Page 104 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

to at least 10
n

. Therefore, one way to effectively reduce or at least partially null or cancel out the
internal coil resistance is to provide a negative resistance in parallel with the coil.
This negative resistance is generated by an active-amplifying device such as a
transistor along with applying positive feedback.
What happens is the amplifying device pumps energy back (via positive feedback)
into the tank circuit to overcome the resistive loss. When a short pulse excites an

inductor capacitor tank circuit, the tank circuit will "ring" at its resonant frequency
but will decay and fail to ring after a period of time. The Q multiplier effect from a
positive-feedback circuit pumps energy back to the coil to sustain a longer ringing
effect when excited by a short pulse.

Thus the Q multiplier increases the original Q of the coil by a factor that is
determined by the amount of positive feedback applied. With no positive feedback,
the Q of the coil is still the original Q. The more positive feedback is applied,
though, the higher is the multiplying effect on the original Q of the coil. (There will
be a follow-up on the subject of Q in the later chapters of this book such as
Chapters 17 and 20.)

Design Considerations for a Regenerative Radio

Two specific characteristics are needed to design a regenerative radio. They are:
1. The RF signal that is being amplified must be "strong" enough to work on its
own without positive feedback to raise the amplitude level. That is, if the RF signal
is too small to begin with, trying to raise its level via positive feedback may lead to
oscillation.
2. The positive feedback must be controllable such that the gain can be raised

easily while not causing oscillation of the RF amplifier.
If you take a look at Figure 7-2, you will see a tickler oscillator circuit, which looks
like a regenerative radio.
In the figure, a resonant circuit is formed by the Ll primary and VC1, which

resonates at a high impedance with 0 degrees of phase shift. The resonant circuit is
amplified by Q1 and Q2, with output current from Q2 fed to the Ll secondary.
Current flowing into the Ll secondary (10 turns) creates a positive-feedback
condition, which causes a sustained oscillation. Any RF signal picked from the Ll
primary winding is now small compared with the continuous-wave (CW) signal it is
generating at the base of Q1 via oscillation. Thus the RF signal is basically "washed
out" by the oscillation signal.

For a larger picture of what's going on, let's take a look at Figure 7-1 again. Note
that the number of turns on Ll's secondary winding is in a range of 1 to 10 turns.
The reason is that when the gain control is set by changing the gain of the RF
amplifier of Q2 via variable resistor VR1, there will be a "sweet" or optimal number

of turns on Ll's secondary. This optimal number of turns on Ll allows the