1.0 Application Notes
1.1 Background
The LMC6035/6 is exceptionally well suited for low voltage
applications. A desirable feature that the LMC6035/6 brings
to low voltage applications is its output drive capability — a
hallmark for National’s CMOS amplifiers. The circuit of Fig-
ure 1 illustrates the drive capability of the LMC6035/6 at 3V
of supply. It is a differential output driver for a one-to-one au-
dio transformer, like those used for isolating ground from the
telephone lines. The transformer (T1) loads the op amps
with about 600Ω of AC load, at 1 kHz. Capacitor C1 functions
to block DC from the low winding resistance of T1. Although
the value of C1 is relatively high, its load reactance (Xc) is
negligible compared to inductive reactance (XI) of T1.
DS012830-45
FIGURE 2. Output Swing Performance of
the LMC6035 per the Circuit of Figure 1
DS012830-44
FIGURE 1. Differential Driver
DS012830-46
The circuit in Figure 1 consists of one input signal and two
output signals. U1A amplifies the input with an inverting gain
of −2, while the U1B amplifies the input with a noninverting
gain of +2. Since the two outputs are 180˚ out of phase with
each other, the gain across the differential output is 4. As the
differential output swings between the supply rails, one of
the op amps sources the current to the load, while the other
op amp sinks the current.
FIGURE 3. Output Swing Performance of
Benchmark Op Amp per the Circuit of Figure 1
Notice the superior drive capability of LMC6035 when com-
pared with the benchmark measurement — even though the
benchmark op amp uses twice the supply current.
Not only does the LMC6035/6 provide excellent output swing
capability at low supply voltages, it also maintains high open
loop gain (A VOL) with heavy loads. To illustrate this, the
LMC6035 and the benchmark op amp were compared for
their distortion performance in the circuit of Figure 1. The
graph of Figure 4 shows this comparison. The y-axis repre-
sents percent Total Harmonic Distortion (THD plus noise)
across the loaded secondary of T1. The x-axis represents
the input amplitude of a 1 kHz sine wave. (Note that T1 loses
How good a CMOS op amp can sink or source a current is
an important factor in determining its output swing capability.
The output stage of the LMC6035/6 — like many op
amps — sources and sinks output current through two
complementary transistors in series. This “totem pole” ar-
rangement translates to a channel resistance (Rdson) at each
supply rail which acts to limit the output swing. Most CMOS
op amps are able to swing the outputs very close to the
rails — except, however, under the difficult conditions of low
supply voltage and heavy load. The LMC6035/6 exhibits ex-
ceptional output swing capability under these conditions.
%
about 20 of the voltage to the voltage divider of RL (600Ω)
and T1’s winding resistances — a performance deficiency of
the transformer.)
The scope photos of Figure 2 and Figure 3 represent mea-
surements taken directly at the output (relative to GND) of
U1A, in Figure 1. Figure 2 illustrates the output swing capa-
bility of the LMC6035, while Figure 3 provides a benchmark
comparison. (The benchmark op amp is another low voltage
(3V) op amp manufactured by one of our reputable
competitors.)
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