IN CONSUMER ELECTRONICS APPLICATIONS, which
tend to be lower in frequency and less demanding
than typical clock-buffering applications, inexpensive
high-speed op amps (~100-MHz bandwidth)
can offer an attractive option in place of
traditional clock buffers. High-speed amplifiers
can be less expensive than traditional clock
buffers, yet they’re able to accommodate a wide
range of design configurations.
For instance, one good alternative for lowcost
clock buffers is the single-supply op amp,
such as those in Analog Devices’ ADA485X
family and the AD8061. These amplifiers all
feature low supply voltage, low supply current,
a power-down mode for power-sensitive applications,
and rail-to-rail outputs, which enable
wide dynamic range.
One advantage of an op amp versus a traditional
clock buffer is flexibility. Op amps allow
clock pulses to be buffered, amplified, offset,
inverted, summed, subtracted, or filtered.
They provide high input impedance, low input
bias current, low supply current, independent
power-down (for multiple amplifiers in a single
package), low output impedance, and low
propagation delay.
Designers must recognize and adhere to some
operating constraints when using op amps in
clock-buffer applications. For example, voltage
feedback amplifiers specify a gain bandwidth
product. As the closed-loop gain of the amplifier
circuit increases, its
bandwidth decreases.
Therefore, large gains
mean less
bandwidth.
Cascading multiple
amplifiers,
each with lower
gain, allows the amplifier to operate at a higher
bandwidth, preserving the overall gain and bandwidth
of the signal path.
Single-supply operation is important for portable
electronics. By definition, the input common-
mode range of a single-supply op amp
includes the negative rail (ground). Most can go
200 mV below ground. However, this doesn’t
mean that the output can swing below ground.
The output stage of a typical rail-to-rail bipolar
amplifier uses a common emitter configuration.
Consequently, the closest the output can come
to the rail is VCE(sat), which can range from tens
of millivolts to hundreds of millivolts, depending
on the output load.
Fortunately, in these applications, the output
doesn’t usually have to swing all the way to
ground. When the input gets too close to ground
(around 100 to 200 mV), though, the output
stage can saturate, introducing distortion and
long recovery times. In dc-coupled systems,
keep the “low” of the signal above 200 mV or
use a –200-mV negative supply voltage. Either
method will prevent the output stage from going
into saturation.
Amplifiers also specify headroom, or how
close they can swing to the positive rail, so care
must be taken to address the high side of the
input common-mode range as well. If the input
voltage gets too high, the input stage will distort
and cut off.
Figure 1a depicts a single-supply non-inverting
op-amp clock buffer with a gain of +2, and Figure
1b shows its transient response. As configured,
the upper limit for the AD8061 is about 33 MHz.
Its 2-ns propagation delay is comparable to
some dedicated clock buffers.
In some applications, ac coupling can be
employed, allowing higher-bandwidth amplifiers
to be used for extended frequency performance.
These amplifiers will fit into single-supply applications
by biasing the amplifier inputs and outputs
to mid-supply.
A clock buffer implementing Analog Devices’
AD8057 high-speed amplifier was configured for
unity gain (Fig. 2a). It features a 325-MHz bandwidth
and 1150-V/s slew rate. The load resistor
is returned to a voltage that’s the dc average of
the input signal. This ensures that the output
will be referenced back to ground. The upper
operating range of this configuration is approximately
100 MHz. Note that in the clock buffer’s
response, good pulse fidelity is maintained at 90
MHz (Fig. 2b).
So when a design calls for a clock buffer, a
high-speed amplifier can often provide more flexibility
at lower cost, enabling high-speed amplifiers
to compete with traditional clock buffers in
many applications. Either single- or dual-supply
amplifiers can be used, depending on the specific
application.