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October 23, 1997
Wideband,
low-noise
amplifier drives 50 ohm loads
Mitchell Lee, Linear Technology
Corp, Milpitas, CA
The circuit
in Figure
1 is a
wideband, high-gain amplifier with two useful features
not often found in combination: The input-referred noise
is 2.6 nV/(square root Hz), yet it can deliver 27 dBm into
a double-terminated 50 ohm load or 30 dBm into a
single-terminated load. The circuit uses two monolithic
op amps to achieve this performance. The input amplifier,
which the circuit configures for a gain of 25, has a
1-GHz gain-bandwidth product and an input noise of only
2.6 nV/(square
root Hz).
The second stage can directly drive 50 ohms, slews
900V/µsec, and has a gain of 8. With the load
double-terminated, the overall gain is 100 (40 dB). The
circuit can serve as a low-noise preamp for spectrum
analyzers and other 50 ohm measurement equipment or as an
output-power amplifier for signal generators and
oscillators.
The
amplifiers collectively contribute perhaps 2-dB peaking
to the small-signal response, and the rest is largely a
function of layout and construction practices. An
air-wired-on-copper-clad construction (components
soldered directly together approximately 1/2 in. above
the copper board that serves primarily as ground with
bypass-capacitor connections as standoffs) results in
total peaking of 3 dB and a 60-MHz, 3-dB bandwidth (Figure 2). The power bandwidth is greater
than 10 MHz. With 5% resistors, the worst-case gain error
can be as high as 1.6 dB. If absolute gain accuracy is
important, use 1% resistors to reduce worst-case gain
error to less than 0.5%. (DI #2101)
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