Figure 14. Output indicator circuit.
degenerative feedback is applied to permit oscillations
49. Audio Oscillator V3 and V4
and to assure stability of operation.
wide-band resistance-coupled audio amplifier with a
b. The frequency determining elements of the Wien
resistance-capacitance regenerative feedback circuit
bridge audio oscillator peculiar to the 1000 ~ position of
called a Wien bridge. The circuit operates in the following
manner: If the positive half-cycle of a 1,000-cps audio
MODULATION switch S2 are capacitor C8 and resistor
signal appears at the grid of V3, the signal is amplified
R26 in parallel and resistor R23 as shown in figure 15.
and inverted by V3. The negative signal, developed
One of the four legs of the Wien bridge oscillator is made
across plate resistor R19, is applied through C4 across
up of C8, R26, and C11 in parallel. The second leg is
R28 to the grid of pentode amplifier V4. The signal again
made by resistor R23 in series with paralleled capacitors
is amplified and inverted and the positive signal,
C9, C10, and C12. The third and fourth legs of the bridge
developed across plate resistors R17 and R18, is coupled
are made by R27 and ballast lamp E1. Trimmer capacitor
by C2B to the grid of V3 through R23, C9, C10, and C12
C8 is used for the exact adjustment of 1,000 cps. The
in parallel, and section 2 of switch S2. Since the original
overall frequency range of the oscillator is adjusted by C9
signal at the grid of V3 is positive, the feed-back voltage
to include 1,000 cps and 20,000 cps within its limits. The
is of the proper polarity to sustain oscillations. The
frequency determining elements peculiar to the 1600 ~
positive feed-back bridge elements R26 with C8 and C11,
position of MODULATION switch S2, are C7 and R25 (fig.
and R23 with C9, C10, and C12, are frequency sensitive
44) in parallel and R22. In the 20KC position, the
and attenuate all except the desired feed-back
frequency determining elements are C6 and R.24 in
frequencies. The oscillator will oscillate at the frequency
parallel and resistor R21. The amplitude of the audio
determined by the bridge elements. The positive output
oscillations is maintained at a constant level by
of V4 also is applied through C2B across R27 and E1.
degenerative feedback controlled by the action of E1. As
Since E1 serves as the cathode resistor of V3, the voltage
the amplitude of audio oscillations increases, the
appearing across E1 is degenerative or out of phase. If
resistance of ballast lamp E1 increases proportionately.
the degenerative voltage applied to the cathode is greater
As a result, the amount of degenerative feedback
than that applied to the grid, the oscillator will not
developed across E1 is increased, and the amplitude of
function.
By adjusting R27, the proper amount of
oscillations is decreased. Additional degenerative
30
