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

Active Element (Op Amp) Impact on the Oscillator
15-6
Gain — dB
Phase — °
ACL
ACL
ÉÉÉÉÉÉÉÉÉ
ÉÉÉÉÉÉÉÉÉ
0°
–45°
–90°
Minimum Desired
Range of fosc
ÉÉÉÉÉÉÉÉÉ
Figure 15–3. Op Amp Frequency Response
0
0.1 fC
Frequency — Hz
–20 dB/
Decade
fC
45°/
Decade
Most op amps are compensated and may have more than the 45 of phase shift at ω 3dB.
The op amp should therefore be chosen with a gain bandwidth that is at least one decade
above the oscillation frequency, as shown by the shaded area of Figure 15–3. The Wien
bridge requires a gain bandwidth greater than 43ω OSC to maintain the gain and frequency
within 10% of the ideal values [2]. Figure 15–4 compares the output distortion versus frequency
of an LM328, a TLV247x, and a TLC071 op amp, which have bandwidths of 0.4
MHz and 2.8 MHz, and 10 MHz, respectively, in a Wien bridge oscillator with nonlinear
feedback (see Section 15.7.1 for the circuit and transfer function) The oscillation frequency
ranged from 16 Hz to 160 kHz. The graph illustrates the importance of choosing the
correct op smp for the application. The LM328 achieved a maximum oscillation of 72 kHz
and was attenuated more than 75%, while the TLV247x achieved 125 kHz with 18% attenuation.
The wide bandwidth of the TLC071 provided a 138 kHz oscillation frequency with
a mere 2% attenuation. The op amp must be chosen with the proper bandwidth or the output
may oscillate at a frequency well below the design specification.
10 fC