op-amps i. practical op-amp -...
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![Page 1: OP-AMPS I. PRACTICAL OP-AMP - engr.usask.caengr.usask.ca/classes/EE/323/2006_slides/opamps1_slide.pdf · EE 323 – Op-amp 14 • The initial slope of a large sinusoidal signal, can](https://reader031.vdocument.in/reader031/viewer/2022041308/5e14cdf999b777660e6d0a73/html5/thumbnails/1.jpg)
EE 323 – Op-amp 1
_____ OP-AMPS _____
I. PRACTICAL OP-AMP
• High gain differential amplifier, A. • High input impedance, zin . • Low output impedance, zout. • Applications: voltage amplitude changes (amplitude and polarity), oscillators,
filter circuits, and many types of instrumentation circuits.
Noninverting input
Inverting input
Output
+VCC
-VEE
A.vin
zout Zin
vin
+
_
vout A + _
vin
Symbol Schematic
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EE 323 – Op-amp 2
• two power supplies: +VCC and –VEE, (some op-amps can operates using a single supply).
• input, vin, difference between two input voltages: non-inverting & inverting . • single-ended output. • very high input impedance, zin, (MΩΩΩΩ) • very low output impedance, zout, (less than 100ΩΩΩΩ) • very high voltage gain, A. • output voltage:
vo=Avin
CLASS B PUSH-PULL AMPLIFIER
GAIN STAGE
DIFF. AMPLIFIER
vin vout
OP-AMP
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EE 323 – Op-amp 3
1. Op-Amp input characteristics
1. Input bias current
• ββββDC of each transistor is slightly different • Base currents in the differential amplifier
are slightly different. • Input bias current is defined as the
average of the DC base currents:
2II
I 2B1B)base(in
++++====
• Typical in nanoamperes (BJT) or picoampares (FET)
• Flows through the resistances between the bases and ground.
• Resistances may be discrete resistances or may be Thevenin resistances of the input sources.
Different base current
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EE 323 – Op-amp 4
2. Input offset current
• Defined as the difference of the DC base currents: Iin(off) = IB1 – IB2 • Indicates how closely the transistors are matched. • Data sheet of an op-amp lists Iin(base) and Iin(off), • Base currents can cause output voltage error in precision applications. • Compensate resistor may be use to eliminate this effect.
a) Base resistor produces unwanted input voltage b) Equal base on other side reduces error voltage
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EE 323 – Op-amp 5
3. Input offset voltage
More errors caused by mismatch of the differential amplifier stage: - collector resistances (RC1≠≠≠≠ RC2) and - base-emitter voltages (VBE1≠≠≠≠VBE2) .
Input offset voltage is defined as the input voltage that would produce the same output error voltage in a perfect differential amplifier.
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EE 323 – Op-amp 6
AV
V error)off(in ====
Output of diff. amp includes desired signal and error voltage
Total error: Vin = v1 – v2
Vout = A(v1 – v2)
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EE 323 – Op-amp 7
DC error inputs: V1err = (RB1 - RB2)Iin(bias)
V2err= (RB1 + RB2)Iin(off)/2 V3err = Vin(off) Verr = A (V1err + V2err + V3err)
Table 1: Sources of Output Error Voltage
Description Cause Remedies
Input bias current Voltage across a single RB Use equal RB on other side
Input offset current Unequal current gains Data sheet nulling methods
Input offset voltage Unequal RC and VBE Data sheet nulling methods
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EE 323 – Op-amp 8
2. The 741 op-amp
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EE 323 – Op-amp 9
a. Final stage The quiescent output is ideally 0V Any deviation from 0V is the output error voltage. Output swing is within 1 to 2V (due to drops inside the op-amp).
b. Frequency compensation
Capacitor CC is a compensating capacitor. Miller effect causes this capacitor become a large input capacitance:
Cin = (A+1)CC A=gain of Q5 and Q6.
CC generates a cutoff frequency of 10Hz The op-amp has an ideal Bode plot.
AOL
100dB
10Hz 1MHz freq.
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EE 323 – Op-amp 10
c. Bias and offsets (741C op-amp compensation and null) • RB neutralizes the effect of input bias current (80nA). • 10K potentiometer is used to null or zero the output voltage (no input
signal). • Eliminates the effect of 20nA input offset current and 2mV input offset
voltage.
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EE 323 – Op-amp 11
d. CMRR
• 90dB at low frequency • Degrades at higher frequency (curve (a)).
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EE 323 – Op-amp 12
e. Maximum peak-to-peak output Depends on the load resistance connected to its output (curve (b)).
f. Short circuit current Max short circuit current 25mA = Max current produced by 741.
g. Frequency response • Unity frequency of 1MHz (curve (c)). • Worthless for higher frequency applications. (Other high frequency op-
amps are available).
h. Slew rate • Compensation capacitor, CC, prevents oscillations (negative feedback)
would interfere with the desired signal. • Also creates a speed limit on how fast the output can change.
