1 Op-Amp Imperfections in The Linear Range of Operations Gain and Bandwidth Limitations Ideal op...
-
Upload
charleen-richard -
Category
Documents
-
view
217 -
download
1
Transcript of 1 Op-Amp Imperfections in The Linear Range of Operations Gain and Bandwidth Limitations Ideal op...
1
Op-Amp Imperfections in The Linear Range of Operations
Gain and Bandwidth Limitations
Ideal op amps have infinite open-loop gain magnitude (AoL is infinite), but the gain of a
real op amp is finite and a function of frequency dc open-circuit differential voltage gain is typically between 104 to 106
The bandwidth is usually limited by the designer to prevent oscillations from
feedbackby a process is called frequency compensation
AOL (f) - open-loop gain as function of frequency
A0OL - dc open-loop gain
fBOL - open-loop break frequency
AOL (f) - constant up to fBOL then it rolls off at 20
dB/decade
The open loop gain function of an op-amp usually has a single dominant pole and is given as:
2
Gain-Bandwidth Limitations
Assuming infinite input impedance and zero input current
Voltage across resistor R1
Thus,
and
Definition of open-loop gain
Hence from (2.27)
(2.27)Therefore, the closed-loop gain
(2.26)
3
In the limit of AOL tending to infinity,
ButFor an ideal op amp
This is the same result as before
Op amp Closed-loop gain is given as,
Putting
We have,
Defining and
we have which is very similar in form to the open loop gain
4
Closed-Loop Bandwidth
But we know that, and
Therefore,
This same formula applies to a non-inverting as well as an inverting amplifier
Substituting,
5
Gain Bandwidth Product
Note that you are trading off the high gain of the op-amp for a higher bandwidth
6
A0CL A0CL
(dB)
fBCL
1 0.999990 0 4 MHz
0.1 9.9990 20 400 kHz
0.01
99.90 40 40 kHz
Closed-Loop Gain Versus Frequency
Not that at ft the gain becomes 1
7
Bandwidths of Inverting and Noninverting Amplifiers
Closed loop bandwidth
Closed loop gain for non-inverting amp
Closed loop gain for inverting amp
Closed loop bandwidth
CL
TBCL A
ff
01
8
Output Voltage Swing
Output Current Limits
Large Signal Operation
Output voltage cannot exceed the maximum supply voltage. It also depends on the type of op-amp design to some extent.
Clipping of the output voltage occurs when either the output voltage or the output current reaches its limit