LMH6555_14 Datasheet, PDF (31/37 Pages) Texas Instruments – LMH6555 Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011
With good matching between the input source impedances, R_S1 and R_S2 shown in Figure 46 and Figure 48, it is
possible to infer the gain and output swing by inspection. The differential input impedance of the LMH6555,
R_{IN_DIFF}, is close to 78Ω.
In differential input drive applications, there is a balanced swing across the input terminals of the LMH6555, V_IN+
and V_IN^−. So, by using the R_{IN_DIFF} value, one determines the differential current flow through the input terminals
and from that the output swing and gain.
R_S1
+
V_IN+-
LMH6555
-
R_S2
R_L
100:
V_OUT-
+
V_OUT =
V_IN x R_F
2R_S + R_{IN_DIFF}
V_OUT
R_F
430:
=
=
V_IN 2R_S + 78: 2R_S + 78:
(43)
For the special case where R_S1 = R_S2 = R_S = 50Ω we have:
for R_S = 50: Ÿ ^VOUT = ⁴³⁰ = 2.42 V/V
V_IN 178
(44)
The following is the expression for the Insertion Gain, A_{V_DIFF}:
A_{V_DIFF}
=
V_IN
V_OUT
x ^100:
2R_S + 100
V_OUT/V_IN
=
100/200
= 2 V_OUT/V_IN = 4.83 V/V
= 13.7 dB
(45)
The expressions above apply equally to the single ended input drive case as well, as long as R_S1 = R_S2 = 50Ω.
For the case of the single ended input drive:
V_OUT
A_{V_DIFF}
=
V_IN
x
50
R_S + 50
V_OUT/V_IN
=
= 2 V_OUT/V_IN = 4.83 V/V
50/100
= 13.7 dB
(46)
This is comparable to A_{V_DIFF} found in Electrical Characteristics.
Copyright © 2006–2011, Texas Instruments Incorporated
Product Folder Links: LMH6555
Submit Documentation Feedback
31