LMH6555 Datasheet, PDF (30/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
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0.3
65.7 mV_PP @
V_{IN- 138 mV DC VIN+}
0.2
0.1
0
V₁
-0.1
V₂
-0.2
-0.3
TIME
Figure 49. Input Voltages for Figure 48 Schematic
Knowing the device input terminal voltages, one can estimate the differential input impedance as follows:
R_{IN_DIFF}
0.131 V_PP
R_{IN_DIFF} + 100 ⁼ 0.3 V_PP ^{Ÿ RIN_DIFF = 78:}
(42)
This is comparable to R_{IN_DIFF} found in Electrical Characteristics.
OUTPUT STAGE AND GAIN ANALYSIS
Differential gain is determined by the differential current flow through the feedback resistors R_F1 and R_F2 as
shown in Figure 24. Current through R_F1 (or R_F2) sets the V_OUT^− (or V_OUT+) swing. The nominal value of these
resistors is close to 430Ω.
The LMH6555 output stage consists of two bipolar common emitter amplifiers with built in output resistances, R_T1
and R_T2, of 50Ω, as shown in Figure 50.
V⁺
R_T2
50:
V_OUT-
V⁺
R_T1
50:
R_L
100:
V_OUT+
Figure 50. Output Stage Including External Load R_L
With an output differential load, R_L, of 100Ω, half the differential swing between the output emitters appears at
the LMH6555 output terminals as V_OUT.
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