"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Sine Wave Oscillator Circuits
15-12
the circuit for R G as shown in Figure 15–10. The lamp resistance, R LAMP, is nominally selected
as half the feedback resistance, R F, at the lamp current established by R F and
R LAMP. When the power is first applied the lamp is cool and its resistance is small, so the
gain is large (> 3). The current heats up the filament and the resistance increases, lowering
the gain. The nonlinear relationship between the lamp current and resistance keeps
output voltage changes small. Figure 15–11 shows the output of this amplifier with a distortion
of 1% for f OSC = 1.57 kHz. The distortion for this variation is reduced over the basic
circuit by avoiding hard saturation of the op amp transistors.
TI–327
Lamp
RL
C
RF
377 Ω
+5 V
_
VREF
0.833 V
+
10 nF
10 nF
TLV247x
R
10 kΩ
C
R
10 kΩ
Figure 15–10. Wien Bridge Oscillator with Nonlinear Feedback
VOUT = 1 V/div
+ –
Time = 500 µs/div
Figure 15–11. Output of the Circuit in Figure 15–10
VOUT