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Chapter 8: OPERATIONAL AMPLIFIERS

# Divided Feedback

If we add a voltage divider to the negative feedback wiring so that only a *fraction* of the output voltage is fed back to the inverting input instead of the full amount, the output voltage will be a *multiple*
of the input voltage (please bear in mind that the power supply
connections to the op-amp have been omitted once again for simplicity's
sake):

If R_{1} and R_{2} are both equal and V_{in} is 6 volts, the op-amp will output whatever voltage is needed to drop 6 volts across R_{1}
(to make the inverting input voltage equal to 6 volts, as well, keeping
the voltage difference between the two inputs equal to zero). With the
2:1 voltage divider of R_{1} and R_{2}, this will take 12 volts at the output of the op-amp to accomplish.

Another way of analyzing this circuit is to start by calculating the magnitude and direction of current through R_{1}, knowing the voltage on either side (and therefore, by subtraction, the voltage across R_{1}), and R_{1}'s resistance. Since the left-hand side of R_{1}
is connected to ground (0 volts) and the right-hand side is at a
potential of 6 volts (due to the negative feedback holding that point
equal to V_{in}), we can see that we have 6 volts across R_{1}. This gives us 6 mA of current through R_{1}
from left to right. Because we know that both inputs of the op-amp
have extremely high impedance, we can safely assume they won't add or
subtract any current through the divider. In other words, we can treat R_{1} and R_{2} as being in series with each other: all of the electrons flowing through R_{1} must flow through R_{2}. Knowing the current through R_{2} and the resistance of R_{2}, we can calculate the voltage across R_{2} (6 volts), and its polarity. Counting up voltages from ground (0 volts) to the right-hand side of R_{2}, we arrive at 12 volts on the output.

Upon examining the last illustration, one might wonder, "where does that
6 mA of current go?" The last illustration doesn't show the entire
current path, but in reality it comes from the negative side of the DC
power supply, through ground, through R_{1}, through R_{2},
through the output pin of the op-amp, and then back to the positive
side of the DC power supply through the output transistor(s) of the
op-amp. Using the null detector/potentiometer model of the op-amp, the
current path looks like this:

The 6 volt signal source does not have to supply any current for the circuit: it merely commands the op-amp to balance voltage between the inverting (-) and noninverting (+) input pins, and in so doing produce an output voltage that is twice the input due to the dividing effect of the two 1 kΩ resistors.

We can change the voltage gain of this circuit, overall, just by adjusting the values of R_{1} and R_{2}
(changing the ratio of output voltage that is fed back to the inverting
input). Gain can be calculated by the following formula:

Note that the voltage gain for this design of amplifier circuit can never be less than 1. If we were to lower R_{2}
to a value of zero ohms, our circuit would be essentially identical to
the voltage follower, with the output directly connected to the
inverting input. Since the voltage follower has a gain of 1, this sets
the lower gain limit of the noninverting amplifier. However, the gain
can be increased far beyond 1, by increasing R_{2} in proportion to R_{1}.

Also note that the polarity of the output matches that of the input,
just as with a voltage follower. A positive input voltage results in a
positive output voltage, and vice versa (with respect to ground). For
this reason, this circuit is referred to as a *noninverting amplifier*.

Just as with the voltage follower, we see that the differential gain of the op-amp is irrelevant, so long as its very high. The voltages and currents in this circuit would hardly change at all if the op-amp's voltage gain were 250,000 instead of 200,000. This stands as a stark contrast to single-transistor amplifier circuit designs, where the Beta of the individual transistor greatly influenced the overall gains of the amplifier. With negative feedback, we have a self-correcting system that amplifies voltage according to the ratios set by the feedback resistors, not the gains internal to the op-amp.

Let's see what happens if we retain negative feedback through a voltage divider, but apply the input voltage at a different location:

By grounding the noninverting input, the negative feedback from the
output seeks to hold the inverting input's voltage at 0 volts, as well.
For this reason, the inverting input is referred to in this circuit as a
*virtual ground*, being held at ground potential (0 volts) by the
feedback, yet not directly connected to (electrically common with)
ground. The input voltage this time is applied to the left-hand end of
the voltage divider (R_{1} = R_{2} = 1 kΩ again), so the
output voltage must swing to -6 volts in order to balance the middle at
ground potential (0 volts). Using the same techniques as with the
noninverting amplifier, we can analyze this circuit's operation by
determining current magnitudes and directions, starting with R_{1}, and continuing on to determining the output voltage.

We can change the overall voltage gain of this circuit, overall, just by adjusting the values of R_{1} and R_{2}
(changing the ratio of output voltage that is fed back to the inverting
input). Gain can be calculated by the following formula:

Note that this circuit's voltage gain *can* be less than 1, depending solely on the ratio of R_{2} to R_{1}.
Also note that the output voltage is always the opposite polarity of
the input voltage. A positive input voltage results in a negative
output voltage, and vice versa (with respect to ground). For this
reason, this circuit is referred to as an *inverting amplifier*. Sometimes, the gain formula contains a negative sign (before the R_{2}/R_{1} fraction) to reflect this reversal of polarities.

These two amplifier circuits we've just investigated serve the purpose of multiplying or dividing the magnitude of the input voltage signal. This is exactly how the mathematical operations of multiplication and division are typically handled in analog computer circuitry.

**REVIEW:**- By connecting the inverting (-) input of an op-amp directly to the output, we get negative feedback, which gives us a
*voltage follower*circuit. By connecting that negative feedback through a resistive voltage divider (feeding back a*fraction*of the output voltage to the inverting input), the output voltage becomes a*multiple*of the input voltage. - A negative-feedback op-amp circuit with the input signal going to the noninverting (+) input is called a
*noninverting amplifier*. The output voltage will be the same polarity as the input. Voltage gain is given by the following equation: A_{V}= (R_{2}/R_{1}) + 1 - A negative-feedback op-amp circuit with the input signal going to
the "bottom" of the resistive voltage divider, with the noninverting (+)
input grounded, is called an
*inverting amplifier*. Its output voltage will be the opposite polarity of the input. Voltage gain is given by the following equation: A_{V}= -R_{2}/R_{1}