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**Unformatted text preview: **g off at very low frequencies. Therefore, to make the
op amp useful at higher frequencies, gain is traded for
bandwidth.
bandwidth.
The Gain-Bandwidth Product (GBW) is given by:
GBW = ANBW
BW Example: For a 741 op amp, a noise gain of 10 k corresponds
to a bandwidth of ~200 Hz
Find: The GBW
Find:
GBW = 10 k * 200 Hz = 2 MHz
GBW Cascaded Amplifiers - Bandwidth
Cascaded
Quite often, one amplifier does not increase the signal enough
Quite
and amplifiers are cascaded so the output of one amplifier is the
input to the next.
input
The amplifiers are matched so:
BWS = BW1 = BW2 = GBW
GBW where, BWS is the bandwidth of all AN the cascaded amplifiers and AN is
the noise gain
the
The Total Bandwidth of the Cascaded Amplifiers is:
BWT = BWs(21/n – 1)1/2
where n is the number of amplifiers
where
that are being cascaded
that
Example: Cascading 3 Amplifiers with GBW = 1 MHz and A N = 15,
Find: The Total Bandwidth, BWT
Find:
BWS = 1 MHz / 15 = 66.7 kHz
BWT = 66.7 kHz (21/3 – 1)1/2 = 34 kHz Common-Mode Rejection Ratio
Common-Mode
The common-mode rejection ratio (CMRR) relates to the ability of
The
the op amp to reject common-mode input voltage. This is very
important because common-mode signals are frequently
encountered in op amp applications.
encountered
CMRR = 20 log|AN / Acm|
Acm = AN log-1 (CMRR / 20)
We solve for Acm because Op Amp data sheets list the CMRR value.
The common-mode input voltage is an average of the voltages that
The
are present at the non-inverting and inverting terminals of the
amplifier.
amplifier.
vicm = v(+) + v(-)
2 Common-Mode Rejection Ratio
Common-Mode
Example
Given: A 741 op amp with CMRR = 90 dB and a noise gain,
Given:
AN = 1 k
Find: The common mode gain, Acm Acm = AN
log-1 (CMRR / 20) = 1000
1000
log-1 (90 / 20) = 0.0316
0.0316
It is very desirable for the common-mode gain to be small. Power Supply Rejection Ratio
One of the reasons op amps are so useful, is that they can
be operated from a wide variety of power supply voltages.
be
The 741 op amp can be operated from bipolar suppl...

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