Page 57 - Op Amps Design, Applications, and Troubleshooting

40 AMPLIFIERS

the output tried to drift in the negative direction. Thus, as long as the input is held
at 0 volts, the output is forced to stay at 0 volts.
Now suppose we allow the input signal to rise to a +2 volt instantaneous
level and freeze it for purposes of the following discussion. With +2 volts applied
to R/ and 0 at the output of the op amp, the voltage divider made up of R F and JRj
will have two volts across it. Since the (-) terminal of the op amp does not draw
any significant current, the voltage divider is essentially unloaded. We can see,
even without calculating values, that the (-) input will now be positive. Its value
will be somewhat less than 2 volts because of the voltage divider action, but it will
definitely be positive. The op amp will now amplify this voltage (%) to produce a
negative-going output. As the output starts increasing in the negative direction,
the voltage divider now has a positive voltage (+2 volts) on one end and a nega-
tive voltage (increasing output) on the other end. Therefore the (-) input may still
be positive, but it will be decreasing as the output gets more negative. If the out-
put goes sufficiently negative, then the (-) pin (V D) will become negative. If, how-
ever, this pin ever becomes negative then the voltage would be amplified and
appear at the output as a positive going signal. So, you see, for a given instanta-
neous voltage at the input, the output will quickly ramp up or down until the out-
put voltage is large enough to cause V D to return to its near-0 state. All of this
action happens nearly instantaneously so that the output appears to be immedi-
ately affected by changes at the input.
We can also see from Figure 2.2 that changes in the output voltage receive
greater attenuation than equivalent changes in the input. This is because the output
is fed back through a 10-kilohm resistor, but the input is applied to the 1.0-kilohm
end of the voltage divider. Thus, if the input makes a 1-volt change, the output will
have to make a bigger change in order to compensate and force V D back to its near-
0 value. How much the output must change for a given input change is strictly
determined by the ratio of the voltage divider resistors. Therefore, since the ratio of
output change to input change is actually the gain of the amplifier, we can say that
the gain of the circuit is determined by the ratio of R F to R/.
Recall that the internal gain (open-loop gain) of the op amp is not a constant.
It varies with different devices (even with the same part number), it is affected by
temperature, and it is different for different input frequencies. Now that we have
added feedback to our op amp, the overall circuit gain is determined by external
components (R F and R f). These can be quite stable and relatively unaffected by
temperature, frequency, and so on.
If the input signal is too large or the ratio of R F to Rj is too great, then the out-
put voltage will not be able to go high enough (positive or negative) to compen-
sate for the input voltage. When this occurs, we say the amplifier has reached
saturation, and the output is clipped or limited at the ±V SAT levels. Under these
conditions the output is unable to rise enough to force V D back to its near-0 level.
From this you can safely conclude the following important rules regarding nega-
tive feedback amplifiers:

1. If the output is below +V SAT and above -V SAT, then V D will be very near 0 volts.
2. If V D is anything other than near 0 volts, then the amplifier will be at one of
the two saturation voltages (±V SAT).