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I was building a receiver circuit ( superhet) for shortwave radio, primarily for 20m ( 14mhz) band. The circuit uses 3N211 or 40673 mosfet for mixer. However I have only these components available at my disposal :

BC547----- BC557------ 2N2222------- 2N3904

I searched this website for mixer circuits using BJT :

http://www.radio-electronics.com/info/rf-technology-design/mixers/bjt-transistor-mixer-multiplier.php

Now my question is

1) Will this circuit suffice practically

2) Since 40673 mosfet is unavailable, can any of the above mentioned bjt be used in its place ( No other component / IC whatsoever is available at my place) ?

The block diagram of the above mentioned scheme is below :

enter image description here

For additional details see :

http://www.sm0vpo.com/_pdf/miscellaneous_tech_info/Design%20tips%20for%20Simple%20Superhet%20Receivers.pdf

( I have decided to remove BFO since I do not plan to communicate on Morse Code. Also IF filter will be used)

Another of my concerns is that the circuits given in the above mentioned link are simply too complicated for me to understand. So if I substitute simple textbook circuits ( or those found on online tutorials) with required component values ( For example as local oscillator, I plan to use a Hartley oscillator with variable capacitor and an active op amp filter ) will this suffice or I need to stick with the same circuits ?

Thank you

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Dual gate MOSFETs (tetrodes) have characteristics that make them very suitable for RF mixers. Typically RF is applied to G1 the first gate, and local oscillator is applied to G2. This maintains good isolation between oscillator and RF circuits, and low third order intermodulation (by which strong signals can wipe out nearby weak signals)

While just about any triode transistor (bipolar or FET) can be pressed into service as some sort of mixer, getting decent performance out of them is quite an art.

Better to build the circuit as designed if possible. Perhaps use a more readily available BF981 (more associated with VHF). perhaps two JFETs in a "cascode" configuration, which operates a bit like the tetrode MOSFET. (Figure1 on the Wiki : RF in to Vin, local oscillator to the grid (cough, gate) of the upper FET, though it is shown grounded in the article.

Or substitute a different mixer; such as a balanced mixer using schottky diodes.

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  • \$\begingroup\$ I only have the transistor models mentioned above. No more. So according to you any of these can be substituted as per my requirements ? \$\endgroup\$ – Plutonium smuggler Jul 2 '14 at 11:34
  • \$\begingroup\$ With some loss in performance, maybe yes. Look at the cascode link for a way to use 2 transistors instead of a tetrode. \$\endgroup\$ – Brian Drummond Jul 2 '14 at 11:35
  • \$\begingroup\$ Ok. Can you also address my concern in the last paragraph ? \$\endgroup\$ – Plutonium smuggler Jul 2 '14 at 11:39
  • \$\begingroup\$ Sorry, no. Build, test, measure, practice. Or simulate. Either way, skills worth learning. \$\endgroup\$ – Brian Drummond Jul 2 '14 at 11:48
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    \$\begingroup\$ @brian Drummond .You can cascode a JFET with a JFET or even a BJT to make a mixer .I used a couple of MPF102 which were still available. \$\endgroup\$ – Autistic Mar 23 '16 at 16:28
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You can get pretty much any transistor acting as an RF mixer (aka 2-quadrant multiplier) you've just got to bias it in the correct region (low collector voltage to the left of the following diagram): -

enter image description here

Why is it important to use this area as a mixer - here's a closer look at a JFET: -

http://www.angelfire.com/planet/funwithtransistors/images/FIG-4-18.gif

This is a JFET's response because it is diagrammatically easier to see what's going on and I didn't want to draw something from scratch. To the left of the pinch-off voltage is where the transistor has to operate and this means biasing the device correctly. You should be able to see that depending on the gate voltage the characteristic is linear like a resistor - a gate-voltage controlled variable resistor.

The BJT has a very similar region where under about 5V (on the top diagram) you can, in effect control the BJT like a variable resistor. This means you can modulate a carrier applied to the collector with a modulating current into the base. DC offsets have to be observed and AC signal applied are not to be very big.

You can do it with any of the transistors listed so if you need more help show the circuit you mean.

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  • \$\begingroup\$ I have added additional details to the questions. Please address my issues. \$\endgroup\$ – Plutonium smuggler Jul 2 '14 at 6:55

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