THS4081, THS4082
175-MHz LOW-POWER HIGH-SPEED AMPLIFIERS
SLOS274C – DECEMBER 1999 – REVISED MAY 2000
APPLICATION INFORMATION
theory of operation
The THS408x is a high-speed, operational amplifier configured in a voltage feedback architecture. It is built
using a 30-V, dielectrically isolated, complementary bipolar process with NPN and PNP transistors possessing
f
T
s of several GHz. This results in an exceptionally high performance amplifier that has a wide bandwidth, high
slew rate, fast settling time, and low distortion. A simplified schematic is shown in Figure 38.
(7) VCC +
(6) OUT
IN – (2)
IN + (3)
(4) VCC –
Figure 38. THS4081 Simplified Schematic
noise calculations and noise figure
Noise can cause errors on very small signals. This is especially true when amplifying small signals, where
signal-to-noise ratio (SNR) is very important. The noise model for the THS408x is shown in Figure 39. This
model includes all of the noise sources as follows:
•
•
•
•
e
n
= Amplifier internal voltage noise (nV/√Hz)
IN+ = Noninverting current noise (pA/√Hz)
IN– = Inverting current noise (pA/√Hz)
e
Rx
= Thermal voltage noise associated with each resistor (e
Rx
= 4 kTR
x
)
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•
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11
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