• This relates to the slew rate, defined as:
where SR is the slew rate and equals to the change in output voltage divided by the change in time.
tvS out
R ====
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EE 323 – Op-amp 13
• Slew rate represents the fastest response that 741 can have (0.5V/µµµµs) (i.e., the output of a 741C can change no faster than 0.5V in a microsecond (figure (c)).
• Slew rate also affects the response
of sinusoidal signal.
• Slew rate limits the large-signal response (specifies in the data sheet.)
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EE 323 – Op-amp 14
• The initial slope of a large sinusoidal signal, can be derived as: SS = 2ππππfVp
• To avoid slew-rate distortion, SS has to be less than or equal to SR • The max frequency at which the signal is on the verge of slew-rate distortion is:
p
Rmax
pSR
V2S
f
fV2SS
====
========
fmax= power bandwidth or large-signal bandwidth of the op-amp. Two bandwidths to be considered when analyzing the op-amp operation:
- The small-signal bandwidth determined by the first order response of the op-amp (by compensation capacitor)
- The large-signal or power bandwidth determined by the slew rate. Examples: 18-1 to 18-4 (page 628).
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EE 323 – Op-amp 15
II. IDEAL OP-AMP
Ideal op-amps are used to simplify the op-amp circuit analyzing.
. Gain A≡≡≡≡∞∞∞∞ . zin=∞∞∞∞
. zout=0 . input currents I+=0
. input current I_=0 . va=v+ - v-=0
vo=Ava ⇔⇔⇔⇔ va=vo/A=vo/∞∞∞∞
Operation depends on external connection
Output
+VCC
-VEE
Avin
zout=0 zin=∞∞∞∞
va=0
I+=0
I_=0
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EE 323 – Op-amp 16
1. INVERTING AMPLIFIER:
• Inverting amplifier uses negative feedback to stabilize the overall voltage gain of the amplifier.
• Gain of the op-amp is too high and unstable to be used without some forms of feedback.
I+=I_=0, va=0 (ideal op-amp):
CL
unity)CL(2
)CL(in
)CL(
ino
in1Ro
inin1R
inina
A
ff
2Rz2R1RA:Gain
2R1Rvv
2R1Rv1Riv
2Rv
ii0_II
2Rv
i0v
====
====
−−−−====
−−−−====
−−−−====−−−−====
================
========
++++
RL
R1
vo vin
R2
iin _ +
iR2
~ va
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EE 323 – Op-amp 17
• Open loop configuration: 1. Open loop gain, A(OL), 2. Open loop cutoff frequency f2(OL).
• Feedback configuration: 1. Close loop gain, A(CL), 2. Close loop frequency f2(CL). 3. Gain = 0dB (i.e., gain=1=unity) f(unity).
Example: 18-7
Gain A(dB)
A(OL)
A(CL)
f2(CL) f2(OL) funity frequency
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EE 323 – Op-amp 18
2. NON-INVERTING AMPLIFIER:
Feedback amplifier provides an output voltage in phase with the input voltage.
Example: 18.10
1R2R1A:Gain
)2R1R1(vv
1R2R
vv1Rivv
ii0I2R
vi
)CL(
ino
inin2Rino
2R1R
in2R
++++====∴∴∴∴
++++====
++++====++++====
========
====
−−−−
R1
vo vin
R2
+ _
iR2
~ va=0
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EE 323 – Op-amp 19
3. SUMMING AMPLIFIER:
(multiple voltage sources: using superposition)
Example: 18-12 Problem: 18-22, 18-26 Try: 18-9, 18-11,18-13,18-21,18-23,18-27
R3
vo v2
R2 _ + ~
)2v2R3R
1v1R3R
(vo +−=
v1
R1
~
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EE 323 – Op-amp 20
4.THE INTEGRATOR
Output is proportional to the integral over time of the input signal (example, a constant input vi yields a ramp output).
−−−−==== dtvRC
1v io
5.THE DIFFERENTIATOR
Output is proportional to the differential over time of the input signal (example, a ramp input vi yields a constant output).
dtdv
RCv io −−−−====
vi
R _ +
~ vo
C
vi
R
_ +
~ vo
C
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EE 323 – Op-amp 21
6. THE INSTRUMENTATION AMPLIFIER
Very High Input Impedance
ca
dabRR
R)RR2(A
++++====
------ DO ASSIGNMENT #1 (posted in the class website) -----
+ _
+ _
_ +
vb
va
Ra Rb
Rc Rd
Rc Rd
Rb
vo