“Recent advances in low-voltage silicon germanium and BiCMOS process technology 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.Since many low-voltage applications are single-ended, the questions arise, “How can I use a differential I/O amplifier in a single-ended application?” and “What are the possible consequences of such use?” This article explores some of the results that actually occurred, And show some specific and differential I/O amplification using 3GHz gain-bandwidth
Recent advances in low-voltage silicon germanium and BiCMOS process technology 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. Since many low-voltage applications are single-ended, the questions arise, “How can I use a differential I/O amplifier in a single-ended application?” and “What are the possible consequences of such use?” This article explores some of the results that actually occurred, And show some specific and single-ended applications using the 3GHz gain-bandwidth differential I/O amplifier LTC6406.
A regular op amp has two differential inputs and one output. Although the nominal value of the gain is infinite, control of the gain can be maintained by feedback from the output to the negative “inverting” input. The output doesn’t go to infinity, but the differential input can stay at zero (as if dividing by infinity). The availability, variety, and benefits of conventional op amp applications are well-documented, but still seemingly inexhaustible. Fully differential op amps have not been studied thoroughly enough.
Figure 1 shows a differential op amp with 4 feedback resistors. In this case, the nominal value of the differential gain is still infinite, and the inputs are connected together through 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 “zero” differential input voltage because the feedback is symmetrical. Therefore, as with 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 5 pins (excluding the supply pins) instead of 4 pins. The equation for differential gain is VOUT(DM) = VIN(DM) • R2/R1. The common mode output voltage is internally forced 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 the convenience of circuit analysis, two inputs are marked with “+” and “-” according to the conventional method, and one output is marked with a dot, indicating that it is the inverting 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 the fully differential op amp shown in Figure 1, there is feedback to both inputs, so there is no high impedance node. This difficulty can be overcome fortunately.
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 inputs are now high impedance.
Figure 2: Feedback is single-ended only. This circuit is stable with a high impedance input like a conventional op amp. The closed loop output (VOUT+ in this case) is low noise. The single-ended output is well obtained from the closed-loop output, providing a 3dB bandwidth of 1.2GHz. The open loop output (VOUTC) has a 2x noise gain relative to VOCM, but performs well up to about 300MHz, above which there is significant passband ripple.
The LTC6406 works well in this circuit and still provides a differential output. However, a simple experiment revealed one of the drawbacks of this configuration. Assume that all inputs and outputs are 1.2V, including VOCM. Now imagine driving the VOCM pin with an additional 0.1V. The only output that may change is VOUT C, because VOUT + must remain equal to VIN, so to raise the common-mode output by 100mV, the amplifier has to raise the VOUT C output by a total of 200mV. This is 200mV differential output drift due to 100mV VOCM drift. This accounts for the fact that the single-ended feedback of the fully differential amplifier introduces a 2x noise gain from the VOCM pin to the “open” output. To avoid this noise, just not use this output, resulting in a thoroughly single-ended application. Alternatively, you can accept a slight noise penalty and use two outputs.
Single-Ended Transimpedance Amplifier
Figure 3 shows the LTC6406 connected as a single-ended transimpedance amplifier with 20kΩ transimpedance gain. 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 will still work 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 C. Calculated differentially, the transimpedance gain is 40kΩ.
Figure 3: Transimpedance amplifier. The ultra-low noise JFET buffers the current noise of the bipolar LTC6406 input, trying to trim the pot without any clues to get a 0V differential output.
New families of fully differential op amps such as the LTC6406 offer unprecedented bandwidth. Fortunately, these op amps also work well in single-ended and 100% feedback applications.