From: Steven Weber (firstname.lastname@example.org)
A couple of people have asked me what values to use in a discret 3028, so
figure might as well post it here for everyone.
In a CA3028 there are two transistors that form a differental amplifier.
The emitors are connected together and go to the collector of a constant
current sink transistor. First the differental pair must be biased. This is
generaly done by placing the bias resistors on the center tap of a
transformer. A 1.5K to ground (R1) and a 5.6K to 10K to 12 volts (R2) is a
good start. Bypass the ct with a 0.1 ufd.
The current sink transistor has a 100 to 330 ohm resistor, (R3) bypass by a
0.1 ufd cap from the emitor to ground. The Base is biased by a 1K (R4)
from base to ground and a 4.7K (R5) to the wiper of a 10K trim pot. One
side goes to ground and the other to 12 volts. Now you can adjust the
trimmer to give the best gain. The adjustment is fairly narrow, but now
that I think about it, if the 4.7K were to be made larger, (say 10K to 33K)
than there would be a wider adjusment range on the trimmer.
This is the best way to go, as now you can use the trimmer as a manual gain
control. It also allows for variations in transistors, which may vary
greatly from device to device or lot to lot. It also makes the other
resistor values reasonably non-critical.
To use this configuration as a mixer, simply feed your LO to the base of
current sink transistor. About 1 v p-p is good. You will get good
conversion gain and good isolation of the LO.
On the output side, you will need to connect a center taped transformer to
the collectors. If it is to be a tuned circuit, I found that putting most
of the capacatince from each collector to ground, instead of across the
coil helps stability. Since you will most likely be winding the
transformer on a toriod, a trimmer cap can be placed across the coil for
tuning. It's probably be best to keep the input side untuned, unless it's a
mixer. If you do tune the input, make that a sperate winding from the one
that drives the base(s), which should be a relativly low impeadance.
To use this as a product detector, use a center taped audio transformer.
Bypass the collectors to ground with 0.001 ufds.
There are ways to make this a single ended input/output, but keeing it
push-pull will give you the best gain.
Hope this is understandable. It should be enough to get you going. Boy, I'm
starting to wish I had a page scanner and a web page!
Steve, KD1JV....In the White Mountains of New Hampshire
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