LMH6555_14 Datasheet, PDF (12/37 Pages) Texas Instruments – LMH6555 Low Distortion 1.2 GHz Differential Driver

LMH6555
SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011
CIRCUIT ANALYSIS
Figure 24 shows the block diagram of the LMH6555.
R_C1
V_{CM_REF}
R_C2
+-
A_CM
V⁺
V⁺
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R_T2
A1
A2
V_OUT-
50:
-A
-A
V_IN+
Q1
Q2
R_G1
D1
D2
R_G2
V_x
V_y
R_T1
50:
V_OUT+
V_IN-
R_E1
R_F1
R_F2
R_E2
R_G1 = R_G2 = R_G = 39Ω
R_E1 = R_E2 = R_E = 25Ω
R_F1 = R_F2 = R_F = 430Ω
I_CQ1 = I_CQ2 = 12.6 mA
Figure 24. Block Diagram
The differential input stage consists of cross-coupled common base bipolar NPN stages, Q1 and Q2. These
stages give the device its differential input characteristic. The internal loop gain from V_x and V_y internal nodes
(Q1 and Q2 emitters) to the output is large, such that these nodes act as a virtual ground. The cross-coupling will
ensure that these nodes are at the same voltage as long as the amplifier is operating within its normal range.
Output common mode voltage is enforced through the action of “A_CM” which servos the output common mode to
the “V_{CM_REF}” input voltage.
The discussion that follows, provides the formulas needed to analyze single ended and differential input
applications. For a more detailed explanation including derivations, please see Appendix at the end of
the datasheet.
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