Gain Insight Into Op-Amp Circuits
Before reading this article, read and consider the question shown in the box.
|Question Posed in the Last Issue (Exam Level: CBT)|
In the schematic, what component or components sets the amplifier stage gain?
a. The gain is set by the chip itself and can be obtained from the IC flysheet
b. The gain is set by VCC as the higher the supply voltage, the higher the gain.
c. By the value of R3 as this resistor to ground sets the offset bias seen by the op-amp hence the gain.
d. By the value of R2 as this resistor sets up a voltage divider with the very small input resistance of the op-amp and thus the feedback level.
e. R1 and R2 as they create a voltage divider setting the feedback level and thus the gain.
The correct answer to the question is e.
The operational amplifier (op-amp) is a high performance gain device with essentially linear qualities. Theoretically the gain of a bare op-amp is infinite, limited only by the voltage of the power supply rails.
In a practical setting, the gain needed is set by the amount of feedback as set by the feedback network. In this very simple case, that feedback network is just a resistive divider that determines the proportion of output signal that is returned to the negative input terminal. Gain in this basic inverting amplifier is equal to R2 over R1 (G=R2/R1) so the last option is the correct one.
Since the example circuit is direct coupled for amplification of a small AC signal, we would want the quiescent output to be at or near zero (midway between the plus and minus supply rails) to provide the best headroom before clipping. R3, which controls the offset but not the gain, can be calculated to "zero" the output by the formula R3 = (R1*R2)/(R1+R2).
There are cases where DC offset would be useful. R3 can be used to adjust for the desired offset voltage with no effect on gain.
The answer b is incorrect as the gain is independent of the supply voltage. As op-amps are high impedance input devices, answer d is wrong on its face.
Op-amps are useful and flexible devices in electronic design, with a nearly unique combination of qualities such as:
For more about op-amps, see Op-Amps Are Used Widely in Broadcast Circuits
- High impedance input such that they take little power from the source so the input does not load the source;
- Low impedance output such that they can produce a voltage (hence power) into very low impedance loads;
- Electrically quiet so they can amplify very low signals with little compromise, e.g. the output of capacitor microphone elements;
- As mentioned, the gain is independent of the supply voltage.
Note: The deadline to sign up for the next SBE certificate exams at the local chapter level in February is Dec. 31, 2008. Details at www.sbe.org.
A CBRE question for the next issue: "At a minimum, what documents should your station engineering records include for the 400-foot-AGL, company-owned FM antenna tower in the studio parking lot?"
a. Only the FAA "Determination of No Hazard"
b. The FAA "Determination of No Hazard" and the FCC "Antenna Structure Registration"
c. The FAA "Determination of No Hazard," the FCC "Antenna Structure Registration" and the latest (last) license on the tower with lighting/marking directives
d. The FAA "Determination of No Hazard," the FCC "Antenna Structure Registration," the latest (last) license on the tower with lighting/marking directives and at least the last two years of quarterly inspections of the tower lighting, marking and structural integrity.
e. No documentation is required
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