Fairchild case: Amplifier characteristic analysis that satisfies both AC and DC performance

Modern system designers face many design challenges, from implementing interfaces to data converters to maintaining signal fidelity when their systems interface with analog systems, and they naturally turn to op amps to solve these problems. As a result, today's amplifiers need to meet a difficult combination of technical specifications. For example, consumer electronic video equipment such as set-top boxes and DVD recorders must have high bandwidth while requiring high output current (for driving 75Ω coaxial cables), good gain accuracy, low supply voltage, and good dynamic range at low supply voltage.

Although high bandwidth amplifiers have been around for decades, they have a reputation for having poor DC characteristics and typically operate from dual (±) power rails. These poor DC characteristics limit the dynamic range that the amplifier can achieve. The following formula is the combined dynamic range of an amplifier:

Dynamic range = 20Log10(VoutMax/VoutMin)

The dynamic range of an amplifier is limited by the supply voltage, which has a direct impact on VoutMax. Modern "rail-to-rail" output amplifiers reduce this effect by making VoutMax ≈ Vsupply-200mV (or less). On the other hand, if VoutMin could be made equal to zero, the dynamic range would be infinite. However, this is not the case in reality because there are many error terms associated with amplifiers, including input offset voltage, input bias current, input offset current, common-mode rejection ratio, power supply rejection ratio, noise, and temperature drift associated with all of these terms.

Figure 1 shows the error terms for an amplifier. All of the error terms are plotted referred to the input (RTI), as is done in almost all manufacturers’ data sheets. Therefore, all terms are multiplied by the noise gain (or noninverting gain) to the amplifier output.

Noise gain = 1 + (feedback resistance / gain resistance)

Let's look at each error term in detail. Noise (amplifier noise) has three basic components: the voltage noise of the amplifier, the current noise of the amplifier, and the thermal noise of the resistive feedback network. For both voltage and current noise, there are two components: broadband noise and low-frequency 1/f noise. Most manufacturers specify noise only at frequencies where broadband noise dominates, but the data sheet should also include a noise graph. The equivalent voltage of current noise is circuit dependent and can be found by multiplying the current noise at the non-inverting terminal by the source resistance and the current noise at the inverting terminal by the equivalent resistance of the feedback network.

Figure 1: Error terms that affect amplifier performance.

1. Input bias current and offset current

Most high speed amplifiers have large input bias currents due to the use of bipolar transistors in their input stages, combined with the high internal bias voltage points of the transistors. Typically, the effect of these error terms is to introduce additional offset voltages through their interaction with the feedback network and/or source resistance. By matching the impedance of the non-inverting and inverting terminals, the effects of the bias currents can be reduced.

2. Offset voltage

Offset voltage is defined as the voltage required between the amplifier inputs to force the output voltage to zero. In practice, it can be simulated as a fixed voltage source at the amplifier inputs and cannot be removed in most modern amplifiers. Offset voltage is generally considered to be the reference point for all amplifier performance and is affected by common-mode voltage (common-mode rejection ratio, CMRR), supply voltage (power supply rejection ratio, ), output voltage swing (open-loop gain, PSRR), load current (indirect open-loop gain, Aol) and temperature.

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3. CMRR

Common-mode rejection ratio is defined as the ratio of differential gain to common-mode gain, usually expressed in dB. In practice, it can be defined as the change in offset voltage caused by a unit change in common-mode voltage:

CMRR=20Log10((Voffset/(Vcm)

4. PSRR

The power supply rejection ratio is defined as the change in offset voltage for a unit change in power supply voltage:

PSRR=20Log10((Voffset/(Vsupply)

5. Aol

For a closed-loop amplifier, it can be defined as the change in output voltage relative to a change in offset voltage:

Aol=20Log10((Vout/(Voffset)

Low Aol will cause distortion, lower VoutMax, and gain error. Because the closed-loop gain of the amplifier is defined as:

Closed-loop gain = noise gain / (1 + (noise gain / open-loop gain))

Open-loop gain has a significant impact on the accuracy of closed-loop gain. Minimizing these errors while maximizing bandwidth, reducing supply voltage, and reducing power consumption is a difficult task for amplifier IC designers, but modern process technologies make it easier to make trade-offs. Fairchild Semiconductor's FHP3130/3230/3430 series amplifiers have very complete specifications: 165MHz bandwidth, 100dB CMRR, PSRR, and Aol, while consuming only 2.5mA of supply current and operating from 2.7V to 12V supply voltage.

Typical performance parameters are shown in Table 1. These amplifiers offer wide bandwidths, combined with high output current and high open-loop gain, resulting in excellent gain accuracy, low supply voltage, and excellent dynamic range on a single 5V supply. These products offer excellent AC performance and excellent DC performance, giving designers the performance advantage of both worlds.

Table 1: Typical amplifier performance parameters

Conclusion

To meet the requirements of today's system designers, IC manufacturers are using new process technologies to develop modern high-speed amplifiers with the right combination of AC and DC performance. Currently, system designers can choose from a variety of high-speed amplifiers that provide the best performance in both AC and DC, enabling better performance, easier implementation, and shorter design cycles in high-speed system designs for a variety of end-market applications.