Model 75A250A
Manual Text
3-1
REV A
3.1
INTRODUCTION
The Model 75A250A amplifier consists of two (2) modules. The first module (A1) consists of three (3)
cascaded mosfet amplifier stages that provide approximately 33dB of gain. This module drives the second
module (A2) which consists of a dual mosfet amplifier operated as a push-pull stage that provides
approximately 16dB of gain. Input and output networks are utilized to provide optimum power transfer of
the RF signal when the amplifier is connected to a source and load impedance of 50 ohms. Interstage
coupling is the accomplished by means of capacitors between the three (3) cascaded stages and
transformer coupling between these stages and the push-pull output stage. This provides an overall flat
frequency response that is further enhanced by utilizing negative feedback around each transistor stage.
The self-contained power supply consists of 2 switching power supplies, which feature universal input
voltage capability. Gate bias stability is provided by a three (3) terminal regulator set for 15 VDC.
3.2
AMPLIFIER SECTION
Refer to Schematic Diagram No. 1007868 and 1007872
The input signal to the amplifier is fed through the gain control assy to the gate of Q1, which is input level
limited by diodes CR2 and CR3. Biasing for Q1 is accomplished by R10, R14 and R16 which drives the
gate of Q1. Bias voltage for Q2 is adjusted via R6 through R11 and R17. Output of Q2 is coupled
through C4 to the gate of Q3. The output of Q3 is coupled to transformer T1 of the A2 module which is
utilized to drive push-pull stage Q1A and Q1B. Resistor R12 sets bias voltages for Q1A and Q1B, in
conjunction with R10 which is used to equalize the bias voltage on gates of Q1A and Q1B.
The output of Q1A and Q1B are coupled through matching transformers T3 and T4 to the coaxial
connector on front panel of the unit.
Power gain of each of the three (3) cascaded stages are approxi11 dB and +16 dB for push-pull
stage, giving an overall gain of better that +49 dB for the amplifier.
U1 and U4 with their associated circuitry provided over-dissipation protection for the FET’s in the final
amplifier. This is done by sensing the DC input current. When the DC input current exceeds a preset
level, the bias voltage on the output transistor is reduced, limiting the current.
3.3
POWER SUPPLY
Refer to Schematic Diagram No 1011806.
The Power Supply Assembly consists of two switching power supplies: PS1 and PS2. PS2 is a 25-watt,
triple-output power supply with three regulated output: +5V, +12V, and –12V, has a universal input range
of 85-264VAC, 50/60Hz. PS2 supplies ±12V and +5V to the operate/control board and to the gain
control. PS1 is a 28V, 11A supply with a Power Factor Corrected front end: it will operate over an input
voltage range of 85-264VAC, 50/60Hz. PS1 su28V to all of the RF Amplifier gain stages. PS1
also supplies to operate Blower B1, which supplies air flow to cool the heat sink and the power supplies.
