Operation amplifier

1
Operation amplifier
The tail, large impedance gives high CMRR
Push-pull class B amp
Mirror as active load. High gain
amplifier
Follower as buffer
2
Operational amplifier

• Op-amp
• Differential amp non-linear amp output driver
• Op-amp has very high gain (A)
• But the gain is non-linear (because IC vs VBE is
  not linear)
• Use global feedback to build linear amplifier
• Exact value of the gain of the op amp is not
  important, as long as it is very large

3
Feedback Op amp golden rules

• Approximations
• 1. Voltage difference between the two inputs is
  zero
• Input draws no current
• Why?
• VOUT A VIN
• If VOUT is around 10V, A10,000, then VIN 1mV,
  voltage difference 1mV (close to zero)
• FET draws virtually zero input current

VIN
VOUT
4
Gain of this amplifier?
Ve
5
An easier method

• Because A is so large that Ve 0 (known as
  virtual earth)

Ve 0
6
Inverting amplifier

• Input impedance (RIN)?
• Ve 0, so RIN R1
• Important
• gain is determined by external parameters that we
  can control (the resistors)
• gain (A) of the op amp can be non-linear, its
  exact value is not important, as long as it is
  very large

7
Negative feedback

• Feedback MUST be applied to the ve terminal
  (ve feedback), so that Ve 0 (IMPORTANT)

B) Doesnt Work
A) Works
8
Positive feedback

• ve VIN produces large ve VOUT
• ve VOUT produces ve Ve
• which produces an even larger ve output.
• output reaches max ve supply voltage very
  quickly
• useless as an amplifier (but good as a switch,
  small ve signal produces large maximum output)

9
Non-inverting amplifier

• RIN?
• infinite !
• V-?
• V- VIN (Ve 0)
• Current through R1?
• IVIN / R1
• Output voltage?

Ve
V-
10
Voltage follower

• Left side circuit (work)
• If output is too large, then feedback to the ve
  terminal will reduce output to a equilibrium
  level until Ve 0
• Right side circuit (doesnt work)
• If output is too large, then feedback to the ve
  terminal will make the output even larger until
  saturation

11
Power booster

• The output current of an op amp is usually small
• If you want larger output current, add an
  external push-pull follower made of discrete
  power transistors

12
Power booster

• Normally push-pull follower has cross-over
  distortion
• Not this one !
• The gain of the amplifier depends on the external
  feedback circuit
• The forward gain of the internal circuit can be
  non-linear but must be large

13
Power of feedback

• Can put in anything in the forward loop, still
  get good result

14
Funny circuits

• What is the function of this circuit?

15
Funny circuits

• Does this circuit work?

16
Practical circuits

• Design an inverting amplifier with a gain of
  -100, to be driven by a source whose output
  impedance is 1MW
• What is the value of R? How about 100R?

17
Practical circuits

• A better solution

18
Practical circuits

• Summing circuit (an adder)

VIN
VOUT
19

V1
V2
I
V3
20
Practical circuits (a simple digital-to-analog
converter)

• VOUT A2B-3C

21
Practical circuits

• Perfect current source using imperfect FET

V-VIN
22
Active rectifier

• The simple passive rectifier has 0.6V drop
  between input and output

23
Active rectifier without the diode drop (optional)

• The active rectifier does not have the 0.6V drop
  !!
• VIN gt0, VOUT is ve, diode is short-circuit
• Feedback gt VOUT VIN (no 0.6V DC offset)

24
Problem

• ve input to V, VOUT , diode is open circuit
• VOUT pushes to ve extreme
• When input becomes ve, takes a long time for
  the output to move from ve to ve (limited by
  the slew rate)

25
Active rectifier (optional)

• This improved circuit prevents the rectifier from
  saturation

26
Negative-impedance converter (NIC)

• An interesting two-terminals active device
• apply a ve voltage, you see a current flowing
  OUT
• a negative impedance (!!)
• i.e. ZIN - Z !!
• (leave the proof as exercise)

27
Gyrator

• The following circuit is a gyrator
• (leave the proof
  as exercise)

28
Gyrator

• Use of gyrator
• can turn a capacitor into inductor !!
• How?
• ZIN behaves as an inductor with L CR2
• The use
• replace bulky inductors by small gyrated
  capacitors
• useful in integrated circuit design and small
  devices

29
Positive feedback

• Apply feedback to the non-inverting (ve)
  terminal
• small ve input produces a larger ve output
• output is feedback to the non-inverting input
• produces an even larger output
• quickly push the output to saturation
• Uses
• Comparators
• Oscillators

30
Comparator a simple switch

• Output 15V if VIN lt 0
• Output -15V if VIN gt 0

15V
VIN
-
VOUT
-15V
31

• A poor comparator because
• if the input is noisy, the output makes several
  transitions (switching noise)

32
311 op amp

• The output stage (emitter follower) is connected
  to external power supply
• User can choose the output voltage they like

33
Schmitt trigger (an inverter switch, high VIN,
low VOUT)

• Hysteresis (circuit has memory)
• the output switches at two separate threshold

34
Analysis

• If VOUT high (5V)
• VOUT 5V,
• Switching threshold V 5V
• When VIN gt 5V
• VOUT 0V
• Switching threshold V4.76V
• Now if VIN drops slightly below 5V
• VOUT wont change state
• Eliminate noisy switching

35
Analysis

• VIN gt 5V, VOUT0V, V 4.76V
• If VIN lt 4.76V, VOUT5V,
• V 5V (the new threshold)

36
Positive feedback

• if phase shift 180o in box X at certain
  frequency, then the feedback becomes ve
  (unstable) !!
• This high frequency has sustained itself even
  there is no input
• oscillation

37
Nasty oscillators

• A follower driving a long cable (a common
  problem)
• phase shift through the op amp 180o
• cables capacitance adds another 90o
• The internal capacitance at high frequency adds
  another 90o
• The follower oscillates !

(cable capacitance)
38
Ways to break the oscillation

• Op amp oscillates at high frequency because of
  the additional phase shift introduced by its
  internal capacitor
• Solution
• Reduce the loop gain AB at high frequency
• At the frequency that may cause oscillation, make
  A 0, so that the loop gain (AB) lt 1
• Op am that does this is called frequency
  compensated op amp
• Disadvantage is that the max bandwidth is reduced

39

• One oscillates, the other not
• 411 is frequency compensated, never oscillates

411
311
May oscillates
Never oscillates
A
A
f
f
focc
40
Slew rate

• The rate that output can change
• Wider bandwidth -gt higher frequency -gt faster
  slew rate
• 311 has faster slew rate, good as a switch

41

• What is the use of these two resistors?