Many applications require the use of low-power, high-performance differential amplifiers to convert small differential signals into readable ground-referenced output signals. The two inputs usually share a large common-mode voltage. The differential amplifier rejects the common-mode voltage, and the residual voltage, after being amplified, appears as a single-ended voltage at the amplifier output. The common-mode voltage can be either AC or DC and is usually greater than the differential input voltage. The rejection effect decreases as the common-mode voltage frequency increases. Amplifiers in the same package have better matching, have the same parasitic capacitance, and do not require external wiring. As a result, high-performance, high-bandwidth dual-channel amplifiers have better frequency performance than discrete amplifiers.
Some common examples are RF DAC buffers or coaxial cable drivers. Most of the time you can do this with a magnetic transformer, but sometimes transformers don’t work. If this is the case, can you use a fully differential amplifier (FDA)? The answer is yes maybe.
As a refresher, the FDA has two different outputs available. The first output is the most commonly used: it consists of the difference between the outputs. The other output is generally considered a spurious output; it is the average of the two outputs. The common mode output DC level is important; however, its derivative should be zero, which means it should have no AC component. In fact, this is not the case.
Let’s look at an example scenario. The LMH5401 is an FDA with ultra-high bandwidth. The impulse response is shown in Figure 1. On the left is the differential output response and on the right is the common mode response.
Now that we’ve reviewed the FDA’s two main output modes, let’s look at two potential alternative outputs: Out+ alone and Out- alone. Now, let’s use the identities and see what happens to the two main responses for just one output. For FDA, closed loop gain = ); given the same loop gain, only one output is used, the closed loop gain. This clearly shows that using only one amplifier output reduces the gain by 6dB, or twice. With an amplifier like the LMH5401, you can mitigate this drawback by setting the amplifier gain to 2x using a different external resistor. Again, this gain reduction is an added benefit if you’re using the FDA as an attenuator.
Using the same method, the common mode of the amplifier will change from .This conversion shows that the amplifier output common mode is no longer a meaningful concept when the single-ended approach is used, since the common mode is simply equal to the output
To gain a deeper understanding of amplifier performance, we need to use more sensitive equipment than Figure 1. Using a spectrum analyzer, we can measure amplifier distortion with very high accuracy under single-tone conditions.
It is clear that single-ended outputs do not provide linearity for differential conditions. The output voltage under both conditions is 2Vpp. Note that under the single-ended output condition, an output running 2Vpp is “same” as the 4Vpp condition of the differential output.
Given the serious pitfalls of single-ended operation, what happens when we reduce the signal amplitude to make the conditions more comparable? Figure 3 shows what happens when you reduce the signal amplitude so that each output swings the same voltage, regardless of the outcome Whether measured in single-ended or differential mode.
It is clear that the third-order distortion products (HD3) for single-ended or differential outputs are very similar—as long as the single-ended output’s amplitude loss is taken into account. However, the results for the second-order distortion product (HD2) are not comparable. This is the main disadvantage of using FDA with only one output. While the HD2 for each output cancels when the outputs are combined into a differential signal, this does not happen with single-ended outputs.
In conclusion, a single output using the FDA may be suitable for some limited applications: when the signal is at the low end of the amplifier’s operating frequency range, when the signal amplitude is small, and when second-order distortion products are not the dominant performance indicator.