LMH6551 Datasheet, PDF (20/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
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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 32. Figure 34 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 LMH6551 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
A_V = 2
V_CM
37.5:
8 V_PP
R_L = 300:
Figure 32. 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 33. Calculating Transformer Circuit Net Gain
20
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