Diagram Guide

The circuit presented here makes amplitude modulation possible, and it also has the significant advantage that it replaces the somewhat exotic and quite expensive OP603AP output opamp with a standard type. Of course, this amplitude modulator can also be used with other models of function generator or for other purposes. As you know, the gain of an NE592 video opamp can be set to 400, 100 or 10 by means of an external jumper. Intermediate settings can be achieved by using a suitable resistance in place of the jumper. This adjustment takes place in the emitter leads of the differential amplifier, directly at the input to the opamp, where the signal amplitude is low.


A BF245B FET is used here as a controllable resistance. With suitably low signal levels, it provides at least 50% of clean amplitude modulation for modulating signals (LF) up to 10 kHz and modulated signals (HF) up to 20 MHz. The FET can also be driven with a DC voltage to control the amplitude of the output signal over a 10:1 range with low distortion. Any slight asymmetry of the modulated signal can be corrected by applying a small correction voltage via P1. P2 is used to bias the FET at around –2.5 V. The output stage is built using discrete transistors and guarantees a 50 Ω output impedance with low DC offset.

The complete circuit can deliver a constant amplitude output signal of up to 2.5 Vpp (unmodulated) for frequencies ranging to over 20 MHz. If the signal is not modulated, the maximum amplitude can be increased somewhat. Output level controls (a potentiometer and/or range switches), if used, should be placed between the NE592 output and the input of the output stage. In such cases, an emitter-follower stage with a high input impedance might be a good idea, since the opamp should operated with a load of at least 1kΩ. Conceivably, the gate of the FET could be driven via an additional opamp, together with the demodulated signal from the output of the NE592 applied as negative feedback, to achieve higher modulation levels.

The Schematic above shows a 10A power suplly with a 5V output and with power 50W. It is a flyback converter operating in the continuous mode. The circuit features a primary side and secondary side controller with full protection from fault conditions such as overcurrent. After the fault condition has been removed the power supply will enter the soft start cycle before recomming normal operation.

Component Values :
Resistor
R1_____100Ω
R2_____1Ω 1W
R3_____10Ω
R4_____100KΩ
R5_____0.33Ω 1W
R6_____10KΩ
R7_____390Ω
R8_____22KΩ
R9_____68Ω
R10____10Ω
R11____3.3Ω
RL_____5Ω 10W

Capacitor
C1_____0.022uF 400V
C2_____470uF  250V
C3_____470uF
C4_____220pF
C5_____470pF
C6_____2200pF
C7_____270pF
C8_____39pF
C9_____11,000uF
C10____10uF
C11____0.047uF


Diode
D1____1N4937
D2____MBR1035
M1____Diode Bridge

Inductor , Transformator
L1_____25uH
T1_____Lp - 9 mH = 1 : 15
T2_____50 uH. n = 1: 3

Transistor
Q1____BUZ80A
Q2____GE IRF823