KH300 Datasheet, PDF (6/7 Pages) Fairchild Semiconductor – Wideband, High-Speed Operational Amplifier

DATA SHEET
To avoid the peaking at low non-inverting gains, place a
resistor R_p in series with the input signal path just ahead
of pin 6, the non-inverting input. This forms a low pass
filter with the capacitance at pin 6 which can be made to
cancel the peaking due to the capacitance at pin 8, the
inverting input. At a gain of +2, for example, choosing
R_p such that the source impedance in parallel with R_i
(see Figure 1), plus R_p equals 175Ω will flatten the
frequency response. For larger gains, R_p will decrease.
Settling Time, Offset, and Drift
After an output transition has occurred, the output
settles very rapidly to final value and no change occurs
for several microseconds. Thereafter, thermal gradients
inside the KH300 will cause the output to begin to drift.
When this can not be tolerated, or when the initial offset
voltage and drift is unacceptable, the use of a compos-
ite amplifier is advised. This technique reduces the off-
set and drift to that of a monolithic, low frequency op
amp, such as an LF356A. The composite amplifier
technique is fully described in the KH103 data sheet.
A simple offset adjustment can be implemented by con-
necting the wiper of a potentiometer, whose end termi-
nals connect to ±15V, through a 20K resistor to pin 8 of
the KH300.
Overload Protection
To avoid damage to the KH300, care must be taken to
insure that the input voltage does not exceed (|V_CC| -
2.5)/A_V. High speed, low capacitance diodes should be
used to limit the maximum input voltage to safe levels if
a potential for overload exists.
If in the non-inverting configuration the resistor R_i, which
sets the input impedance, is large, the bias current at
pin 6, which is typically a few pA but which may be as
large as 18µA, can create a large enough input voltage
to exceed the overload condition. It is therefore recom-
mended that R_i < [(|V_CC| -2.5)/ A_V]/(18µA).
KH300
Distortion and Noise
The graphs of intercept point versus frequency on the
preceding page make it easy to predict the distortion at
any frequency, given the output voltage of the KH300.
First, convert the output voltage (V_o) to V_rms = (V_pp/2√2)
and then to P = (10log₁₀(20V_rms2)) to get output power in
dBm. At the frequency of interest, its 2nd harmonic will
be S₂ = (I₂ - P) dB below the level of P. Its third harmon-
ic will be S₃ = 2 (l₃ = P) dB below P as will the two tone
third order intermodulation products. These approxima-
tions are useful for P < -1dB compression levels.
Approximate noise figure can be determined for the
KH300 using the Equivalent Input Noise graph on the
preceding page. The following equation can be used to
determine noise figure (F) in dB:
F = 10log
1+
v_n2
+ ^in2Rf2
A _v2
4 kTR _s ∆f
Where v_n is the rms noise voltage and in is the rms noise
current. Beyond the breakpoint at the curves (i.e.,
where they are flat), broadband noise figure equals spot
noise figure, so ∆f should equal one (1) and vn and in
should be read directly off of the graph. Below the
breakpoint, the noise must be integrated and ∆f set to
the appropriate bandwidth.
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REV. 1A January 2004