkhausen Criterion
Figure Linear oscillator with external signal Xin injected.

5. Barkhausen Criterion

6. How does the oscillation get started?
Noise signals and the transients associated with the circuit turning on provide the initial source signal that initiate the oscillation

7. Practical Design Considerations
Usually, oscillators are designed so that the loop gain magnitude is slightly higher than unity at the desired frequency of oscillation
This is done because if we designed for unity loop gain magnitude a slight reduction in gain would result in oscillations that die to zero
The drawback is that the oscillation will be will be slightly distorted (the higher gain results in oscillation that grows up to the point that will be clipped)

8. Example of Oscillator circuit (1)
Figure Typical linear oscillator.

9. Example of Oscillator circuit (2)
Figure Feedback network. Note that the input to the network
is on the right-hand side and the response is on the left-hand side.

10. Wien-Bridge Oscillator
Figure Wien-bridge oscillator.

11. Wien-Bridge oscillator output
Figure Example of output voltage of the oscillator.

12. Methods for Amplitude Stabilization
In a linear oscillator, amplitude stabilization below the amplifier clipping level is needed to reduce distortion
We can reduce the amount of distortion by reducing the amplifier gain
However, if the gain becomes too small the oscillations die out
Several approaches can be used to resolve the conflict
The easiest approach is to make the gain adjustable by using a potentiometer in place of the resistors that determine the gain

13. Inverting Amplifier R vo Rf R v R
Inverting Amplifier o
10 V i Rf i Ri 
-10 V
10 V i o i +
Slope = -R / Ri f
(a)
-10 V R vo Rf
(b) f i R
v R i i i
(a) An inverting amplified. Current flowing through
the input resistor R also flows through the i
feedback resistor R . f
(b) The input-output plot shows a slope of -R / R in f i
the central portion, but the output saturates at
about ±13 V.

14. Non-inverting AmplifierRi Rf
Slope = (R + R )/ R f i i
-10 V
10 V i - o
-10 V i +
R  Ri
R  R  R  f f i f
vo  vi
G 
 1    Ri Ri R   i

15. Summing Amplifier

16. Summing Amplifier  v v  v  R     R R   Rf R1 1 R2 o 2 1 2+ 
v v 
v  R  1  2   o f
R R   1 2

17. Summing Amplifier

18. Non Inverting Summer

19. Adder-Subtractor

20. Op-amp Differentiator
Applying KCL at inverting node of opamp, we get
(0-V )/R + I = 0
out c
I = V /R c
out
where I = C*d(0-V )/dt. Hence we get V = -R*C*dV /dt. c in
out in

21. Op-amp Differentiator

22. Improved Op-amp Differentiator

23. Opamp Integrator Applying KCL at inverting node of opamp, we get
(0-V )/R + I = 0
out c
Ic = Vout/R= 1/C

24. Practical Integrator

26. Differential Amplifier

28. Differential Amplifiers
Differential Gain Gd  v3 vo R4
G   d
v  v R 3 v4 4 3
Common Mode Gain Gc 
◦ For ideal op amp if the inputs are equal
then the output = 0, and the G = 0. c R 4
◦ No differential amplifier perfectly rejects
v  (v  v ) o 4 3
the common-mode voltage. R 3
Common-mode rejection ratio CMMR 
◦ Typical values range from 100 to 10,000
CMRR Gd Gc
Disadvantage of one-op-amp differential amplifier
is its low input resistance

32. An instrumentation amplifier is used to measure and
control physical quantities such as temperature,
humidity, light intensity, and waterflow.

33. signal from the transducer
 Instrumentation amplifier is the front end component of every measuring
instrument which improves the signal to noise ratio of the input electrical
signal from the transducer

34. Instrumentation Amplifiers
Differential Mode Gain
v  v  i(R  R  R ) 3 4 2 1 2
v  v  iR 1 2 1
v  v 2R  R 1
G  3 4  2
Advavntagves: HigRh input impedance, High CMRR, d 1 2 1
Variable gain

35. Instumentation Amplifier

39. Follower ( buffer) resistance from being loaded down by a
 Used as a buffer, to prevent a high source
resistance from being loaded down by a
low-resistance load. In another word it
prevents drawing current from the source.  o i +
vo  vi
G 1

40. Current to Voltage Converter
(Transresistance Amplifier)

42. Voltage to Current Converter
(Transconductance Amplifier)

43. Voltage to Current Converter
(Transconductance Amplifier)

44. Transconductance amplifier with floating Load

45. Filter passes signal of specified Band of frequencies and the band
 Filter is a frequency selective circuit that
passes signal of specified Band of frequencies
and
 attenuates the signals of frequencies outside
the band
 Type of Filter
 1. Passive filters
 2. Active filters

46. low Q,resulting in high power dissipation
 Passive filters
 Passive filters works well for high frequencies. But at audio frequencies, the
inductors become problematic, as they become large, heavy and expensive.
 For low frequency applications, more number of turns of wire must be used
which in turn adds to the series resistance degrading inductor’s performance ie,
low Q,resulting in high power dissipation
 Active filters
 Active filters used op- amp as the active element and resistors and capacitors as
passive elements.
 By enclosing a capacitor in the feed back loop , inductor less active filters can be
obtained

47. • Active filters use op-amp(s) and RC components.
• Advantages over passive filters:
– op-amp(s) provide gain and overcome circuit losses
– increase input impedance to minimize circuit loading
– higher output power
– sharp cutoff characteristics some commonly used
active filters
1. Low pass filter
2. High pass filter
3. Band pass filter
4. Band reject filter
Active Filters
can be produced simply and efficiently without bulky
inductors

48. Review of Filter Types & Responses
• 4 major types of filters: low-pass, high-pass, band
pass, and band-reject or band-stop
• 0 dB attenuation in the pass band (usually)
• 3 dB attenuation at the critical or cutoff frequency,
fc (for Butterworth filter)
• Roll-off at 20 dB/dec (or 6 dB/oct) per pole outside
the passband (# of poles = # of reactive elements).
Attenuation at any frequency, f, is
• Bandwidth of a filter: BW = fcu - fcl
• Phase shift: 45 /pole at fc; 90o/pole at >> fc o
• 4 types of filter responses are commonly used:

49. Types of filters •Butterworth - maximally flat in passband;
highly non-linear phase response with
frequency
•– Bessel - gentle roll-off; linear phase shift
with freq.
•– Chebyshev - steep initial roll-off with
ripples in passband
•– Cauer (or elliptic) - steepest roll-off of the
four types but has ripples in the passband
and in the stop band

50. Frequency response of filters

51. I order Active LPF

54. I order Active HPF

56. Wide BPF