Laboratory Manual - King Fahd University of Petroleum and Minerals

vo
( s)
R2
/ R1
v ( )
s
i
s
1
R
w /(1
2
T
)
R1
This means that the inverting amplifier has frequency response of a low pass function
with DC gain of -R 2 /R 1 and –3dB pole at w
T
( 1 R 2
/ R1
) . Also, it can be seen that as
the gain increases the bandwidth decreases. In fact, this is considered to be the most
serous disadvantage of using op amp (i.e. the conflict between gain and bandwidth of op
amp based circuits). Also, it is worth mentioning that the unity gain inverting amplifier
has 3dB frequency of f T /2. One solution to circumvent this problem is through cascading
several (N) simple amplifiers as shown in Figure 1.
+
v s
R 1
R 2
v o1
R 3
R 4
v o2
R 5
R 6
. . .
+
v oN
Figure 1: Cascaded amplifier using inverting amplifier
If each sub-amplifier is modeled by a single pole, it can be shown easily that the dc gain
of the amplifier is equal to the product of dc gains of individual amplifiers and the
1/ N
bandwidth of the cascade amplifier is w w
1
2 1
H H
. Noman Check this
relation This implies that distributing the gain over several amplifiers will results in
wider bandwidth than using a single stage amplifier.
PRELAB WORK
Students must perform the following calculations and SPICE before the lab.
Hand Calculation:
1. For a single stage inverting amplifier let R 1 =1kΩ and complete Table I assuming
f t =1MHz for op amp 741.
39