LMH6555 Datasheet, PDF (28/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
V_IN
0.3 V_PP
R_S1
50:
R_S2
50:
V_IN+
LMH6555
V_IN-
V_OUT-
R_L
100:
V_OUT+
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Figure 46. Single Ended Input Drive
The first task would be to derive the internal transistor emitter voltages based on the schematic of Figure 45
(assuming that there is no interaction between the stages.) Here is the derivation of V_X and V_y:
Vx Vx 0.15
+
25
89
= 12.6 mA Ÿ Vx =
0.279V
0.213V
Vy Vy
+ = 12.6 mA Ÿ Vy = 0.246V
25 89
(36)
V_X varies with V_IN+ (0.213V with negative V_IN swing and 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_x and V_y are equal to the average of V_y with either end of the
swing of V_X. This is calculated below along with the derivation of V_IN+ and V_IN^− based on this new average
emitter voltage (the average of V_X and V_y.)
Vx + Vy 0.279 + 0.246
=
= 0.262V
2
2
0.213 + 0.246
= 0.229V
2
Emitter
= Voltage
Swing
V_IN+ = ± 0.15V ± 50
0.262V
±0.15V - _0.229V
89
V_IN+ =
0.213V
63.2 mV
;
V_IN^- =
50
89
x
0.262V
0.229V
V_IN^- =
0.147V
0.129V
(37)
With 0.3 V_PP V_IN, V_IN+ experiences 150 mV_PP (213 mV - 63.2 mV) of swing and V_IN^− will swing by about 18.6
mV_PP in the process (147 mV – 129 mV). The input voltages are shown in Figure 47.
0.3
V_IN+
0.2
0.1
V_IN
0
150 mV_PP @
138 mV DC
V_IN-
18.6 mV_PP @
138 mV DC
-0.1
-0.2
-0.3
TIME
Figure 47. Input Voltages for Figure 46 Schematic
28
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