2-4
RC Network
Figure 2-2, view (A), shows the block diagram of an amplifier with an RC network through which
regenerative feedback is provided. The RC network also acts as the frequency-determining device. View
(B) shows a vector analysis of the signal E at various points in the circuit.
Figure 2-2A.—RC oscillator. AMPLIFIER WITH AND RC FEEDBACK NETWORK.
Figure 2-2B.—RC oscillator. VECTOR ANALYSIS
To analyze the operation of the circuit in view (A), assume that the amplifier is a common-emitter
configuration. The signal on the collector (M) is 180 degrees out of phase with the signal (input) on the
base (R). For the circuit to produce regenerative feedback, the RC network must provide a 180-degree
phase shift of the collector signal. When power is applied to the circuit, a noise voltage (noise contains
many different frequencies) will appear on the collector. This noise signal is represented by vector LM in
view (B). As the signal couples through C1 and across R1 (view (A)), a phase shift occurs. The voltage
across R1 (E
R1), represented by vector LN, has been shifted in phase (about 60 degrees) and reduced in
amplitude. The signal at point N (view (A)) is then coupled to the next RC section (R2 and C2). Using the
same size resistor and capacitor as before will cause another 60-degree phase shift to take place. The
signal at point P is the voltage across R2, represented by vector LP. Now the signal at point P has been
shifted about 120 degrees and its amplitude is reduced still further. The same actions occur for the last
section (R3 and C3). This signal experiences another 60-degree phase shift and has further amplitude
reduction. The signal at point R (ER3) has been shifted 180 degrees and is represented by vector LR.
Notice that point R is the input to the base of the common-emitter amplifier. Also, vector LR shows
that the signal on the base is regenerative (aiding the circuit operation). This meets the regenerative
feedback requirement. An exact 60-degree phase shift per stage is not required, but the sum of the three
phase shifts must equal 180 degrees.
