DC Characteristics of Opampddddddddddddddddddddddd.ppt

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1. A
.S
.R
athore
General Objectives
To understand the dc characteristics of operational amplifier.

2. A
.S
.R
athore
DC Characteristics
The non ideal dc characteristics that add error
components to the dc output voltage are,
Input Bias Current
Input Offset Current
Input offset Voltage
Total Output offset Voltage
Thermal drift

3. A
.S
.R
athore
Input Bias Current
In an ideal op-amp, we assumed that no current is drawn from the input
terminals.
 The base currents entering into the inverting and non-inverting terminals
(I
B
+
& I
B
-
respectively).
 Input bias current IB is the average value of the base currents entering
into terminal of an op-amp

2



BB
B
II
I

4. A
.S
.R
athore
Input Bias Current
The effect can be compensated with compensation resistor R
comp.
By
KVL,Vo = V2 – V1
By selecting proper value of
Rcomp, V2 can be cancelled with
V1 and the Vo = 0.
The value of Rcomp is derived as,

5. A
.S
.R
athore
Input Bias Current
f
f
f
comp
compf
f
f
f
f
RR
RR
RR
R
R
V
RR
RR
V
RR
RR
V
R
V
R
V
II
||
)(
)(
get we,II Assume
)(
I
gives a nodeat KVL
1
1
1
1
1
1
1
B
-
B
1
1
1
1
11
12
-
B










6. A
.S
.R
athore
Input offset current
The difference between the bias currents at the input terminals of the op-
amp is called as input offset current.
The input terminals conduct a small value of dc current to bias the input
transistors.
Since the input transistors cannot be made identical, there exists a
difference in bias currents
Even with bias current compensation, offset current will produce an
output voltage when Vi = 0.
V
1
= I
B
+
Rcomp
I
1 = V
1/R
1


BB
IIIos

7. A
.S
.R
athore
Input offset current
 
osfBBfo
compBf
comp
BB
compBffo
comp
BBB
IRIIRVHence
RIR
R
R
II
RIRIVRIVAgain
R
R
IIIII

















,

,
)()(
gives, a nodeat KCL
1
212
1
12
So even with bias current compensation and with
feedback resistor of 1M, a BJT op-amp has an output
offset voltage
Vo = 1M Ω X 200nA
Vo = 200mV with Vi = 0

8. A
.S
.R
athore
Input offset current
T-feed back network is a good solution
Provides a feedback
signal as if the network
were a single feedback
resistor
By T to π conversion,
tf
t
s
f
s
stt
f
RR
R
R
R
to
R
RRR
R
2
calculateThen ,
2
R
pickfirst network, T adesign
,
2
2
t
2





9. A
.S
.R
athore
Input offset voltage
A small voltage applied to the input terminals to make the output voltage
as zero when the two input terminals are grounded is called input offset
voltage, Vos

10. A
.S
.R
athore
Input offset voltage
Let us determine the Vos on the output of inverting and non-inverting
amplifier. If Vi = 0 (Fig (b) and (c)) become the same as in figure (d).
V2 at the –ve input terminal is given by,
ios
f
oios
iiios
ff
o
o
f
V
R
R
VVV
andVVVVce
V
R
R
V
R
RR
V
V
RR
R
V






































1
2
2
2
1
2
1
1
1
1
2
1,
0,sin
1
,

11. A
.S
.R
athore
Total Output Offset Voltage
The maximum offset voltage at the output of an
inverting and non-inverting amplifier without any
compensation technique used is given by
With Rcomp in the circuit, total output offset voltage
will be given by
Bfios
f
oT
IRV
R
R
V 









1
1
osfios
f
oT
IRV
R
R
V 









1
1

12. A
.S
.R
athore
THERMAL DRIFT
Bias current, offset current and offset voltage change with temperature.
 A circuit carefully nulled at 25
o
c may not remain so when the
temperature rises to 35
o
c.
This is called drift.
Offset current drift is expressed in nA/ºC.
Offset voltage drift is expressed in mV/ºC.
Indicates the change in offset for each degree celsius change in
temperature
12

13. A
.S
.R
athore
ApplicationsApplications
Audio and radio communication
Medical Electronics
Instrumentation control
Voltage comparator
Filter
Waveform generator