operational amplifier (opamp)
DESCRIPTION
For Engineering StudentsTRANSCRIPT
Opamp
The 741 Op-Amp Circuit
• Schematic diagram of OP-AMP consists:
• The input stage
• The intermediate stage
• The output stage
• The biasing circuits
Schematic diagram of lm741
• 24 transistors, few resistors and only one capacitor
• Two power supplies
• Short-circuit protection
General Description
• The input stage consists of transistors Q1 through Q7.
• Q1-Q4 is the differential version of CC and CB configuration.
• High input resistance.• Current source (Q5-Q7) is the active load of
input stage. It not only provides a high-resistance load but also converts the signal from differential to single-ended form with no loss in gain or common-mode rejection.
The Input Stage
• The intermediate stage is composed of Q16, Q17 and Q13B.
• Common-collector configuration for Q16
gives this stage a high input resistance as well as reduces the load effect on the input stage.
• Common-emitter configuration for Q17 provides high voltage gain because of the active load Q13B.
The Intermediate Stage
• The output stage is the efficient circuit called class AB output stage.
• Voltage source composed of Q18 and Q19 supplies the DC voltage for Q14 and Q20 in order to reduce the cross-over distortion.
• Q23 is the CC configuration to reduce the load effect on intermediate stage.
The Output Stage
(a) The emitter follower is a class A output stage.
(b) (b) Class B output stage.
The Output Stage
Wave of a class B output stage fed with an input sinusoid.
Positive and negative cycles are unable to connect perfectly due to the turn-on voltage of the transistors.
This wave form has the nonlinear distortion called crossover distortion.
To reduce the crossover distortion can be implemented by supplying the constant DC voltage at the base terminals.
The Output Stage
QN and QP provides the voltage drop which equals to the turn-on voltages of QN and QP.
This circuit is call Class AB output stage.
The Output Stage
• Short-circuit protection circuitryForward protection is implemented by R6 and
Q15.Reverse protection is implemented by R7, Q21,
current source(Q24, Q22) and intermediate stage.
Short-circuit protection
• Reference current is generated by Q12, Q11 and R5.
• Wilder current provides biasing current in the order of μA.
• Q13B provides biasing current for intermediate stage, Q13A for output stage.
• Q5, Q6 and Q7 is composed of the current source to be an active load for input stage.
The Biasing Circuits
Ideal Opamp
Equivalent Circuit of the Ideal Op Amp
Characteristics of the Ideal Op AmplifierThe ideal OPAMP has the
following characteristic :
Differential Input resistance Ri=
Output resistance Ro = 0
Differential voltage gain Av=-
Bandwidth =
Offset voltage and current is zero.a) No difference voltage
between inverting and noninvertying terminals.
b) No input currents.
Perfect balance Vo=0 when V-= V+
A
Vo = (A V -A V ) = A (V - V )
+
+
--
OP AMP is a direct coupled high gain amplifier to which feedback is added to control its overall response characteristic
Operational Amplifier (OP AMP)
Basic and most common circuit building device. Ideally,
1. No current can enter terminals V+ or V-. Called infinite input impedance.
2. Vout=A(V+ - V-) with A →∞
3. In a circuit V+ is forced equal to V-. This is the virtual ground property
4. An opamp needs two voltages to power it Vcc and -Vee. These are called the rails.
A
Vo = (A V -A V ) = A (V - V )
+
+
--
OPAMP: COMPARATOR
Vout=A(Vin – Vref)
If Vin>Vref, Vout = +∞ but practically hits +ve power supply = Vcc
If Vin<Vref, Vout = -∞ but practically hits –ve power supply = -Vee
Compare the voltage of one input with the voltage with other input
Two types:
inverting comparator when the reference voltage apply to the inverting terminal
non inverting comparator when the reference voltage apply to the non inverting terminal
A (gain) very high
24
(a) The unity-gain buffer or follower amplifier.
(b) Its equivalent circuit model.
V+ = VIN.
By virtual ground, V- = V+
Thus Vout = V- = V+ = VIN !!!!
OPAMP: VOLTAGE FOLLOWER
SJTU Zhou Lingling 25
The inverting closed-loop configuration.
Virtual ground.
OPAMP: The Inverting Configuration
26
OPAMP: The Inverting Configuration
SJTU Zhou Lingling 27
OPAMP: The Inverting Configuration
OPAMP: INVERTING AMPLIFIER
1. V- = V+
2. As V+ = 0, V- = 0
3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2. 4. I1 = (VIN - V-)/R1 = VIN/R1
5. I2 = (V- - VOUT)/R2 = -VOUT/R2 => VOUT = -I2R2
6. From 3 and 5, VOUT = -I2R2 = -I1R2 = -VINR2/R1
7. Therefore VOUT = (-R2/R1)VIN
8. Gain = Vout / Vin = - R2 / R1
SJTU Zhou Lingling 29
The noninverting configuration.
Series-shunt negative feedback.
OPAMP: The Non Inverting Configuration
30
OPAMP: The Non Inverting Configuration
OPAMP: NON – INVERTING AMPLIFIER
1. V- = V+
2. As V+ = VIN, V- = VIN
3. As no current can enter V- and from Kirchoff’s Ist law, I1=I2. 4. I1 = VIN/R1
5. I2 = (VOUT - VIN)/R2 => VOUT = VIN + I2R2
6. VOUT = I1R1 + I2R2 = (R1+R2)I1 = (R1+R2)VIN/R1
7. Therefore VOUT = (1 + R2/R1)VIN
SUMMING AMPLIFIER
VOUT = -Rf (V1/R1 + V2/R2 + … + Vn/Rn)
If
Recall inverting amplifier and If = I1 + I2 + … + In
Summing amplifier is a good example of analog circuits serving as analog computing amplifiers (analog computers)!
Note: analog circuits can add, subtract, multiply/divide (using logarithmic components, differentiate and integrate – in real time and continuously.
33
SUMMING AMPLIFIER
34
)()())(())((4
43
32
21
1 R
Rv
R
Rv
R
R
R
Rv
R
R
R
Rvv cc
b
ca
b
cao
SUMMING AMPLIFIER
Difference AMPLIFIER
•This type is of the same characteristic of the inverting and non inverting OPAMP.
•Vo is the differences between the two inputs
• Rin in both inputs must be equal, and equal to Rf
Vo = Rf (V1 –V2)/ Rin
Rf
Rin
Rin
V2
V1
Rf
Vo
37
Linear amplifier.
Theorem of linear Superposition.
Difference AMPLIFIER
38
Application of superposition
Inverting configuration
11
21 Io v
R
Rv
Difference AMPLIFIER
39
Application of superposition.
Non inverting configuration.
234
4
1
22 )(1( Io v
RR
R
R
Rv )
Difference AMPLIFIER
40
The inverting configuration with general impedances in the feedback and the feed-in paths.
Integrators AMPLIFIER
SJTU Zhou Lingling 41
The Miller or inverting integrator.
The Inverting Integrators AMPLIFIER