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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
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
GBW = 10 k * 200 Hz = 2 MHz
GBW Cascaded Amplifiers - Bandwidth
Quite often, one amplifier does not increase the signal enough
and amplifiers are cascaded so the output of one amplifier is the
input to the next.
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 Total Bandwidth of the Cascaded Amplifiers is:
BWT = BWs(21/n – 1)1/2
where n is the number of amplifiers
that are being cascaded
Example: Cascading 3 Amplifiers with GBW = 1 MHz and A N = 15,
Find: The Total Bandwidth, BWT
BWS = 1 MHz / 15 = 66.7 kHz
BWT = 66.7 kHz (21/3 – 1)1/2 = 34 kHz Common-Mode Rejection Ratio
The common-mode rejection ratio (CMRR) relates to the ability of
the op amp to reject common-mode input voltage. This is very
important because common-mode signals are frequently
encountered in op amp applications.
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
are present at the non-inverting and inverting terminals of the
vicm = v(+) + v(-)
2 Common-Mode Rejection Ratio
Given: A 741 op amp with CMRR = 90 dB and a noise gain,
AN = 1 k
Find: The common mode gain, Acm Acm = AN
log-1 (CMRR / 20) = 1000
log-1 (90 / 20) = 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.
The 741 op amp can be operated from bipolar suppl...
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