Operational amplifiers and it's Parameters.ppt

Operational amplifiers and it's Parameters.ppt

1. Op-Amp Parameters
Input Bias Current
Ideally should be zero
The dc current required by the inputs of the
amplifier to properly operate the first stage.
Is the average of both input currents
1
I
B1
I
B12
2
21BIBI
biasI



2. Op-Amp Parameters
Input Bias Current
Range 10 to 100 nA for BJT
Range 1 to 10 pAfor JFET
Reason : It is undesirable voltage drop in the resistors forming feedback network
2

3. Op-Amp Parameters
3
I
B1
I
B12
21 BBOS III 
Input Offset Current (I
OS)
Is the difference of input bias currents
Reason : It is due to lack of perfect symmetry in
input differential stage

4. Op-Amp Parameters
Input Offset Current (I
OS)
Range 20 to 60 nA for BJT
4
T
OS
I
DriftCurrentOffsetInput
lationeTemperaturIos



Re

5. Op-Amp Parameters
5
Input Offset Voltage [V
os
]
Ideally, output of an op-amp is 0 Volt if the
input is 0 Volt.
Realistically, a small dc voltage will
appear at the output when no input
voltage is applied.
Thus, differential dc voltage is required
between the inputs to force the output to
zero volts.

This is called the Input Offset Voltage, V
os.

6. Op-Amp Parameters
6
Input Offset Voltage [V
os
]
It is input voltage which must be applied
across the input terminals to obtain zero
output voltage.
It can be nulled by a external
potentiometer.

7. Op-Amp Parameters
7
Input Offset Voltage [V
os
]
Range between 2 mV to 5 mV.
T
OS
V
DriftVoltageOffsetInput
lationeTemperaturVos



Re

8. Op-Amp Parameters
Input Offset Current
Is the difference of input bias currents
21
III
os

8
 
inininos RIIRIRIV
2121 
inosos RIV
inosverrorout
RIAV 
)(
Input offset current Offset voltage
Thus, error

9. Op-Amp Parameters
Common-Mode Rejection Ratio (CMRR)
The ability of amplifier to reject the common-mode
signals (unwanted signals) while amplifying the
differential signal (desired signal)
Ratio of open-loop gain, A
dm to common-mode gain, A
cm
The open-loop gain is a datasheet value
cm
dm
A
A
CMRR
9
The higher the CMRR, the better, in which the open-loop
gain is high and common-mode gain is low.

10. Op-Amp Parameters
Common-Mode Rejection Ratio (CMRR)









cm
dm
A
A
log20CMRR
10
CMRR is usually expressed in dB
Range 60 to 100 dB

11. Op-Amp Parameters
Common-Mode Input Voltage
It is the maximum range of input voltage that can be
simultaneously applied to both inputs without causing cut off or
saturation of amplifier stages.
The range of input voltages which, when applied to both inputs,
will not cause clipping or other output distortion.
11

12. Op-Amp Parameters
Difference -Mode Input Voltage
It is the maximum range of differential signal
voltage that can be safely applied to the
OpAmp input signal.
12

13. Supply Voltage Rejection Ratio (SVRR)
One of the reasons OpAmps are so
useful, is that they can be operated
from a wide variety of power supply
voltages.
The 741 op amp can be operated from
bipolar supplies ranging from 5V to 18V
without too many changes to the
parameters of the op amp.

14. Supply Voltage Rejection Ratio
The supply voltage rejection ratio (SVRR)
refers to the slight change in input offset
voltage per volt change in power supply of
the opamp.
SVRR = V
OS /  V
CC)
Range 10
-5
to

7  10
-5

15. Op-Amp Parameters
Input Impedance
Is the total resistance between the inverting and non-
inverting inputs.
Differential input impedance : total resistance between
the inverting and non-inverting inputs
Common-mode input impedance: total resistance
between each input and ground
15
Range 100 k to 1 M

16. Op-Amp Parameters
Output Impedance
Ideally should be zero
Is the resistance viewed from the output terminal of the
op-amp. In reality, it is non-zero.
16
Range 40 k to 100 

17. Op-Amp Parameters
Slew Rate
Is the maximum rate of change of the output voltage in
response to a step input voltage.
t
V
SlewRate
out



17
)( where
maxmax
VVV
out


18. Op-Amp Parameters
Slew Rate
It’s a measure of how fast the output can “follow” the input
signal.
18
Range 50 mV/Sec to 500 mV/Sec

19. Op-Amp Parameters
Example
Determine the slew rate:
t
V
SlewRate
out



19
sV
s
VV
SlewRate 

/18
1
)9(9




20. Output Voltage Swing
Max p-p output which
can be obtain without
waveform clipping
Range 50 to 80 % of
supply voltage
It depends on supply voltage
This parameters is useful in
switching application.

21. Maximum Output Voltage Swing

22. Unity Gain bandwidth
In practice, the voltage gain decrease
at high frequency.
The frequency at which the open loop
voltage gain becomes equal to Unity
Gain bandwidth.

Range 1 to 20 MHz

23. Unity Gain bandwidth
Frequency at which the open loop
voltage gain becomes equal to unity
Range 1 to 20 MHz

24. Full power bandwidth
The maximum frequency over which
the full output voltage swing can be
obtained.

25. Overload Recovery time
It is time required for the output stage
to return to active region when drives
to active hard saturation.
It depends upon the degree of
overload and feedback impedance.

26. Slew Rate at Unity Gain

27. V
E
= V
IN+
- V
IN-
V
OUT
= a * V
E
V
IN+
V
IN-

28. Input Offset Voltage

29. Input Current

30. Input Common Mode Voltage Range
V
ICR - Average voltage at the inverting and
noninverting input pins

31. Differential Input Voltage Range
Absolute Maximum Rating