LMH6555 Datasheet, PDF (27/37 Pages) Intersil Corporation – Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1D – NOVEMBER 2006 – REVISED MARCH 2013
EVALUATION BOARD
Texas Instruments suggests the following evaluation board as a guide for high frequency layout and as an aid in
device testing and characterization.
Device
LMH6555
Package
16-Pin WQFN
Evaluation Board Ordering ID
LMH6555EVAL
The evaluation board can be ordered when a device sample request is placed with Texas Instruments.
Appendix
Here is a more detailed analysis of the LMH6555, including the derivation of the expressions used throughout
APPLICATION INFORMATION.
INPUT STAGE
Because of the input stage cross-coupling, if the instantaneous values of the input node voltages (V_IN+ and V_IN^−)
and current values are required, use the circuit of Figure 45 as the equivalent input stage for each input (V_IN+ and
V_IN^−).
V⁺
V⁺
12.6 mA
12.6 mA
V_IN+
Q1
R_G1
39:
V_x
R_E1
25:
Q2
R_G2
39:
V_y
V_IN-
R_E2
25:
Figure 45. Equivalent Input Stage
Using this simplified circuit, one can assume a constant collector current, to simplify the analysis. This is a valid
approximation 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_IN+ and V_IN^−.
With the component values shown, it is possible to analyze the input circuits of Figure 45 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_CM”, shown in Figure 24, 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^st derived emitter voltages. Using this
new emitter voltage, one could recalculate V_IN+ and V_IN^− voltages. The values derived in this fashion will be
within ±10% of the measured values.
Single Ended Input Analysis
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 46.
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