Q3 and Q4 are wired as diodes in the feedback of the driver op-amp. This means for output
voltages between -0.6V and +0.6V the gain of the op-amp is very high indeed as it is pretty
much set solely by the ratio of R12 and R11. Thus even a small voltage on the Y input will
lead to 0.6V or -0.6V depending on the polarity of the input. Once this voltage has
been achieved any increase in input voltage will decrease the gain as either Q3 or Q4 turns
increasingly on. Eventually, the gain tends to the ratio of R15 and R11, a much smaller figure.
Unlike the original circuit application, the Oakley Ring Modulator allows you to add a positive
offset voltage to the Y input voltage. Thus effectively increases the gain of the left hand
transconductance amplifier thus allowing the non inverted X signal to pass through to the
output even when no positive Y input is present.
Power is supplied via the usual 10-way 0.1” header. As is the custom for Oakley modules, I
have used ferrite beads to act as high frequency filters on the power lines. Decoupling at the
point of entry is provided by C13 and C15 for the positive rail, and C14 and C16 for the
negative rail. Additional decoupling is also provided elsewhere on the board by the other
capacitors shown at the bottom of the schematic. All these capacitors keep the power supply
clean of noise, and provide a reservoir for the little bursts of current that the circuit takes in
normal operation.
D7 and D8 provide some protection against the power lead being connected in reverse.
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