11.8 Summary
In this chapter we have taken a closer look at op amp characteristics. First of all, we find that the upper frequency limit is a function of the op amp parameter 𝑓𝑢𝑛𝑖𝑡𝑦 , also known as the gain-bandwidth product, and the circuit’s noise gain. The higher the gain is, the lower the upper break frequency will be. Op amps are capable of flat response down to DC. If coupling capacitors are used, the lower break frequency may be found by using standard lead network analysis. When stages are cascaded, the results echo those of cascaded discrete stages. The lowest 𝑓2 is dominant and becomes the system 𝑓2 . The highest 𝑓1 is dominant and sets the system 𝑓1 . If more than one stage exhibits the dominant critical frequency, the actual critical frequency will be somewhat lower for 𝑓2 and somewhat higher for 𝑓1 .
In order to make the op amp unconditionally stable, a compensation capacitor is used to tailor the open loop frequency response. Besides setting 𝑓𝑢𝑛𝑖𝑡𝑦 , this capacitor also sets the slew rate. Slew rate is the maximum rate of change of output voltage with respect to time. Slewing slows down the edges of pulse signals and distorts sinusoidal signals. The highest frequency that an amplifier can produce without slewing is called the power bandwidth. In order to optimize 𝑓𝑢𝑛𝑖𝑡𝑦 and slew rate, some amplifiers are available without the compensation capacitor. The designer then adds just enough capacitance to make the design stable.
Due to slight imperfections between the input transistors, op amps may produce small DC output voltages called offsets. Offsets may be reduced through proper resistor selection. Simple nulling circuits may be used to completely remove the offset. A variable offset due to temperature variation is called drift. The larger the temperature variation, the larger the drift will be. The transistor mismatch also means that common-mode signals will not be completely suppressed. Just how well common mode signals are suppressed is measured by the common-mode rejection ratio, CMRR. Similar to CMRR is PSRR, the power-supply rejection ratio. PSRR measures how well power-supply noise and ripple are suppressed by the op amp. Both PSRR and CMRR are frequency-dependent. Their maximum values are found at DC and then they decrease as frequency increases.
Finally, noise is characterized as an undesired random output signal. The noise in op amp circuits may be characterized by three components: the thermal noise of the input and feedback resistors, the op amp’s input noise voltage density, 𝑣𝑖𝑛𝑑 , and its input noise current density, 𝑖𝑖𝑛𝑑 . The combination of these elements requires an RMS summation. In order to find the output noise voltage, the input noise voltage is multiplied by the circuit’s noise gain. The ratio of the desired output signal and the noise voltage is called, appropriately enough, the signal-to-noise ratio. Normally, signal-to-noise ratio is specified in decibels.
