LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 20

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

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Appendix
Here is a more detailed analysis of the LMH6555, including
the derivation of the expressions used throughout the Appli-
cation Information.
INPUT STAGE
Because of the input stage cross-coupling, if the instanta-
neous values of the input node voltages (V
current values are required, use the circuit of Figure 23 as the
equivalent input stage for each input (V
Using this simplified circuit, one can assume a constant col-
lector current, to simplify the analysis. This is a valid approx-
imation as the large open loop gain of the device will keep the
two collector currents relatively constant. First derive Q1 and
Q2 emitter voltages. From there, derive the voltages at V
and V
With the component values shown, it is possible to analyze
the input circuits of Figure 23 in order to determine Q1 and
Q2 emitter voltages. This will result in a first order estimate of
Q1 and Q2 emitter voltages. Since Q1 and Q2 emitters are
cross-coupled, the voltages derived would have to be equal.
With the action of the common mode amplifier, “A
in Figure 2, these two emitters will be equalized. So, one other
iteration can be performed whereby both emitters are set to
be equal to the average of the 1
Using this new emitter voltage, one could recalculate V
V
±10% of the measured values.
Here is an actual example to further clarify the procedure.
Consider the case where the LMH6555 is used as a single
ended to differential converter shown in Figure 24.
IN
Single Ended Input Analysis
−
voltages. The values derived in this fashion will be within
IN
−
.
FIGURE 24. Single Ended Input Drive
FIGURE 23. Equivalent Input Stage
st
derived emitter voltages.
IN
+
and V
IN
+
and V
IN
CM
−
).
”, shown
IN
20127709
IN
20127710
−
) and
+
and
IN
+
20
The first task would be to derive the internal transistor emitter
voltages based on the schematic of Figure 23 (assuming that
there is no interaction between the stages.) Here is the deriva-
tion of V
V
0.279V with positive.) The values derived above assume that
the two halves of the input circuit do not interact with each
other. They do through the common mode amplifier and the
input stage cross-coupling. V
of V
below along with the derivation of V
new average emitter voltage (the average of V
With 0.3 V
mV) of swing and V
process (147 mV – 129 mV). The input voltages are shown in
Figure 25.
X
FIGURE 25. Input Voltages for Figure 24 Schematic
varies with V
y
with either end of the swing of V
X
and V
PP
V
IN
y
, V
:
IN
IN
+
IN
+
(0.213V with negative V
−
experiences 150 mV
will swing by about 18.6 mV
x
and V
y
IN
are equal to the average
+
and V
X
. This is calculated
PP
IN
(213 mV - 63.2
−
X
IN
based on this
20127764
and V
swing and
PP
y
in the
.)