Feedback System Design
Instead of calculating the bandwidth, PSPICE was used with a linearized circuit (see Figure 4−5) to simulate and adjust the component values to approximately
Figure 4−5. PSPICE Circuit for Simulating the Feedback
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Integrator for APA100
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100 VDB
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adds 80kHz pole for TPA2001D1
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filterdivide by 45 in |
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feedback of APA100 |
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First, resistor R18 was removed to give an open−loop response, with the APA100 output being simulated by the output of the RC filter after the third op amp. Taking the gain and phase after the RC filter takes into account the 252−kHz filtering before the feedback op amp.
Then, R24 was set low and C25 was adjusted to make the output of the third op amp equal to the closed−loop gain (27 dB) at 40 kHz; C25 was kept less than one−tenth of C25. Once the open−loop frequency was approximately 40 kHz, R24 was adjusted to set the zero to 48.2 kHz (needing to stay lower than the pole of the TPA2001D1). The zero was set much lower than 80 kHz for compensation, so that the cutoff frequency of the filter before the op amp (R22, R23, C20, C23, and C24) could be reduced from 400 kHz to 252 kHz. Resistor R24 was set to 1000 Ω, and capacitor C25 set to 3.3 nF.
The second pole, Fp from Equation 8, was set to 770 kHz by adjusting capacitor C21 to 220 pF.
The circuit was simulated to show
The green curve is the output of the integrator. Notice that the green curve’s slope levels off at 48 kHz, showing that the zero is properly placed. The zero does not cause the TAS5111 output (red curve) to level off at the zero frequency because the pole of the TPA2001D1 at 80 kHz keeps the overall slope constant. The red curves slope increases after 770 kHz due to the integrator pole from C21.