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.