Question

12.19 The pedagogical circuit of Problem 12.17 is used in place of the operational amplifier shown in Example 12.1. The pole frequencies are at 1 MHz, 12 MHz, and 50 MHz. Determine the phase margin.

For reference:

12.17 Figure P12.17 shows a “Pedagogical Circuit” that is useful for finding phase-margin and stability conditions with the help of SPICE. Box zero uses a voltage-dependent voltage source (E0) to set the AF product, while boxes 1, 2, and 3 use a similar source and an RC network to set pole frequencies fp1, fp2, and fp3. These dependent sources are described with statements of the form E0 a b c d g where a and b denote positive and negative nodes for the dependent output, c and d denote positive and negative nodes for the controlling input, and g is the gain (unity for E1, E2, and E3). Subject to C = 159.2 pF, a pole frequency is 1 GHz/R, with R expressed in ohms. Let fp1 = 10 kHz, fp2 = 1 MHz, and fp3 = 100 MHz. Simulate the pedagogical circuit with SPICE to find the phase margin for feedback systems with AF = 5000, AF = 10, 000, and AF = 20, 000.

Answer 1

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