Page 229 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

that carry the difference- and summing-frequency terms are intermodulation

distortion products or signals.
And in superheterodyne receivers, the IF signal from the mixer has a difference
frequency such as (Fl - F2), where Fl is the frequency of the local oscillator, and
F2 is the frequency of the input RF signal.
Distortion Can Be a Good Thing (for Mixing)

We have seen how an FET with an approximate square-law characteristic can act

as a linear summer and as a mixer/multiplier. This square-law characteristic
generates distortion in terms of harmonic and intermodulation distortion products
at the drain of the FET.
FETs are used as mixers, and often to provide good mixing action. Typically the
local oscillator's voltage (Le., combined with the RF signal) is in the amplitude level
of volts peak to peak. The reason for this is that the FET is such a linear device that

huge amounts of input voltage are required to drive the FET into "usable" distortion
that provides adequate levels of the IF signal.
Single-Bipolar-TranisistorDistortion

Now suppose that we would like to use a bipolar transistor for mixing. As
mentioned previously, a bipolar-transistor amplifier with its emitter grounded
AC-wise has an exponential function:

I -
~
where VBE is the total voltage across the base emitter junction of the transistor,
which includes a DC bias voltage VBEQ plus the AC signal voltage V sig • Thus

+
I . i.
The collector current can be expressed equivalently as

( , l~'" v ~/(!S!J
I
s
The DC or quiescent collector current is just

Figure 14-5 presents an example of a common emitter amplifier. In the figure, a

common emitter amplifier with J3 > lOO, the DC emitter current is essentially equal
to the DC collector current. Thus VE/R E = I cQ .