Page 38 - Op Amps Design, Applications, and Troubleshooting

Introduction to Practical Op Amps 21

Whether this value is good or bad, high or low, acceptable or unacceptable is
determined by the particular application being considered. For now, you should
strive to understand the meaning of common-mode voltage gain and how it dif-
fers from the differential voltage gain.

1.4.3 Bandwidth
You will recall from your studies of basic amplifier and/or filter theory that the
bandwidth of a circuit is defined as the range of frequencies that can be passed or
amplified with less than 3 dB power loss. In the case of the ideal op amp, we said
that the bandwidth was infinite because it could respond equally well to frequen-
cies extending from DC through infinitely high frequencies. As we saw in our dis-
cussion of differential voltage gain, however, not all frequencies receive equal
gains in a practical op amp.
If you examine the behavior of the op amp itself with no external circuitry, it
acts as a basic low-pass filter. That is, the low frequencies (all the way to DC) are
passed or amplified maximally. The higher frequencies are attenuated. The band-
width of practical op amps nearly always begins at DC. The upper edge of the
passband, however, may be as low as a few hertz. This would seem to represent a
serious op amp limitation. We will see, however, that this apparently restricted
bandwidth can be dramatically increased by the addition of external components.
Now let us determine the bandwidth of a 741 op amp by examining the spec-
ification sheets in Appendix 1. The bandwidth often cited for 741 op amps is 1.0
megahertz (MHz). This seems like a fairly respectable value but it is misleading
when viewed from the basic definition of bandwidth. In the case of op amps, the
true open-loop (i.e., no external components) bandwidth is of very little value
since it is so very low (a few hertz). The bandwidth generally cited in the data
sheet is more appropriately labeled the gain-bandwidth product. Recall that the dif-
ferential gain decreases as the frequency increases. The gain-bandwidth product
indicates the frequency at which the differential gain drops to 1 (unity). This fre-
quency is also called the unity gain frequency.
To further illustrate the bandwidth characteristics of the 741, examine the
graph showing open-loop voltage gain as a function of frequency. You can see that
the amplifier has a gain of about 100 dB at 1 hertz, but the gain has dropped dra-
matically by the time the input frequency reaches 10 hertz. In fact, the actual
upper edge of the passband (the half-power or 3 dB frequency) is about 5 hertz.
This is the actual bandwidth of the open-loop op amp. Observe that the gain drops
steadily until it reaches unity at a frequency of 1.0 megahertz. This is the unity-
gain frequency. This same value (1.0 MHz) is obtained by multiplying the DC gain
(200,000) by the bandwidth (5 hertz). Thus it is also called the gain-bandwidth prod-
uct. It is the gain-bandwidth product that is labeled "bandwidth" in some manu-
facturers' data sheets.

1.4.4 Slew Rate
Although the output of an ideal op amp can change levels instantly (as required
by changes on the input), a practical op amp is limited to a rate of change specified
by the slew rate of the op amp. The slew rate is specified in volts per second and
indicates the highest rate of change possible in the output.