Texas Instruments APA100 manual 5. Pspice Circuit for Simulating the Feedback

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 40-kHz bandwidth. Then, Equations 7 and 8 were used to set the poles and zeros. The first op amp (U1) in the simulation circuit of Figure 4−5 is the integrator; the second op amp (U2) sets the 80-kHz pole; the third (U3) adds the gain from the TPA2001D1 and TAS5111 (56 V/V); and the final op amp (U4) is the divide−by−45 feedback amplifier.

Figure 4−5. PSPICE Circuit for Simulating the Feedback

1.5

0

C10

V1 3

1

0

V2

R121		U10A		V+
3	3	+	4
20
					1
	2		OUT
		−	11	V−	TLV2464A
				0
				R21
			200k
				C21
R14				220p
2k			R24		C25
			1k		3.3n

Integrator for APA100

R1

100 VDB

R18

1k

			U11A				3
				4
		3 3	+		V+
						1
	R112	2			OUT
						TLV2464A
			−	11	V−
							C117
C1	1k
					0
1n							20u

0

R133

1k

C116	R120
2nF	10

adds 80kHz pole for TPA2001D1

	V3	3 3	U12A		4
			+		V+
					1
				OUT
	0	2	−		TLV2464A
R119
			11		V−
1k
					0
			R115
			56k
		adds 56V/V of gain from
		TPA2001D1 + TAS5111.
		Output of this opamp is
		simulating output of
		TAS5111.

		U13A
			4
3	3	+		V+
					1
			OUT
R16	2	−	11	TLV2464A
				V−

40k

0

	VDB	R17
	55
R23	VP	1k
4.7k	C24	C26	C27
	56p	56p	22p
	0
filterdivide by 45 in
feedback of APA100

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 40-kHz bandwidth with 49_ phase margin (see Figure 4−6). The red curve (simulating APA100 output) hits 27 dB at 40 kHz, and at 40-kHz frequency the phase margin (blue curve) is 49_.

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.

4-6