使用寬帶電壓和電流反饋運算放大器時的應用基礎
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Figure 16. DC-coupled single to differential frequency response.
Application C
Differential-input-to-differential-output circuits with very low distortion can clearly benefit from the FDA topology. Where a low IF requirement needs the best third-order intermodulation spurious suppression using modest quiescent power levels, a transformer-coupled FDA implementation provides a surprisingly low-noise figure with exceptionally low harmonics.
Figure 17shows an example of this type of circuit using the very wideband THS4509.
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Figure 17. Very low noise and distortion IF amplifier using the THS4509.
This example gives a noise figure of 8.2 dB (from a 50 Ω source) while also giving >100 dB two-tone spurious free dynamic range (SFDR) through 70 MHz for 2 Vppoutputs. This performance is equivalent to a 54 dBm third-order intercept in an FDA using approximately 200 mW quiescent power.
The 1:1.4 input turns-ratio transformer reflects the 100 Ω differential input impedance of the FDA circuit to a 50 Ω termination (Reference 3discusses this circuit and measured performance in detail). In this case, the transformer becomes the bandwidth limiting element.Figure 18shows the simulated performance, including the transformer model.
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Figure 18. Small-signal bandwidth for the transformer-coupled FDA.
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