AM25
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L7 and varactor diode D2. The Phase-Locked-Loop portion of the circuit
controls the capacitance of the varactor diode to keep the frequency exactly as
selected. We’ll examine that in more detail later. A buffer amplifier, Q5
increases the output of the oscillator enough to drive final RF amplifier Q3.
The audio input path is routed from J2, the audio input source, to opamp U1
which both amplifies and level shifts the incoming signal. Transistors Q8, Q9
are used to increase the current output capacity of the opamp in order to
modulate the final RF amplifier stage. These transistors are biased to be
operating in the linear region with a no signal output level of exactly one-half
the supply voltage as set by pot R23. The incoming audio signal is therefore
amplified undistorted (for great sounding audio). The audio input level to the
amplifier can be adjusted using R31, the input level adjustment.
Power FET transistor Q3, is the “power amplifier” section of the circuit. Its drain
voltage is supplied through matching auto-transformer L4 from Q8 and Q9,
thus producing an AM output waveform. This signal is then low pass filtered
using C1 - C7 and inductors L1 - 3.
Notice that the audio information is applied at the power amplifier stage. This is
referred to as “high level” modulation, and is commonly used for high power
AM broadcast stations. The distinct advantage to this is that the RF amplifier
need not be biased for linear operation, which is not only very inefficient but
somewhat complicated also. It is much easier and cheaper to manufacture a
linear amplifier for the relatively low frequency audio range, than to perfectly
linear amplify a low level AM RF waveform to a high power RF level. The main
disadvantage of high level modulation is that the audio modulator’s power must
be half that of the final transmitter, not too tough for our low power kit, but try to
imagine the amplifier for a 50,000 watt AM broadcast station! Boy, that audio
amp would sure crank the ‘ol car stereo!
It should also be stated that, due to the linear operation of the amplifiers in this
circuit (transistors Q8 and Q9 biased partly “on”), this circuit will consume a fair
Low Frequency Audio Wa veform
High Freque ncy Radio Carrier
Amplitude Modulated Signal
Summary of Contents for AM25
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