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4. CIRCUIT DESCRIPTION
The FTA820A Amplifier has a noninverting gain of
200. The FTA420C has a noninverting gain of 20.
The FTA820A is composed of eight separate
amplifiers; the FTA420C has four separate
amplifiers. The amplifiers are constructed on a
single printed wiring board. Each section is
identical, and the circuit description that follows
relates to any of the four sections. Component
designations are the same for each channel except
that channel 1 components have a prefix of “1”,
channel 2 components have a prefix of “2”, and so
on for all four channels.
The “A” version has five gain stages, 1Q1 through
1Q4, connected in the Common-Emitter
configuration, and 1Q5 connected in the Emitter-
Follower configuration. Each stage is ac-coupled to
isolate bias voltages and to strongly reject low-
frequency components. Input protection is furnished
by diode 1D1.
Consider the first stage formed around 1Q1.
Resistor 1R2 furnishes base-injection bias for the
transistor, the series combination 1C2 and 1R3
provides shunt feedback, and resistor 1R4 is an
unbypassed emitter resistor which, along with 1C3,
peaks the high-frequency response of the stage.
Each of the other Common-Emitter stages operated
in a similar manner with specific component values
set for optimum gain distribution and rise time. The
final two Common-Emitter stages, 1Q3 and 1Q4,
have variable peaking capacitors, 1C20 and 1C21,
which are set in test to give an overall rise time of
<1 ns. Transistor 1Q5 is connected in the Emitter-
Follower configuration and provides excellent
output drive capability, 0 to
5 V, and low output
impendence, <1
.
The grounding for each section of the amplifier is
isolated from the frame except at the front panel.
Each individual amplifier section receives its circuit
ground via coax to the front panel, and a high-
quality ground connection must be maintained for
proper operation.
The “C” version has two fewer Common-Emitter
amplifier stages than the “A” version. There is no
signal inversion from input to output, and the
overall gain is nominally 20. The operation of each
stage is similar to that of the “A” version except for
the distribution of gain and rise time among the
various amplifier stages. Peaking capacitors 1C3
and 1C7 are set in test to give an overall rise time
<1 ns.
5. MAINTENANCE
The FTA820A and FTA420C require very little maintenance other than routine removal of dust and tightening
of mechanical connections. Be sure to observe the cleaning instructions and safety warnings at the front
of this manual.
