Page 200 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The second part of this book will concentrate on tutorials of various subjects related

to electronics, so I hope that readers will stick around for that. I will do my best to
explain some of the principles of electronics with no more than high school math.
So hang on, there is more to come.
Chapter 13

Oscillator Circ1uits

In Chapters 8, 9, and 10, oscillator circuits were used as local OSCillators and
converters (mixer and oscillator combination). In this chapter we will explore some

"simple" analyses of a one-transistor oscillator circuit and a differential-pair
OSCillator. Both of these types of inductor/capacitor (LC) circuits have been used in
some of the superheterodyne radio deSigns, and in a sense, this chapter is a
"tutorial" on how these circuits work in more detail.
Although the oscillator circuits presented in this chapter are "simplified" for analysis
sake and for ease of building and experimentation, the basic oscillator principles

still apply. For example, the one-transistor circuit will use capacitors for "stepping
down the voltage" instead of an oscillator coil/transformer.
The objectives for the analysis of oscillator circuits are as follows:
1. Introduce the reader to transconductance,which is related to the

voltage-to-current gain.
2. Show how transconductance determines whether a circuit will oscillate reliably.
3. IUustrate that there are two types of transconductances-small-signal
transconductance and large-Signal transconductance.
4. Explain the role of the inductor and capacitors used in the oscillator circuit to
determine oscillator frequency and equivalent turns ratio for a transformer.

But first let's take a systems approach to how oscillators work in general.
A condition for oscillation includes the following characteristics:
1. The total gain from the amplifier to the LC circuit and back to the input of the

amplifier must exceed 1; typically, the gain is at least 2.
2. The total phase shift around the system must total to 0 or 360 degrees. So, if
the amplifier has an inverting gain (e.g., common-emitter amplifier), the other
components must deliver 180 degrees of phase shift. And if the amplifier has a
noninverting gain (e.g., common or grounded-base amplifier), the other
components typically must have a net phase shift of 0 degree.

Figure 13-1 shows an oscillator system. A gain amplifier's output terminal is
connected to a resonant filter, and the output of the resonant filter is fed back to
the input terminal of the gain amplifier. The filter is commonly implemented as a
parallel LC circuit.