The easiest way to see the effects of increasing the transformer's turns ratio is to use the equations derived above at several turns ratios. This was done for an example design of a full-power ADSL upstream driver. The table shows the result for the required amplifier output voltage, current, minimum supply voltage, quiescent power dissipation, output-stage power dissipation, and total power dissipation. Turns ratios of from one to four were used. The device used was the OPA2681, a very-wideband, dual, current-feedback op amp.
The analysis computes the supply voltage needed to support the required output voltage from each output. But it doesn't address whether the amplifier can deliver the needed current. One design approach is to look for the turns ratio that will require a supply voltage suitable to the amplifier selected (and the desired system power supplies), while requiring less than the maximum available output current for that amplifier.
In this design, a turns ratio of two will require a 7.5-V p-p output from each amplifier with a peak-output current of 150 mA and a minimum supply voltage of 11.44 V. With a single +12-V supply, this output current is within the capability of the OPA2681.
The table also shows that the required total supply voltage across the two output-driver op amps initially decreases rapidly with an increasing turns ratio. But as the impedance-matching resistors decrease to a level similar to that of the R1 and R2 resistors in series with the output-stage devices, the required supply voltage stops falling as the turns ratio increases. This again shows the diminishing utility to increasing the ratio.
Furthermore, the table reveals that the quiescent power dissipation initially decreases with supply voltage more than the output-stage power increases. This stops being the case after a turns ratio of about 1.8, because the increase in the output-stage power exceeds the decrease in quiescent power.
It's interesting, however, to observe the impact of increasing CF on power efficiency. The design point in the example delivers 20 mW to the line while drawing a total power from the supply of 420 mW + 40 mW = 460 mW. This is a 4.3% power efficiency. By keeping the same line-power targets and optimizing the turns ratio for minimum driver power in each case, the power efficiency would increase to about 8.4% if the crest factor decreases from 5.3 to 2.5 (Fig. 4).
A basic driver design was created using the information from the table for a 1:2 turns ratio. The maximum amplifier output must swing 7.5 V p-p into what appears to be a 25-Ω load while delivering 150-mA peak current (Fig. 5).
The example circuit includes some input and output components added only for test purposes. One is the 1:1 input transformer, which was added to supply the differential input expected by the driver. The center-tap on the input transformer secondary is set to the optimized +6.5-V bias point by a resistor divided off the +12-V supply. The 50 Ω across the secondary is for test purposes only, to get the required test-signal input impedance matching.
The design is set up for a differential gain of 7.5 from a maximum 2-V p-p differential input to the maximum required 15-V p-p differential output swing. Because the OPA2681 is a wideband, high-slew-rate, current-feedback design, ac performance holds up very well as the gain is increased to this 7.5 value. The output stage for this device (patents pending) also has been optimized to deliver high currents with minimal voltage headroom while retaining very low distortion on a low 6-mA/channel quiescent supply current.
The dc gain is set to 1 by the 1-µF blocking capacitor in series with the 100-Ω gain resistor. This is so the +6.5-V input bias voltage can be passed on to the output as an optimized center point for the output swing. With no input signal present, both outputs are sitting at +6.5 V with no current through the output transformer. At the amplifier outputs, two 12.5-Ω resistors (both labeled R in Figure 1) match into the 1:2 transformer, whose secondary emulates a 100-Ω load while still matching into a 50-Ω measurement system. In an actual application, the two 71-Ω resistors would not be used.
Driving a full 15-V p-p single-tone output, this circuit delivers over 60 MHz of bandwidth (Fig. 6). The circuit also has very low spurious levels at the ADSL upstream frequencies, which max out in the 138-kHz region. Two-tone, full-power (7.5 V p-p on each tone) intermodulation, spurious-free dynamic range is shown in Figure 7.
To support PC-based applications with single +5-V supplies, a different design point for the ADSL line driver is required. Given a 5-V supply to work with and a full upstream power of 13 dBm, the circuit would require peak currents and transformer turns ratios exceeding system design constraints. The G.Lite upstream power of 10 dBm with a 5.33 crest factor is, however, within reach of a single +5-V design.
Because every bit of voltage swing from the supply will be needed, a CMOS amplifier with rail-to-rail output swing appears to be the best design. Using the output-stage headroom (0.2 V) and series resistor values (R1 = R2 = 5.8 Ω for the DRV1101 high-current output differential driver) in the design spreadsheet gives a solution at a 1:3.2 turns ratio where a 6.7-VPP differential voltage is required with 170-mA peaks. The apparent load to the driver for this design is a very demanding 20 Ω. The patent-pending output stage design of the DRV1101 will deliver this power while meeting the distortion targets of G.Lite.
The 24-mA quiescent current on a 5-V supply, along with the internal power dissipated to deliver the load power, as discussed previously, gives this design a 260-mW internal power dissipation. The receiver-channel amplifier requirement can again be met by the low-noise OPA2680, this time operating on one +5-V supply. Since the voltage on each output of the OPA2680 can swing 3-V p-p (with greater than 150-MHz bandwidth), even the most demanding AFE input requirements can be met.
Reference:
- Harris Application Note AN9718, "Analog Amplifier Linearity Characterization Via Probability-Weighted Multitone Power Ratio Testing (HI5905)."