AR 10046246, Operating And Service Manual

Page: 35 / 46

Rev B 17
3. THEORY OF OPERATION
3.1 INTRODUCTION
The Model 125A250 RF amplifier consists of a .01-250 MHz RF amplifier assembly. The RF amplifier
assembly consists of a Pre-Amplifier (Pre-Amp), and one final amplifier module.
The power supply section consists of an AC input filter, a circuit breaker, +5V, +15V, and -15V power
supplies; a +34V power supply, voltage regulator circuity, an Operate/Control circuit, and an interface board.
The control system consists of a Control/Fault Board, an Interface Board and remote interfaces for IEEE-488,
RS-232, USB, and Ethernet.
3.2 RF AMPLIFIER OPERATION
3.2.1 A1 2W Pre-Amplifier Assembly (Schematic No. 10041189,
10041352, 10041054)
The 2W Pre-Amplifier Assembly consists of 3 sub-assemblies: the A1 Pre-Amplifier Assembly, the A2
Switch Assembly, and the A3 2W Amplifier Assembly.
3.2.1.1 A1 Pre-Amplifier Assembly (Schematic 10041189)
The Pre-Amplifier PWB Assembly consists of a variable attenuator circuit (Q1-Q4), a gain stage (U4), a
resistive splitter (R13, R14, R16), an RF power detector (U6), and associated control circuitry. The overall
gain of the pre-amplifier assembly is approximately 6-8 dB at minimum attenuation. The power detector (U6)
is used to sense the input RF power and it increases the attenuation of the variable attenuator circuit if an
input overdrive condition is detected.
3.2.1.2 A2 Switch Assembly (Schematic 10041352)
The Switch Assembly is made up of a variable attenuator circuit (Q1-Q4) and associated control circuitry.
The switch can either be in an on or off state depending on the Inhibit input signal (E4). In the off state, when
E4 is pulled low, the switch will reduce the amplifier gain by approximately 40 dB.
3.2.1.3 A3 2W Amplifier Assembly (Schematic 10041054)
The 2W Amplifier Assembly has two gain stages (U1 and Q1) that have a combined gain of approximately 30
dB. The output RF power is greater than 2W at the 1 dB compression point.