Page 75 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

always will beat field-effect transistors. This gain is usually characterized by
transconductance or mutual transconductance.

The transconductance of a device can be described as the ratio of the
alternating-current (AC) output current divided by the AC input-signal voltage. From
Ohm's law, we know the V = JR, where V is the voltage in volts, J is the current in
amps, and R is the resistance in ohms. Thus, via Ohm's law, R= L1 J or something
that has volts divided by amps. Transconductance is measured by current/voltage

or amps/volts. 50 transconductance is measured in mhos (ohm spelled in reverse)
to signify that its unit is the reciprocal of an ohm. It should be noted that today the
mho is replaced by siemens, usually denoted by 5.
The higher the transconductance, the higher is the gain. 50 let's take a look at the
transconductance of a bipolar transistor versus a field-effect transistor. For an

operating collector current of 2 mA direct current (DC), the transistor has a
transconductance of 0.076 mho or 0.076 5 (5 = siemens, where 1 5 = 1 mho = 1
amp/1 volt), whereas a 2N3819 field-effect transistor at 2 mA gives out only 0.002
5. For a bipolar transistor to yield 0.002 5 of transconductance, we just need to
operate it at 52 ~A, which is much lower than the 2N3819 FET operating at 2,000
~A or 2 mA.

An inherent advantage of a field-effect transistor is that its input terminal (gate)
has close to infinite resistance; that is, a FET does not load down a signal, whereas
a bipolar transistor has a finite input resistance across its base and emitter. But we
will see that this finite resistance does not pose much of a problem at very low
operating currents (e.g., <50 ~A collector current).

To summarize a couple of the goals for low-power designs, we want the following:
1. Operating voltages from 1.2 volts to 2.4 volts or better
2. Bipolar transistors for their high gain at low operating currents (i.e.,
transconductance)
The TRF radiO, as seen in Figure 1-1, consists of an antenna and tunable RF filter.

In the very early radios from the 1920s, the TRF design had the antenna as an
external long wire antenna or an external loop antenna.
One of the first TRF designs shown in this book will use ferrite bar or rod antenna
coils. These antenna coils can be compact, less than 2 inches long. Or for higher

sensitivity, they can be longer, on the order of 3 inches or more (see Figure 3-1).
We also will show TRF designs with an external loop antenna, which is the type
used currently in stereo high-fidelity (hi-fi) receivers (see Figure 3-2). There is an
advantage to using an external loop antenna, in that virtually any example can be
used as long as there are sufficient windings. This loop antenna does not need to
be wound to a specific inductance value to work with the tuning capacitor. Instead,
the external loop antenna is connected to a "stepped down" smaller winding of an

RF transformer, and the larger winding of the RF transformer is connected to the
tuning capacitor.