电子产品世界

, arial; WHITE-SPACE: normal; ORPHANS: 2; LETTER-SPACING: normal; COLOR: rgb(0,0,0); WORD-SPACING: 0px; PADDING-TOP: 0px; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px">The THS4511 shows a very high full-power bandwidth with its 2 GHz GBP and 4900 V/μsec differential slew rate. These two characteristics together give an exceptional pulse response in a dc-coupled single to differential conversion.Figure 16shows the simulated frequency response.

(Click to Enlarge Image)
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.

(Click to Enlarge Image)
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 V_ppoutputs. 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.

(Click to Enlarge Image)
Figure 18. Small-signal bandwidth for the transformer-coupled FDA.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16