LMH6555 Datasheet, PDF (29/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
Using the calculated swing on V_IN+ with known V_IN, one can estimate the input impedance, R_IN as follows:
'V_IN+
150 mV
R_IN =
'I_IN+
=
= 50:
(-1.26 + 4.26) mA
(38)
Differential Input Analysis
Assume that the LMH6555 is used as a differential amplifier with a transformer with its Center Tap at ground as
shown in Figure 48:
R_S1
V₁
50: _VIN+
V_OUT-
LMH6555
V_IN-
V₂
R_S2
50:
R_L
100:
V_OUT+
Assuming transformer secondary, V_IN, of 300 mV_PP
Figure 48. Differential Input Drive
The input voltages (V_IN+ and V_IN^−) can be derived using the technique explained previously. Assuming no
transformer output and referring to the schematic of Figure 45:
Vx Vx
25 ^{+ 50 + 39 = 12.6 mA Ÿ Vx = Vy = 0.246V}
V_IN+ =
50
50 + 39
x
0.246
Ÿ _VIN+
= V_IN- =
0.138V
(39)
The peak V_IN+ and V_IN^− voltages can be determined using the transformer output voltage. Assuming there is 0.3
V_PP of signal across the transformer secondary, ½ of that, or 0.15 V_PP (±75 mV peak), would appear at each
input side (V₁ or V₂ in Figure 48). Here is the derivation of the LMH6555 input terminal’s peak voltages.
Vx Vx ± 0.075
+
= 12.6 mA Ÿ Vx =
25
89
262.4 mV
229.5 mV
(40)
When V₁ swings positive, V₂ will go negative by the same value, and vice versa. Therefore, the values derived
above for V_x can be used to determine the average emitter voltage, as described earlier:
Vx +
2
Vy
=
262.4
mV + 229.5
2
mV
= 245.9
mV
=
Emitter
Voltage
V_IN+ = ±75 mV ± 50
±75 mV ± 245.9 mV
89
V_IN+ =
171.0 mV
105.3 mV
and by symmetry: V_IN- =
105.3 mV
171.0 mV
(41)
With the transformer voltage of 0.3 V_PP, each input (V_IN+ and V_IN^−) swings from 105.3 mV to 171.0 mV or about
65.7 mV_PP. The input voltages are shown in Figure 49.
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