LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 21

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

Using the calculated swing on V
estimate the input impedance, R
Assume that the LMH6555 is used as a differential amplifier
with a transformer with its Center Tap at ground as shown in
Figure 26:
Assuming transformer secondary, V
The input voltages (V
technique explained previously. Assuming no transformer
output and referring to the schematic of Figure 23:
The peak V
transformer output voltage. Assuming there is 0.3 V
nal across the transformer secondary, ½ of that, or 0.15 V
(±75 mV peak), would appear at each input side (V
Figure 26). Here is the derivation of the LMH6555 input
terminal’s peak voltages.
When V
value, and vice versa. Therefore, the values derived above
for V
as described earlier:
Differential Input Analysis
x
can be used to determine the average emitter voltage,
1
swings positive, V
FIGURE 26. Differential Input Drive
IN
+
and V
IN
−
IN
voltages can be determined using the
+
and V
2
IN
, of 300 mV
will go negative by the same
IN
IN
IN
−
) can be derived using the
+
as follows:
with known V
PP
IN
, one can
1
PP
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or V
of sig-
2
PP
in
21
With the transformer voltage of 0.3 V
V
mV
Knowing the device input terminal voltages, one can estimate
the differential input impedance as follows:
This is comparable to R
teristic table.
OUTPUT STAGE AND GAIN ANALYSIS
Differential gain is determined by the differential current flow
through the feedback resistors R
ure 2. Current through R
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
R
IN
T2
FIGURE 28. Output Stage Including External Load R
FIGURE 27. Input Voltages for Figure 26 Schematic
−
PP
, of 50Ω, as shown in Figure 28.
) swings from 105.3 mV to 171.0 mV or about 65.7
. The input voltages are shown in Figure 27.
IN_DIFF
F1
(or R
found in the Electrical Charac-
F2
F1
) sets the V
and R
PP
, each input (V
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F2
as shown in Fig-
OUT
20127765
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−
(or V
IN
T1
+
OUT
and
and
L
+
)