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HFA1106 Datasheet(PDF) 5 Page - Intersil Corporation

Part No. HFA1106
Description  315MHz, Low Power, Video Operational Amplifier with Compensation Pin
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Maker  INTERSIL [Intersil Corporation]
Homepage  http://www.intersil.com/cda/home
Logo INTERSIL - Intersil Corporation

HFA1106 Datasheet(HTML) 5 Page - Intersil Corporation

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Application Information
Optimum Feedback Resistor
All current feedback amplifiers (CFAs) require a feedback
resistor (RF) even for unity gain applications, and RF in
conjunction with the internal compensation capacitor sets
the dominant pole of the frequency response. Thus the
amplifier?s bandwidth is inversely proportional to RF. The
HFA1106 design is optimized for RF = 150? at a gain of +1.
Decreasing RF decreases stability resulting in excessive
peaking and overshoot - Note: Capacitive feedback causes
the same problems due to the feedback impedance
decrease at higher frequencies. At higher gains, however,
the amplifier is more stable, so RF can be decreased in a
trade-off of stability for bandwidth (e.g., RF = 100? for
AV =+2).
Why Use Externally Compensated Amplifiers?
Externally compensated op amps were originally developed
to allow operation at gains below the amplifier?s minimum
stable gain. This enabled development of non-unity gain sta-
ble op amps with very high bandwidth and slew rates. Users
needing lower closed loop gains could stabilize the amplifier
with external compensation if the associated performance
decrease was tolerable.
With the advent of CFAs, unity gain stability and high perfor-
mance are no longer mutually exclusive, so why offer unity
gain stable op amps with compensation pins?
The main reason for external compensation is to allow users
to tailor the amplifier?s performance to their specific system
needs. Bandwidth can be limited to the exact value required,
thereby eliminating excess bandwidth and its associated
noise. A compensated op amp is also more predictable;
lower lot-to-lot variation requires less system overdesign to
cover process variability. Finally, access to the internal high
impedance node allows users to implement external output
limiting or allows for stabilizing the amplifier when driving
large capacitive loads.
Noise Advantages - Uncompensated
The HFA1106 delivers lower broadband noise even without
an external compensation capacitor. Package capacitance
present at the Comp pin stabilizes the op amp, so lower
value feedback resistors can be used. A smaller value RF
minimizes the noise voltage contribution of the amplifier?s
inverting input noise current - INI x RF, usually a large con-
tributor on CFAs - and minimizes the resistor?s thermal noise
contribution (4KTRF). Figure 1 details the HFA1105 broad-
band noise performance in its recommended configuration
of AV = +2, and RF = 510?. Adding a Comp pin to the
HFA1105 (thereby creating the HFA1106) yields the 23%
noise reduction shown in Figure 2. In both cases, the scope
bandwidth, 100MHz, limits the measurement range to pre-
vent amplifier bandwidth differences from affecting the
Offset Advantage
An added advantage of the lower value RF is a smaller DC
output offset. The op amp?s inverting input bias current (IBI)
flows through the feedback resistor and generates an offset
voltage error defined by:
Reducing RF reduces these errors.
Bandwidth Limiting
The HFA1106 bandwidth may be limited by connecting a
resistor, RCOMP (required to damp the interaction between
the compensation capacitor and the package parasitics),
and capacitor, CCOMP , in series from pin 8 to GND. Typical
performance characteristics for various CCOMP values are
listed in the specification table. The HFA1106 is already
unity gain stable, so the main reason for limiting the band-
width is to reduce the broadband noise.
Noise Advantages - Compensated
System noise reduction is maximized by limiting the op amp to
the bandwidth required for the application. Noise increases as
the square root of the bandwidth increase (4x bandwidth
increase yields 2x noise increase), so eliminating excess
EN = 456µ VRMS
RF = 510?
EN = 350µ VRMS
() V

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