Download Semiconductor Basics

The Operational Amplifiers
Dr. Farahmand
Opamps
Properties
Ideal
Architecture
Practical
overview
Circuits
Open Loop
Closed Loop
Parameters
Frequency Response
Modes of operation
Negative Feedback
Frequency Response
Inverting
Non-inverting
Operational Amplifiers



Historically built using vacuum tubes and
used for mathematical operations
Today, opamps are linear integrated circtuis
(ICs)
Terminal
–
–
–
Inverting and non-inverting inputs
Dc supplies
Single output
Opamps

Ideal opamps
–
–
–
–

Infinite BW
Infinite voltage gain
Infinite input impedance
Zero output impedance
Practical opamps
–
–
–
–
wide BW
Very high voltage gain
Very high input impedance
Very low output impedance
Architecture

3 stages
Differential amplifier input stage:
-Take the difference between the input signals
-If the input base voltage is different:
-Vb1 > Vb2
-Ic1 > Ic2
-VRc1 > VRc2
-Vc1 < Vc2
Modes of Operations

Differential amplifiers can be connected in difference
ways
–
Single-ended mode

–
Differential mode


–
Single input
Out of phase inputs
Unwanted noise on both inputs is cancelled
Common mode

In phase inputs
Parameters

Common mode input voltage
–
–

Input offset voltage (in mV)
–

Total resistance between the inverting and noninverting inputs
Output impedance (in ohm)
–

Dc current required by the inputs of the amplifier
to properly operate the first stage ( Ibias = (I1 +
I2)/2 ); I1 and I2 are the current into inverting and
non-inverting inputs
Input impedance (in Mega ohm)
–

Differential dc voltage required between the
inputs to force the output to zero volt
Input bias current (in nA)
–

Input voltage range limitation
Typically +/- 10 V with dc voltages of +/- 15 V
Total resistance at the output
Slew rate (in V/usec)
–
How fast the output voltage changes in response
to the input voltage change
Parameters

Common mode input voltage
–
–

Input offset voltage (in mV)
–

Total resistance between the inverting and noninverting inputs
Output impedance (in ohm)
–

Dc current required by the inputs of the amplifier
to properly operate the first stage ( Ibias = (I1 +
I2)/2 ); I1 and I2 are the current into inverting and
non-inverting inputs
Input impedance (in Mega ohm)
–

Differential dc voltage required between the
inputs to force the output to zero volt
Input bias current (in nA)
–

Input voltage range limitation
Typically +/- 10 V with dc voltages of +/- 15 V
Total resistance at the output
Slew rate (in V/usec)
–
How fast the output voltage changes in response
to the input voltage change (Dt)
Refer to Table 12-1
CMRR

Common-mod-rejection ratio (CMRR)
–
–
–
–
The measurement of how the amplifier can reject
common more signals
CMRR = Open loop voltage gain / Common mode
gain
Often expressed in dB
The larger the better
From data
sheet
Ideally zero/ indicate how
much of input noise is
passing through
Open Loop Frequency Response

Aol(OL) : Open loop gain
AOL  AOL( mid ) (
Vout
)
Vmid
AOL( mid )
f2 
1 
2
In practice
f
c 

Vmid = Vin x AOL(mid)
AOL(mid
)
Vout
Xc


Vmid Xc  R
1
f2 
1 
2
f
c 

Open Loop Frequency Response
Frequency response: Aol(OL) = Aol(mid)
Critical frequency is the roll-off point
Phase response: q = -arctan (R/Xc) = -arctan (f/fc)
Delay = Period x Phase shift / 360
Open Loop Frequency Response
For multiple stages
qtotal = q1  q2  q3  ......
Av(dB) = Av1 + Av2 + Av3 + ….
Closed Loop Frequency Response

Non-inverting
–
–
–
–

Source is connected to the noninverting input
Feedback is connected to the
inverting input
If Rf and Ri are zero, then unity
feedback used for buffering
Vo=
Inverting
–
Feedback and source are
connected to the inverting input
Comparators




Determines which input is
larger
A small difference
between inputs results
maximum output voltage
(high gain)
Zero-level detection
Non-zero-level detection
Max and minimum
Example
Vref = Vin(max).R2/(R1+R2)=1.63 V
Comparator – Impact of noise
(unwanted voltage fluctuation)
No Noise
With Noise
Inaccuracy!
Hysteresis
(Schmitt triggers)

Making the
comparator less
sensitive to the
input noise
–
–
–
Effectively higher
reference level
Upper Trigger Point
Lower Trigger Point
VUTP = Vout(max).R2/(R1+R2)
VLTP = -Vout(max).R2/(R1+R2)
VHYS= VUTP – VLTP
Zener Bounding

The output voltage can be
limited using Zener diodes
–
–

Vout >0  Vz
Vout < 0  Forward biased
(0.7)
Note that the output signal
is inverted
Virtual Ground
Zener Bounding

Combined effect
?
Bounding
the
negative
values /
Resources

Applets
–
–
http://www.chem.uoa.gr/Applets/AppletOpAmps/A
ppl_OpAmps2.html
http://www.falstad.com/circuit/directions.html