LMH6550 Datasheet, PDF (20/33 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6550
SNOSAK0I – DECEMBER 2004 – REVISED JANUARY 2015
www.ti.com
Typical Applications (continued)
9.2.5 Using Transformers
Transformers are useful for impedance transformation as well as for single to differential, and differential to
single-ended conversion. A transformer can be used to step up the output voltage of the amplifier to drive very
high impedance loads as shown in Figure 30. Figure 32 shows the opposite case where the output voltage is
stepped down to drive a low-impedance load.
Transformers have limitations that must be considered before choosing to use one. Compared to a differential
amplifier, the most serious limitations of a transformer are the inability to pass DC and balance error (which
causes distortion and gain errors). For most applications the LMH6550 will have adequate output swing and drive
current and a transformer will not be desirable. Transformers are used primarily to interface differential circuits to
50-Ω single-ended test equipment to simplify diagnostic testing.
300: TWISTED PAIR
500
37.5: 1:2 (TURNS)
250
+
a 4 VPP
V_CM
-
250
500
V_CM
37.5:
8 V_PP
R_L = 300:
A_V = 2
ENABLE
Figure 30. Transformer Out High-Impedance Load
V_IN * A_V * N
V_L =
§
¨
2
R_OUT
*
N²
¨©
R_L
+1
WHERE V_IN = DIFFERENTIAL INPUT VOLTAGE
N = TRANSFORMER TURNS RATIO =
§
¨
SECONDARY
¨© _PRIMARY
A_V = CLOSED LOOP AMPLIFIER GAIN
R_OUT = SERIES OUTPUT MATCHING RESISTOR
R_L = LOAD RESISTOR
V_L = VOLTAGE ACROSS LOAD RESISTOR
Figure 31. Calculating Transformer Circuit Net Gain
20
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