■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest
to the hobbyist. Feel free to participate with
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Join us as we delve into the
basics of electronics as applied
QAs you may know, dual gate
JFETs/MOSFETs have been
phased out, and those that
can be found are very
expensive. Somewhere I have seen a
circuit that duplicates, say a 3N201
or a 40573 being used as a mixer in a
receiver, using two MPF102s. I would
appreciate a circuit, if possible.
— Barry J Maloney
AI was not aware that dual gate
FETs were being phased out.
I think it is that thru-hole parts
are not in demand because
the electronic world is going to surface-mount. I found three dual gate
MOSFETs presently available: BF1211,
BF908, and BF991, all in SMD packages.
A mixer consists of a non-linear
device which handles two signals of
different frequency. The output has
the original two signals plus the sum
and difference frequencies. A filter is
required at the output to select
the desired frequency, usually the
difference frequency. A diode makes a
good mixer but the best you can do is
- 6 dB gain from RF input to IF output.
An XOR logic gate can also be used as
a mixer within its frequency range.
Your request for a two transistor
■ FIGURE 1
circuit brought a cascode circuit to
mind, so I threw together the circuit of
Figure 1 in the LTspice simulator. The
RF input to J1 is 2 mV peak-to-peak at
100 MHz; the local oscillator (LO)
input to J2 is five volts p/p at 110
MHz. The output circuit is tuned to
the difference frequency of 10 MHz.
Figure 2 is the output signal. The 10
MHz is about 70 mV p/p, a gain of
over 30 dB. The LO output is significant, more filtering is needed.
A single transistor can be used as
a mixer and oscillator at the same
time, but the problem is that a strong
RF signal can “pull” the LO off
frequency, causing increased interference and reduced gain. This problem
increases as the RF and LO signals get
There is a lot of interest nowadays
in balanced mixers which will reject
the LO signal, making the output filtering easier. A single balanced mixer
rejects just the LO, a double balanced
mixer rejects both the RF and the LO.
Figure 3 is a single balanced mixer.
The input transformer is center-tapped
to provide push-pull drive to the
output transformer, through the JFETs.
The local oscillator drive is common
mode and does not couple to the
output. The output transformer must
be well balanced, such that when the
same signal is applied to both sides,
the result is cancellation. As you can
see in Figure 4, there is no LO in the
output waveform. The 10 MHz is 70
mV peak, a gain of over 36 dB.
I used the 2N5484 because it was