APPENDIX D --- USE OF THE L-17C WITH SCHOTTKY DIODE DETECTORS
Although the L-17C input amplifier was originally optimized for use with tunnel diode
detectors, it can also be used very effectively with Schottky diode detectors, as outlined in
this appendix.
Tunnel diodes are DC stable over temperature, and they have low impedance in the video
band. For these reasons, the L-17C's front- end amplifier does not use emitter followers
in front of the input differential pair. These would add both noise and DC drift. In addition,
the output of a tunnel diode detector is not large enough to overdrive the L-17C input
amplifier (for reasonable choices of gain), a fact which we shall see is important.
Schottky diodes, on the other hand, are high-Z devices which are rarely used in DC-
coupled configurations because of long-term DC stability problems. Because of their high
Z, little or no noise penalty results from putting an emitter-follower buffer between the
detector and the input differential pair. Furthermore, at high RF input powers, the output
voltage of many Schottky detectors will overdrive the L-17C's front-end amplifier (when its
gain is set high enough to give the desired start of logging). Nevertheless, the high end of
the Schottky output can be used to drive additional log stages provided that this output is
not limited by the front end amplifier. This may be accomplished by using a path to the
additional log stages which is attenuated, rather than amplified. This in turn can be
achieved using buffering, as illustrated below. This approach allows the optimal use of a
Schottky with the L-17C, even at high RF power.
It is important to understand the architecture of the front-end amplifier in order to make the
best use of the L-17C with Schottky detectors. A brief description of the relevant factors
follows.
The amplifier has two stages: a full-differential stage with an input bias current of ~500 na
and a gain of ~40, followed by a single- ended stage with a gain of 20 (see Fig. B1 in
Appendix B). Thus, as long as the amplifier is not overdriven, there is very little voltage
swing across the collectors of the first differential pair. Also, the front pair is biased so that
even if there is some swing, the power expended in each transistor in the front pair is
unaltered. This is essential for good thermal recovery of the amplifiers. The quiescent
voltages at the collectors of the front pair are ~500 mV above ground. Consequently, if
these amplifiers are overdriven, the input transistors will saturate! It is imperative that this
not happen, and this is a critical factor to be considered when using Schottky detectors.
Two factors which may help are the following:
1)
because of the high K of Schottky detectors, the front amplifier will probably be set
at fairly low gain; and
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