Using Differential I/O Amplifiers in Single-Ended Applications

introduction

Recent advances in low voltage SiGe and BiCMOS process technologies have allowed the design and production of very high speed amplifiers. Because these process technologies are low voltage, most amplifier designs incorporate differential inputs and outputs to recover and maximize the total output signal swing. Because many low voltage applications are single-ended, the question arises, “How can I use a differential I/O amplifier in a single-ended application?” and “What are the possible results of doing so?” This article explores some of the actual results and demonstrates some specific single-ended applications using the LTC6406 3GHz gain-bandwidth differential I/O amplifier.

background

A conventional op amp has two differential inputs and one output. Although the gain is nominally infinite, it is kept under control by feedback from the output to the negative "inverting" input. The output does not go to infinity, but the differential input is held to zero (as if divided by infinity). The utility, variety, and advantages of conventional op amp applications are well documented, yet seem inexhaustible. The fully differential op amp has been less thoroughly explored.

Figure 1 shows a differential op amp with four feedback resistors. In this case, the nominal value of the differential gain is still infinite, and the inputs are tied together through the feedback, but this is not sufficient to determine the output voltage. The reason is that the common-mode output voltage can be any value and still result in a differential input voltage of "zero" because the feedback is symmetrical. Therefore, for any fully differential I/O amplifier, there is always another control voltage that determines the output common-mode voltage. This is the purpose of the VOCM pin and explains why fully differential amplifier devices have at least five pins (not including the supply pins) instead of four. The equation for differential gain is VOUT(DM) = VIN(DM) • R2/R1. The common-mode output voltage is internally forced to be equal to the voltage applied to VOCM. A final conclusion is that there is no longer a single inverting input: both inputs are inverting and non-inverting, depending on which output is considered. For ease of circuit analysis, the two inputs are labeled "+" and "-" in the conventional way, and one output is labeled with a dot to indicate that it is the inverted output of the "+" input.

Anyone familiar with conventional op amps knows that non-inverting applications have inherently high input impedance at the non-inverting input, approaching GΩ or even TΩ. But in the case of a fully differential op amp as shown in Figure 1, there is feedback to both inputs, so there is no high impedance node. Fortunately, this difficulty can be overcome.

Fully Differential Op Amp Simple Single-Ended Connection

Figure 2 shows the LTC6406 connected as a single-ended op amp. Only one output is fed back, and only one input receives feedback. The other input is now high impedance.

Figure 2: Feedback is single-ended only. This circuit is stable with a high impedance input like a regular op amp. The closed-loop output (VOUT+ in this case) is low noise. The single-ended output is nicely derived from the closed-loop output, providing a 3dB bandwidth of 1.2GHz. The open-loop output (VOUT–) has a noise gain of 2x relative to VOCM, but behaves well until about 300MHz, above which there is significant passband ripple.

The LTC6406 works just fine in this circuit and still provides a differential output. However, a simple experiment reveals one of the shortcomings of this configuration. Imagine that all inputs and outputs are 1.2V, including VOCM. Now imagine driving the VOCM pin an additional 0.1V higher. The only output that can change is VOUT –, because VOUT + must remain equal to VIN, so in order to raise the common-mode output by 100mV, the amplifier has to raise the VOUT – output by a total of 200mV. This is a 200mV differential output drift caused by a 100mV VOCM drift. This illustrates the fact that the single-ended feedback of a fully differential amplifier introduces a noise gain of 2 from the VOCM pin to the “open” output. To avoid this noise, simply unuse this output, resulting in a completely single-ended application. Alternatively, accept the slight noise penalty and use both outputs.

Single-ended transimpedance amplifier

Figure 3 shows the LTC6406 connected as a single-ended transimpedance amplifier with a transimpedance gain of 20kΩ. The BF862 JFET buffers the input of the LTC6406, greatly mitigating the effects of its bipolar input transistor current noise. The JFET’s VGS is considered as an offset, but it is typically 0.6V, so the circuit still works well on a single 3V supply, and the offset can be removed with a 10k potentiometer. The time domain response is shown in Figure 4. The total output noise over a 20MHz bandwidth is 0.8mVRMS at VOUT + and 1.1mVRMS at VOUT –. Calculated differentially, the transimpedance gain is 40kΩ.

Figure 3: Transimpedance Amplifier. Ultralow Noise JFET Buffers Bipolar LTC6406 Input Current Noise, Without Any Clues Trying to Trim the Potentiometer to Get 0V Differential Output.

in conclusion

A new family of fully differential op amps, such as the LTC6406, offers unprecedented bandwidth. Fortunately, these op amps also work well in single-ended applications and in 100% feedback applications.